I have before expressed my sentiments upon several of these Orders[1443]: I shall not here repeat them, but shall merely observe, with respect to those I have not adopted, that, though perhaps not entitled to rank as Orders, most of them form natural groups. His Orders, however, of Arachnida must be excepted from this remark, since they are evidently artificial. His analyses of his Orders, though in general they give natural groups, are usually not carried so far as those of M. Latreille, so as seldom to indicate what may properly be denominated families. He has made his nomenclature for his so-called families more uniform and satisfactory than that of the French Entomologist: and we may say, with respect to the extent and effect of his zoological labours,—Nihil non tetigit, et omnia quæ tetigit ornavit.
7. Era of MacLeay, or of the Quinary System. I have more than once stated to you in my former letters the bases upon which the system which I am in the last place to explain to you is built. You know the Sub-kingdoms and Classes into which its learned and ingenious author, upon a novel and most remarkable plan, has divided the Animal Kingdom[1444]. I shall now copy for you his diagram of the Annulosa.
I have before sufficiently noticed these Classes, or Orders as Mr. MacLeay terms them, of the Sub-kingdom Annulosa: I shall here therefore only throw out a few remarks on their composition. With regard to their circular distribution in the Crustacea, Mr. MacLeay thinks the series runs from the Branchiopods or Monoculus L. to the Decapods or Cancer L.; and so on, till by means perhaps of the genus Bopyrus, which Fabricius regards as a Monoculus, it returns to the Branchiopods again. This circle, through Porcellio, a kind of wood-louse, &c., which has only a pair of antennæ and at first but six legs, is connected with the Ametabola Class, which beginning with Glomeris goes by the other Chilognatha (Iulus L.), having also six legs at first, and certain Vermes to the Anoplura, and terminates in the Chilopoda (Scolopendra L.) their cognate tribe[1445]. From the Ametabola Mr. MacLeay proceeds to the Mandibulata, between which two groups he has discovered no osculant one, but he takes the Anoplura of the former as the transit to the Coleoptera in the latter; from whence passing to the Orthoptera, &c., he finally returns by the Hymenoptera. Between the Mandibulata likewise and Haustellata he finds no osculant class: but as the affinity between the Trichoptera and Lepidoptera is evident, proceeding by the Homoptera he returns to the Lepidoptera by certain Diptera, as Psychoda, &c. From the Aptera Lam. or Pulex L. he passes by the osculant class Nycteribida to the Arachnida; and beginning with the Acaridea, he goes to the Scorpionidea, and so to the Aranidea or spiders, which he connects with the Decapod Crustacea;—thus forming his great circle of five smaller ones, each of which, as well as that which they form, returns into itself[1446].
We next take his Circles of Mandibulata: thus—
In this arrangement of the tribes, as he calls them, of Mandibulata, Mr. MacLeay sets out from the Coleoptera, which he distributes, according to the supposed typical forms of their larvæ, into five minor groups, sufficiently noticed on a former occasion[1447]. From this tribe or Order he proposes to pass by Atractocerus to the osculant Order Strepsiptera, and from thence by Myrmecodes and the Ants to the Hymenoptera. From hence he next proceeds to his Trichoptera; in which, as we have seen[1447], he places not only Phryganea L., but also Tenthredo L. and Perla Geoffr., making his transit by Sirex L.; forming an osculant Order which he denominates Bomboptera. From this his way to the Neuroptera is by the Perlides, with Sialis as an osculant Order under the name of Megaloptera: he enters by Chauliodes, and leaves it by Panorpa or Raphidia by means of Boreus, forming also an osculant Order (Raphioptera) for the Orthoptera; which he enters by Phasma, Mantis, &c., and leaves by Gryllus, entering the Coleoptera again by the osculant Order Dermaptera formed of Forficula L.: and thus returning to the point from which he set out[1448]. He has not, however, made this return of the series into itself so clear in each order, excepting in the Orthoptera, as he has done in the whole Class or Sub-class. Thus in the Coleoptera there appears no particular affinity between the Predaceous and Vesicant beetles, his first and fifth forms[1449], or his Chilopodimorphous Coleoptera, and his Thysanurimorphous.
To enter fully into his doctrine of Analogies would lead us into a very wide field, and occupy a larger space than I can afford; I must therefore refer you to his work for more particular and detailed information on that subject. With regard to the analogy between opposite points of contiguous circles, you may get a very good idea of it from his diagram of Saprophagous and Thalerophagous Petalocerous beetles, which I here subjoin.
It is a very singular circumstance that in these two circles we have two sets of insects,—one impure in its habits and feeding upon putrescent food, and the other clean and nourished by food that has suffered no decay,—set in contrast with each other, and that in each of the opposite groups, the one has its counterpart in some respect in the other. In none is this more striking than the Scarabæidæ and Cetoniadæ, both remarkable for having soft membranous mandibles unfit for mastication, and both living upon juices, the one in a putrescent and the other in an undecayed state[1450]. Our learned author in subsequent works has stated every circle to be resolvable into two superior groups, which he denominates normal or typical, and three inferior ones, which he calls aberrant or annectent[1451].
Before I conclude this account of the various general systems that have distinguished the different entomological eras, i must say a few words on those partial ones which have been founded on the neuration of the wings of insects. Frisch, who died in 1743, attempted something in this way[1452]: Harris, in his Exposition of English Insects published in 1782, had arranged his Hymenoptera and Diptera according to characters derived from this same circumstance[1453]: Mr. Jones in the Linnean Transactions had made good use of it in dividing the Diurnal Lepidoptera into groups[1454]: and in the Monographia Apum Angliæ, the characters exhibited by the various groups into which Linné's genus Apis was resolvable, as to the neuration of their wings, were described[1455]. But M. Jurine was the first Entomologist who made that circumstance the keystone of a system; which indeed he restricted to Hymenopterous and Dipterous insects, but which might be extended much further. As this system has been before sufficiently enlarged upon[1456], I need here only mention it.
To particularize the various entomological works in every department of the science, that have appeared since the commencement of the era of Fabricius, would require a volume. Such was its progress and spread, that in every corner of Europe the pens and pencils of able and eminent men, whose works have almost all been quoted in the course of our correspondence, have been employed to illustrate it[1457]. I may observe, however, that the Internal Anatomy of Insects, a branch of Entomology which on account of its difficulty, from the extreme nicety required in dissecting them, had before been cultivated by scarcely more than a single student in an age, has now attracted numerous votaries. In Germany—Carus, Gaede, Herold, Posselt, Ramdohr, Rifferschweils, Sprengel, and others, have distinguished themselves in this arena: and in France, besides the illustrious Baron Cuvier (himself a host), Marcel de Serres, Leon Dufour, and very recently, by his elaborate essays On the Flight of Insects and its wonderful apparatus, one of the most acute of anatomical physiologists, M. Chabrier,—have all contributed greatly to the elucidation of this interesting part of the science. In our own country very little has hitherto been effected in this line; but a learned Oxford Professor (Kidd) has presented to the Royal Society an account of the anatomy of the Mole-cricket, which entitles him to an eminent station amongst the above worthies.
I may likewise further observe, that the pictorial department of Entomology was, during the period I am speaking of, carried to its greatest perfection. Painters of insects formerly were satisfied with giving a representation generally correct, without attempting a faithful delineation of all the minor parts, particularly as to number;—for instance, the joints of the antennæ and tarsi, the areolets of the wings, &c.: but now no one gives satisfaction as an entomological artist unless he is accurate in these respects.
Though no subject is more worthy of the attention of the Entomologist than the Geographical Distribution of insects, yet perhaps there is none connected with the science, for the elucidation of which he is furnished with fewer materials. The geographer of these animals sitting by his fireside, even supposing his museum as amply stored as that of Mr. MacLeay, and the habitats of its contents as accurately indicated, still labours under difficulties that are almost insuperable; so that it is next to impossible, with our present knowledge of the subject, to give satisfactory information upon every point which it includes. Had he the talents and opportunities of a Humboldt, and could, like him, traverse a large portion of the globe, he would endeavour to note the elevation, the soil and aspect, the latitude and longitude, the mean temperature and meteorological phænomena, the season of the year, the kind of country, and other localities connected with the insects he captured, and so might build his superstructure upon a sure basis. But these are things seldom registered by travellers that take the trouble to collect insects; who, if they specify generally the country in which any individual was found, think they have done enough. But to say that an insect was taken in India, China, New Holland, and North or South America,—when we consider the vast extent of those regions,—is saying little of what one wishes to know even with respect to its habitat. You must regard therefore, after all, what I have been able to collect,—and for which I am greatly indebted to the labours of my few but able precursors in this walk,—as merely approximations to an outline, rather than as a correct map of insect Geography.
Amongst the numerous obligations that he conferred upon Natural History, Linné was the first Naturalist who turned his attention to the Geographical Distribution of its objects, especially that of the Vegetable Kingdom[1458]: and the accomplished traveller Baron Humboldt, by the observations he made on this subject in the course of his peregrinations in tropical America, has furnished the Botanist with a clue which, duly followed, will enable him to perfect that part of his science; an end to which the learned observations of Messrs. R. Brown and Decandolle have greatly contributed[1459]. With regard to animals, Mr. White, so long ago as 1773, had observed that they, as well as plants, might with propriety be arranged geographically[1460]: and in 1778 Fabricius in his Philosophia Entomologica applied the principle to insects[1461]. Nearly forty years elapsed before any improvement or enlargement of this last department was attempted; when in 1815 M. Latreille, stimulated by what had been effected in Botany, in a learned and admirable memoir[1462] endeavoured to place Entomology in this respect by the side of her more fortunate sister: and subsequently Mr. W. S. MacLeay, in the memorable work so often quoted in our correspondence, has viewed the subject in another light, and added some important information to what had been before collected[1463].
The point now under consideration naturally divides itself into two principal branches;—the numerical distribution of insects, and the topographical.
I. By the numerical distribution of insects I mean not only the number which Providence has employed to carry on its great plan on this terraqueous globe, or any given portion of it; or of the species of which each group or genus may be supposed to consist; or of the comparative number of individuals furnished by each species,—points of no easy solution: but more particularly their distribution according to their functions, whether they prey upon animal or vegetable matter, and in its living or decaying state.
We have no data enabling us to ascertain with any degree of accuracy the actual number of species of insects and Arachnida distributed over the surface of the globe; but it is doubtless regulated in a great degree by that of plants. We should first then endeavour to gain some just though general notion on that head. Now Decandolle conjectures that the number of the species of plants, 60,000 being already known, may be somewhere between 110,000 and 120,000[1464]. If we consider with reference to this calculation, that though the great body of the mosses, lichens, and sea-weeds are exempt from the attack of insects, yet as a vast number of phanerogamous plants and fungi are inhabited by several species, we may form some idea how immense must be the number of existing insects; and how beggarly does Ray's conjecture of 20,000 species[1465], which in his time was reckoned a magnificent idea, appear in comparison! Perhaps we may obtain some approximation by comparing the number of the species of insects already discovered in Britain with that of its phanerogamous plants. The latter,—and it is not to be expected that any large number of species have escaped the researches of our numerous Botanists,—may be stated in round numbers at 1500, while the British insects, (and thousands it is probable remain still undiscovered,) amount to 10,000; which is more than six insects to one plant. Now though this proportion, it is probable, does not hold universally; yet if it be considered how much more prolific in species tropical regions are than our chilly climate, it may perhaps be regarded as not very wide of a fair medium. If then we reckon the phanerogamous vegetables of the globe in round numbers at 100,000 species, the number of insects would amount to 600,000. If we say 400,000, we shall perhaps not be very wide of the truth. When we reflect how much greater attention has been paid to the collection of plants than to that of insects, and that 100,000 species of the latter may be supposed already to have a place in our cabinets[1466], we may very reasonably infer that at least three fourths of the existing species remain undiscovered.
Certain groups and genera are found to contain many more species than others: for instance, the Coleoptera and Lepidoptera Orders than the Orthoptera and Neuroptera; the Rhincophora than the Xylophagi: the Dytiscidæ than the Gyrinidæ; Aphodius than Geotrupes; Carabus than Calosoma. Again, some insects are much more prolific than others. Thus the Diptera Order, though not half so numerous with respect to species as the Coleoptera, exceeds it greatly in the number of individuals, filling the air in every place and almost at every season with its dancing myriads. We rarely meet with a single individual of the most common species of Calosoma or Buprestis; whilst the formicary, the termitary, the vespiary, and the bee-hive send forth their thousands and tens of thousands; and whole countries are covered and devastated by the Aphides and the Locusts. An all-wise Providence has proportioned the numbers of each group and species to the work assigned to them. And this is the view in which the numerical distribution of insects is most interesting and important: and we are indebted to Mr. W. S. MacLeay for calling the attention of Entomologists more particularly to this part of our present subject.
With regard to their functions, insects may be primarily divided into those that feed upon animal matter and those that feed upon vegetable. At first you would be inclined to suppose that the latter must greatly exceed the former in number: but when you reflect that not only a very large proportion of Vertebrate animals, and even some Mollusca[1467], have more than one species that preys upon them, but that probably the majority of insects, particularly the almost innumerable species of Lepidoptera, are infested by parasites of their own class, sometimes having a different one appropriated to them in each of their preparatory states[1468], and moreover that a large number of beetles and other insects devour both living and dead animals,—you will begin to suspect that these two tribes may be more near a counterpoise than at first seemed probable. In fact, out of a list of more than 8000 British insects and Arachnida taken several years ago, and furnished chiefly by Mr. Stephens, I found that 3894 might be called carnivorous, and 3724 phytiphagous[1469]; so that, speaking roundly, they might be denominated equiponderant.
Carnivorous and phytiphagous insects may be further subdivided according to the state in which they take their food,—whether they attack it while living, or not till after it is dead. To adopt Mr. W. S. MacLeay's phraseology, the former may be denominated thalerophagous, and the latter saprophagous. The British saprophagous carnivorous insects, compared with those that are thalerophagous, are about as 1:6; while the phytiphagous ones are as 1:9. The thalerophaga in both tribes may be further subdivided as they take their food by suction or mastication: in the carnivorous ones, the suckers to the masticators in Britain are nearly as 1:6; but with respect to the phytiphagous tribe you must take into consideration that some insects imbibing their food by suction in their perfect state (as the great body of the Lepidoptera), masticate it when they are larvæ: deducting therefore from both sides the insects thus circumstanced, the masticators will form about three fourths of the remaining British thalerophagous insects. Another circumstance belonging to this head must not be passed without notice:—there are certain insects feeding upon liquid food that do not suck, but lap it. This is the case with the Hymenoptera, who, though they are mandibulate, generally lap their food (the nectar of flowers) with their tongue, and may be called lambent insects: nor is this practice confined to that order, but all the mandibulate insects that feed on that substance merit the same appellation. The absorption of this nectar is so important a point in the economy of nature, that a very large proportion of the insect population of the globe in their perfect state, are devoted to it. Considerably more than half the species indigenous to Britain fulfill this function, and probably in tropical countries the proportion may be still larger.
To push this analysis still further—Amongst our carnivorous thalerophaga, aphidivorous insects are about as 1:14; and amongst the phytiphagous, the fungivorous ones form about a twentieth; and the granivorous about a twenty-fifth part of the whole. Again: in the saprophaga the lignivorous tribes form more than half, and the coprophagous ones more than a third.
If you wish to know further the relative proportions of the different Orders to each other—The Coleoptera may be stated as forming at least 1:2 of our intire insect population; the Orthoptera and Dermaptera as about 1:160; the Hemiptera as 1:15; the Lepidoptera as more than 1:4; the Neuroptera with the Trichoptera as 1:29; the Hymenoptera as about 1:4; the Diptera as not 1:7; and the Aptera and Arachnida as perhaps amounting to 1:19[1470].
To extend this inquiry to exotic and more particularly to extra-European insects, in the present state of our knowledge, would lead to no very satisfactory results. The lists we have are so imperfect, that those which tell most in this country,—I mean the more minute insects and the Brachyptera—have hitherto formed a very small, if any part, of the collections made out of Europe. Mr. W. S. MacLeay however, who, besides his father's (particularly rich in Petalocera), has had an opportunity of examining the Parisian and other cabinets, finds that the species of coprophagous insects within the tropics, to those without, are nearly in the proportion of 4:3; and that the coprophagous Petalocera, to the remainder of the saprophagous ones, may be represented by 3:2[1471]. It may be inferred, from the superabundance of plants and animals in equinoctial countries, that the number of species of insects in general is greater within than without the tropics: the additional momentum produced by the vast size of many of the tropical species must also be taken into consideration.
II. There are three principal points that call for attention under the second branch of our present subject—the topographical distribution of insects; namely, their Climates, their Range, and their Representation.
i. Entomologists, taking heat for the principal regulator of the station of insects, have divided the globe into entomological climates. Fabricius considers it as divisible into eight such climates, which he denominates the Indian, Egyptian, Southern, Mediterranean, Northern, Oriental, Occidental, and Alpine. The first containing the tropics; the second, the northern region immediately adjacent; the third, the southern; the fourth, the countries bordering on the Mediterranean sea, including also Armenia and Media; the fifth, the northern part of Europe interjacent between Lapland and Paris; the sixth, the northern parts of Asia where the cold in winter is intense; the seventh, North America, Japan, and China; and the eighth, all those mountains whose summits are covered with eternal snow[1472]. M. Latreille objects to this division, as too vague and arbitrary and not sufficiently correct as to temperature; and observes, with great truth, that as places where the temperature is the same, have different animals, it is impossible, in the actual state of our knowledge, to fix these distinctions of climates upon a solid basis. The different elevations of the soil above the level of the sea, its mineralogical composition, the varying quantity of its waters, the modifications which the mountains, by their extent, their height, and their direction, produce upon its temperature; the forests, larger or smaller, with which it may be covered; the effects of neighbouring climates upon it,—are all elements that render calculations on this subject very complicated, and throw a great degree of uncertainty over them[1473]. This learned Entomologist would judiciously consider entomological climates under another view,—that which the genera of Arachnida and insects exclusively appropriated to determinate spots or regions would supply[1474]. Linné's dictum with regard to genera will here also apply; "Let the insects point out the climate, and not the climate the insects." If you expect invariably to find the same insects within the same parallels of latitude, you will be sadly disappointed; for, as our author further observes, "The totality or a very large number of Arachnida and insects, the temperature and soil of whose country are the same, but widely separated, is in general, even if the countries are in the same parallel, composed of different species[1475]." The natural limits of a country,—as mountainous ranges, rivers, vast deserts, &c.,—often also say to its insect population, "No further shall ye come;" interposing a barrier that it never passes[1476]. Humboldt observes, with respect to the Simulia and Culices of South America, that their geographical distribution does not appear to depend solely on the heat of the climate, the excess of humidity, or the thickness of forests; but on local circumstances that are difficult to characterize[1477]: and Mr. W. S. MacLeay makes a similar observation upon that of Gymnopleurus[1478]. So that the real insect climates, or those in which certain groups or species appear, may be regarded as fixed by the will of the Creator, rather than as certainly regulated by any isothermal lines. Still, however, under certain limitations, it must be admitted that the temperature has much to do with the station of insects. The increase of caloric is always attended with a proportional increase in the number and kind of the groups and species of these beings. If we begin within the polar regions of ice and snow, the list is very meager. As we descend towards the line, their numbers keep gradually increasing, till they absolutely swarm within the tropics. Something like this takes place in miniature upon mountains. Tournefort long since observed at the summit of Mount Ararat the plants of Lapland; a little lower, those of Sweden; next, as he descended, those of Germany, France, and Italy; and at the foot of the mountain, such as were natural to the soil of Armenia. And the same has been observed of insects. Those that inhabit the plains of northern regions have been found on the mountains of more southern ones; as the beautiful and common Swedish butterfly Parnassius Apollo, on the mountains of France, and Prionus depsarius on those of Switzerland[1479].
M. Latreille, having given a rapid survey of the peculiar insect-productions of different countries, next attempts a division of the globe into climates, which he thinks may be made to agree with the present state of our knowledge, and be even applicable to future discoveries. He proposes dividing it primarily into Arctic and Antarctic climates, according as they are situated above or below the equinoctial line; and taking twelve degrees of latitude for each climate, he subdivides the whole into twelve climates. Beginning at 84° N. L. he has seven Arctic ones, which he names polar, subpolar, superior, intermediate, supratropical, tropical, and equatorial: but his antarctic climates, as no land has been discovered below 60° S. L., amount only to five, beginning with the equatorial and terminating with the superior. He proposes further to divide his climates into subclimates, by means of certain meridian lines; separating thus the old world from the new, and subdividing the former into two great portions,—an eastern, beginning with India, and a western, terminating with Persia. He proposes further that each climate should be considered as having 24° of longitude, as well as 12° of latitude[1480]. In this chart of insect Geography he states that he has endeavoured to make his climates agree with the actual distribution of insects[1481]; and it should seem that in many cases such an agreement actually does take place: yet the division of the globe into climates by equivalent parallels and meridians, wears the appearance of an artificial and arbitrary system, rather than of one according with nature.
He has also pointed out another index to insect climates, borrowed from the Flora of a country. Southern forms in Entomology, he observes, commence where the vine begins to prosper by the sole influence of the mean temperature; that they are dominant where the olive is cultivated; that species still more southern are compatriots of the orange and palmetto; and that some equatorial genera accompany the date, the sugar-cane, the indigo and banana[1482]. The idea is very ingenious, and, under certain limitations, supplies a useful and certain criterion. For though none of these plants are universal in isothermal parallels of latitude; yet, as plants are more conspicuous than insects, the Entomologist, furnished with an index of this kind, may by it be directed in his researches for them; and in all countries in which there is a material change of the climate, as in France, there will be a proportional change in the vegetable accompanied by one in the insect productions.
ii. In considering the range of insects I shall first advert to that of individual species. At the extreme limits of phanerogamous vegetation we find a species of humble-bee (Bombus arcticus), which, though it is not known to leave the Arctic circle, has a very extensive range to the westward of the meridian of Greenwich, having been traced from Greenland to Melville Island; while to the eastward of that meridian it has not been met with. In Lapland its place appears to be occupied by B. alpinus and lapponicus, with the former of which, though quite distinct, it was confounded by O. Fabricius; but whether these range further eastward of that meridian has not been ascertained. From its being found in the Lapland Alps[1483], it may be conjectured that B. alpinus ranges as high on this side as B. arcticus on the other, and may perhaps be found in Nova Zembla. Some species that have been taken in Arctic regions are not confined to them. Of this kind is Dytiscus marginalis, which appears common in Greenland, abundant in Britain, and is dispersed over all Europe; while D. latissimus is more confined, neither ranging so far to the north or south; and though found in Germany, not yet discovered in Britain. Other species have a still more extensive range, and are common to the old world and the new. Thus Dermestes murinus, Brachinus crepitans, Tetyra scarabæoides[1484], Pentatoma juniperina, Cercopis spumaria, Vanessa Antiopa, Polyommatus Argiolus, Hesperia Comma, Vespa vulgaris, Ophion luteus, Helophilus pendulus, Oscinis Germinationis, and many besides, though sometimes varying slightly[1485], inhabit both Britain and Canada: and though vast continents and oceans intervene between us, New Holland, and Japan; yet all have some insect productions in common. With the former we possess the painted-lady butterfly (Cinthia Cardui), with scarcely a varying streak: and Thunberg, in his list of Japan insects, has mentioned more than forty species that are found also in this country. Whether any species has a universal range may be doubted, unless indeed the flea and the louse may be excepted. On the other hand, some are confined within very narrow limits. Apion Ulicis for instance, abundant upon Ulex europæus in Britain, has not, I believe, been found upon that plant on the continent.
The geographical distribution of groups, is, however, far more interesting than that of individual species: for in considering this we see more evidently how certain functions are devolved upon certain forms, and can scan the great plan of Providence, in the creation of insects, more satisfactorily than by confining our attention to the latter. Groups, according to their range, may be denominated either predominant, dominant, sub-dominant, or quiescent.
1. M. Latreille has observed, that where the empire of Flora ceases, there also terminates that of Zoology[1486]. Phytiphagous animals can only exist where there are plants; and those that are carnivorous and feed upon the former, must of necessity stop where they stop. Even the gnat, which extends its northern reign so high[1487], must cease at this limit; while, where vegetation is the richest and most abundant, there the animal productions, especially the insect, must be equally abundant. I call that, therefore, a predominant group, members of which are found in all the countries between these points, or from the limits of animal-depasturing vegetation in the polar regions to the line.
Generally speaking, the carnivorous insects, whether thalerophagous or saprophagous, are of this description. Calosoma, which devours Lepidopterous larvæ, though poor in species and individuals, is widely scattered. Captain Frankland found C. calidum in his Arctic journey; C. laterale and curvipes inhabit tropical America[1488]: C. Chinense, as its name indicates, is Chinese[1489]; Mr. MacLeay has an undescribed species from New Holland; and C. retusum was taken in Terra del Fuego. Another genus, equally universal and richer in numbers, is the lady-bird (Coccinella), which keeps within due limits the Aphides of every climate from pole to pole. The Libellulina pursue their prey both in Greenland and New Holland. The saprophagous carnivora are also similarly predominant;—the Silphidæ, the Dermestidæ, the Brachyptera, the Muscidæ, prey on carcases wherever the action of the solar beam causes them to become putrid. Many of the above insects have probably their capital station, or that where the species are most numerous, in or near the tropics; but the metropolis of the Brachyptera, at least as far as we can judge from our present catalogues, is within the temperate zone, particularly in Britain[1490]. The coprophagous Petalocera are most abundant in the hottest climates; but the Aphodiadæ form a predominant group: Professor Hooker took one species in Iceland[1491], and it probably ascends higher; others are found in India and China: but the metropolis of the group is within the temperate zone. Perhaps no genus is more completely universal than Bombus (Bremus Jur.), which, although its centre or metropolis is likewise in the northern temperate zone, extends from Melville Island to the line. It is remarkable that some of the tropical Bombi wear the external aspect of Xylocopæ, the kindred genus most prevalent in warm climates; and, vice versâ, some Xylocopæ resemble Bombi. I have a Brazilian undescribed species of the latter genus, whose black body and violet-coloured wings would almost cause it to be mistaken for a variety of X. violacea; and B. antiguensis and caffrus F., (though their aspect belies it,) which misled Fabricius, are true Xylocopæ. I shall mention only one other predominant group, but that one of no common celebrity, formed of the gnats, or genus Culex. These piping pests, with their quiver—"venenatis gravida sagittis"—annoy man almost from the pole to the line. What remarkably distinguishes them, (as was formerly observed[1492],) and also the Simulium or true mosquito,—they appear to prevail most in the coldest and the hottest climates, and the Laplander and the tropical American are equally their prey; while the inhabitants of the temperate zone, with some exceptions, suffer but little from them: so that they may be stated to have both an arctic and a tropical metropolis.
2. There are other groups which, though their empire extends to the tropics, fall short of the polar circles:—these I call dominant groups. Of this description are some of the Scarabæidæ. Onthophagus is found both in the old world and in the new, and in the temperate and torrid zones. Its principal seat appears to be within the tropics, but it may almost be said to have also a northern metropolis. More than one species have been taken in New Holland. In general, tropical insects exceed those of colder climates in size; but in the genus we are speaking of, the European species are usually larger than the Indian. Copris seems more abhorrent of cold than its near relation Onthophagus. C. lunaris, which ranges northward as far as Sweden, is the only recorded species found in Europe out of Spain. Latreille says, that all the large species of this genus are equinoctial: but C. Tmolus, described and figured by Fischer[1493], found in Asia near Orenburg, north of 50° N. L., is as big as C. Gigas or bucephalus. Another dominant group of Petalocera, remarkable for the bulk and arms of its tropical species, are the mighty Dynastidæ, the giants and princes of the insect race. Though their metropolis is strictly tropical, yet the scouts of their host have wandered even as far as the south of Sweden, where one of them, Oryctes nasicornis, is extremely common. O. Grypus[1494] and some other species are found in South Europe; but though in a torpid state they can endure unhurt the severity of a Scandinavian winter, they cannot when revived stand the cold that often pinches Britons in the midst of summer, and therefore are unknown in our islands[1495]. The Sphæridiadæ, whose metropolis is within the northern temperate zone, extend from thence beyond the line, since Dr. Horsfield found two species in Java[1496]. It is probable, indeed, that this group is predominant. Some dominant groups begin at a lower latitude. Of this description are the carpenter-bees (Xylocopa), whose larvæ are preyed upon by that of the Horiadæ[1497] under two forms, which extend from the tropics to about 50° N. L. Others are not common to both worlds. Thus, while Cantharis is the gift of Providence to America as well as the old world, Mylabris is confined to the latter, where its range is very extensive;—in Europe, from South Russia to Italy and Spain; in Asia, from Siberia to India; and in Africa, from the shores of the Mediterranean to the Cape of Good Hope; which last continent, to judge from our present lists, especially the vicinity of the Cape, may be called the metropolis of the group[1498]. On the other hand, the Rutelidæ and Chlamys, which have a range from Canada to the tropics, (within which is their metropolis,) are purely American groups. Many more might be named under this head, but these will suffice for examples.
3. I call those subdominant groups, which either never enter the tropics, or those tropical ones whose range does not exceed 50° of N. L. in the old world, or 43° in the new. I make this difference because, as M. Latreille observes, the southern insects which in Europe begin between 48° and 49° N. L., in America do not reach 43°.[1499] But though the winters in Canada, within the same parallel as France, are longer and more severe than those even of Great Britain or of Germany, yet the summers are intensely hot; so that though tropical species do not range so high, those of a tropical structure, as Mr. W. S. MacLeay has intimated[1500], may be found at a higher latitude in the new world than in Europe.
The genus Melöe affords an instance of a subdominant group of the first description. It ranges from Sweden to Spain and the shores of the Mediterranean, and seems a tribe almost confined to Europe, where it is not very unequally distributed. Of registered species Britain possesses the largest proportion; but Mr. W. S. MacLeay is of opinion that Spain is its true metropolis[1501]. I have a species of this genus, taken in North America by Professor Peck. The splendid genus Carabus ranges still further north than Melöe[1502]. A very fine species (C. cribellatus) inhabits the polar regions of Siberia[1503]; but the metropolis of the group appears to be the temperate zone: some, however, have been found in northern Africa; and Sir Joseph Banks captured one in Terra del Fuego. Of those whose range is between the tropics and 50° N. L. we may begin with Cicada. One species, indeed, has been found by Mr. Bydder and others, a little higher, near the New Forest, Hampshire. We may take Scolia for an example of a subdominant group beginning more southward. Its species first appear about 43° N. L., and abound in warm climates. In general most of those insects which M. Latreille denominates meridional,—such as Scarabæus, Onitis, Brentus, Scarites, Mantis, Fulgora, Termes, Scorpio, &c.—come under the present head, and in fact all tropical forms that wander to any distance within the above limits from their metropolis.
4. By quiescent groups I mean those that have none, or no high range as to latitude, from their centre or metropolis. I say as to latitude, because these groups have often an extensive one as to longitude. Thus, Mr. W. S. MacLeay has remarked to me, that Goliathus appears to belt the globe, but not under one form. The types of the genus are the vast African Goliaths (G. giganteus, &c.), which, as well as G. Polyphemus, and another brought from Java by Dr. Horsfield, have, like Cetonia[1504], the scapulars interposed between the posterior angles of the prothorax and the shoulders of the elytra[1505]: while the South American species (G. micans, &c.) have not this projection of the scapulars; in this resembling Trichius. Mr. MacLeay further observes, that the female of the Javanese Goliathus is exactly a Cetonia, while that of the Brazilian is a Trichius. But quiescent groups have not generally this ample longitudinal range. Thus, Euglossa, in both its types,—one represented by Eu. cordata, and the other by Eu. surinamensis,—is confined to the tropical regions of America. Doryphora, likewise American, seems equally confined. Asida, though a southern genus, is not found to enter the tropics; and Manticora and Pneumora are in nearly the same predicament.
Under the present head we may consider what may perhaps be denominated without much impropriety endemial groups; by which I mean those groups that are regulated, as to their limits, not so much by the temperature, or the northing and southing of the latitude, as by the general aspect and circumstances of the country. Thus, the vast and nearly insular continent of Africa, almost as wide as it is long, and situated in or near the tropics, instead of inland seas or sea-like rivers, is intersected by parched sandy deserts, extending far and wide; circumstances which, though in the vicinity of its streams it is humid, impart an unusual degree of aridity as well as heat to its general atmosphere; so that it well merits the poet's epithet, Leonum arida nutrix; and is also peculiarly fitted for all such animals, especially insects, as delight in a dry, sandy, hot country, particularly such as are predaceous in their habits. America, on the other hand, exhibits quite an opposite character. It is long, and comparatively narrow; surrounded, and almost divided into two continents, by immense circumfluent oceans; watered every where by rivers and lakes that emulate seas: in some parts covered by interminable forests; in others, intersected by ridges of the loftiest mountains. These circumstances, except in its Llanos, Pampas, or table-land, give a general character of humidity to its atmosphere, and fit it particularly for the production of a vast variety of peculiar plants, and for the residence of numerous and peculiar phytiphagous insects and other animals[1506]. Midway between these two continents lies a third (for so the vast island of New Holland may be denominated), which presents new features in its general aspect, and consequently new forms both in its Flora and Fauna, mixed with many old ones parallel to those both of the new world and the old. Perhaps Europe and Asia, with several that are peculiar, agree more in their animal productions than the continents just described.
Let us next particularize a few of the peculiar types that distinguish particular continents and countries. The genera Manticora, Graphipterus, Glaphyrus, Eurychora, Pneumora, Masaris, and many others, are peculiar to Africa. In Asia alone we find Mimela[1507], Euchlora[1508], Colliuris, Catascopus[1509], Apogonia[1510], a peculiar type of Horia, &c. In America, Agra, Galerita, Nilio, another type of Horia, Tetraonyx, Rutela, Doryphora, Alurnus, Erotylus, Scotinus[1511], Cupes, Corydalis, Labidus, Heliconius, Castnia, &c. And in New Holland, Helluo, Elephastomus, Anoplognathus, Diphucephala[1512], Cerapterus, Heleus, Adelium, Notoclea, Achilus, Thynnus, Nycterobius, &c.
The countries bordering upon the Mediterranean, the Black, and the Caspian seas, agree in producing similar insects. These countries, and the Cape of Good Hope though so distant from them, appear to be the principal seat of Heteromerous Coleoptera, of the genera Lixus and Brachycerus, and of the conical Buprestes[1513]. But the insects of Guiana, on one side the Cordilleras, differ from those of New Granada and Peru on the other; and similar differences are observed in other neighbouring countries separated by natural boundaries.
iii. Another head connected with the topographical distribution of insects relates to their representation of each other. Here we may observe, that some insects represent each other only in their form; others also in their function; and others in both. I shall give some instances of each. In Brazil there is a group of petalocerous beetles (Chasmodia), one of the Rutelidæ, which in New Holland has a representative, as to form, in one of the Cetoniadæ (Schizorhina[1514]), which, having soft mandibles, must have a different function:—it is to be observed, however, that these insects appear to approach each other in the series of affinities. Again, the Carabidæ may in the same country be said to have a representative in the remarkable heteromerous genus Adelium[1515], which is altogether an analogy. Others are representative only in their function. The general function of insects is to remove nuisances and to check redundances,—the saprophagous tribes do the one, and the thalerophagous the other. In going from the poles to the line,—in proportion as the heat increases, the quantum of work of both kinds increases; and new forms are either added to the old ones, so as to increase their momentum; or new ones, more powerfully talented, replace the old ones, and act in their stead: thus we see a gradual and interesting change take place in proportion as we approach the maximum of heat and of insect population. At the Cape, the universal Cicindelæ are aided by Manticora; in North America, the Silphidæ by a new group, the type of which is Silpha Americana (Necrophila, K.MS.); in South America, Copris by Phanæus. Again: Colliuris and Drypta of the old world, in the new give place to Ctenostoma and Agra. The honey and wax of Europe, Asia, and Africa, is prepared by bees congenerous with our common hive-bee (Apis); while in America this genus is not found as a native, but is replaced by Melipona and Trigona[1516]; and in New Holland by a still different but undescribed type. The Melolonthidæ and Rutelidæ of the old and new world appear to have their work done in that country by the brilliant and numerous Anoplognathidæ. The Rhipicera of Brazil is of a different type from that of New Holland. The singular genus Cremastocheilus of North America has its representative in Africa in Genuchus[1517]. The Lucani of the rest of the world give place in New Holland to Lamprima and Ryssonotus.—I could produce a much greater number of examples, but these are sufficient to explain my meaning.
Having thus given you some, though an imperfect account, of the geographical distribution of insects, I am next to say something concerning their local distribution in any district, or their favourite haunts; a knowledge of which, with respect to those of our own country, is indispensable to the collector.
The surface of a country consists either of mountains, hills and valleys, or of plains. It is diversified by forest, wood, or copse; and watered by rivers, rivulets, lakes, and pools. Those parts that are not clothed with wood are either open or inclosed, forming grassy downs, heaths, pastures, meadows, morasses, and arable land. The soil also is equally various:—we find clay, loam, marl, chalk, vegetable mould, moor, sand, &c. The mountains and hills are either covered with a stratum of soil, or are rocky and bare; the arable lands are divided by living or dead fences, the latter formed of various materials,—or else they are open, and the property only marked out by grassy balks, &c. All these places abound in shrubs and plants; some local, and some generally distributed. But besides the land and its fresh waters, we must look also to the sea, and its sandy, pebbly, or rocky shores, and the sea-wrack that is cast up upon them; the estuaries that receive its tides; the brackish waters and saline marshes in its vicinity. All the above places, when opportunity serves, the Entomologist should explore, for in almost all he will find peculiar kinds of insects.
As mountains and hills have usually their own Flora, the insects appropriated to alpine plants can only be met with where the pabulum is found. Here also those northern insects that are impatient of a warmer climate will take their station, if they migrate to the southward[1518]. The predaceous beetles likewise sometimes frequent a mountainous district. Carabus glabratus was first taken by Professor Hooker on Ingleborough; and probably, if the Welsh and Scotch mountains were duly investigated by an Entomologist, many novelties would reward his toils. The valleys and plains, especially those of a sunny exposition, abound in insects. When the heat of the atmosphere indisposes you for motion, you will find it no unprofitable or unpleasant employment, lying on the grass, to search for minute beetles, which you will there find coursing about amongst the tufts and roots of the herbage. Thus you may procure many of the Pselaphidæ, which you would not otherwise meet with. Even when the grass is grown up, insects are fond of alighting upon its spikes, and thence drop or run to the ground. Should circumstances ever carry you abroad to the steppes or grassy plains of Tartary, or to Hungary, you would find there two or three species of the singular genus Lethrus, which burrows in the soil. Every hole is inhabited by a male and female;—from it they issue to attack the plants or vines; and having cut out the heart of a plant, go backwards like a crab with the prize to their burrow. At the time of pairing, sometimes violent battles, encouraged by the female, take place between the male and a stranger of that sex desirous of admission, which cease only with the death or flight of the stranger[1519]. The vicinity and borders of woods generally abound in insects of every Order; and if you proceed, as hereafter directed, will furnish you with numerous prizes, especially of Lepidoptera. Here alone you can meet with the purple emperor butterfly (Apatura Iris); and if properly equipped you may readily secure him.
The waters you will find nearly as prolific in insects as the land. In them, amongst the beetles, you may expect to meet with Dytiscus, Haliplus, Pælobius, Hyphydrus, Hydroporus, Noterus, Colymbetes, and other Dytiscidæ; the Gyrini, Hydrophili, Hydrænæ, Elophori, &c.: under stones, the Elmis; and in the mud, the Parni and Heteroceri. Some Sphæridiadæ are also aquatic: I have taken more than once Cercyon hæmorrhoidale from the under side of a piece of wood immersed in a canal[1520]. Even a few of the weevil tribes are to be met with in water. Lixus paraplecticus, Tanysphyrus Lemnæ, Bagous atrirostris, are of this description. A species of Ceutorhynchus of Germar's third family (C. Natator K.) swims well. On aquatic plants you must look for Helodes and the splendid Donaciæ, which, living on submerged shoots and roots of these plants in their larva state, continue to attend them when perfect. Amongst the Eutrechina[1521],—Elaphrus, Notiophilus, and Bembidium frequent humid places, as the banks of rivers and ponds; and in such a station, under the roots of Potentilla anserina, Polygonum, &c. if you should be fortunate enough to find Omophron limbatum, which connects the Eutrechina with the Eunechina, you will make a valuable addition to the list of British insects. In the waters also you will meet with many Heteropterous Hemiptera; as Gerris, Hydrometra, and Velia, and all the Hydrocorisæ or water-bugs. On aquatic plants the larvæ of some Lepidoptera feed, as Hydrocampa stratiotata, potamogata, &c. Those also of the Trichoptera must be sought for in the water: and if you should feel inclined to see an interesting collection of their very curious cases, Mr. Sheppard of Wrabness can gratify your curiosity. Though few or no Hymenoptera frequent this element, vast numbers of Diptera are there alone to be met with in their preparatory state, particularly the gnats. We learn from Humboldt a curious fact with respect to those of South America, or the Zancudos; that, with some exceptions, these pests do not frequent those rivers called by the natives black waters, but only those which they name white waters[1522]. Of the Aptera, the genera Hydrachna, Eylaïs and Limnochares are purely aquatic. Several spiders will walk over the water; and one species (Argyroneta aquatica) inhabits it[1523]. The stagnant waters in your vicinity will produce different species from running ones. Thus Haliplus elevatus, &c. inhabits only the latter, while the majority of the Dytiscidæ abound most in the former: the more minute ones may be sought for with success amongst the duckweed that covers a pool. I do not recollect finding any insect in waters absolutely salt[1524]; but brackish waters produce peculiar species: in these only, Hydræna marina occurs; and many of those large-eyed Cimicidæ (Acanthia), as A. saltatoria, littoralis, and Zosteræ occur in places where salt water has been. Latreille observes, that the genus Pimelia is to be met with only where the soil is impregnated with saline particles, or where the species of the genus Salsola abound[1525].
Heaths, though they do not afford numerous insects, have their rarities. Cicindela sylvatica, Carabus nitens and arvensis, frequent them, and are not elsewhere to be seen. Curculio nebulosus is also to be found on them, in places where the turf has been peeled; and some scarce Lepidoptera. In their vicinity, in sunny sandy banks, some of the rarer Ammophilæ and Pompili may be taken; and it is here only that I have ever met with Panurgus[1526]. Meadows and pastures are not to be neglected. Early in the year, when they are yellow with the blossoms of Ranunculus bulbosus, Leontodon Taraxacum, &c., many minute beetles, and not a few Hymenoptera and Diptera, frequent them. Morasses also have their peculiar insects. In these you will meet with some of the scarcer Eutrechina; as Chlænia holosericea and nigricornis, Blethisa multipunctata, various Bembidia, &c. In this kind of district in the Isle of Ely Aphodius plagiatus has been taken, and that scarce and beautiful butterfly Lycæna Virgaureæ. Where land is cultivated the Entomologist as well as the farmer may expect a harvest. Insects in general are fond of perching on the summit of a blade of grass or corn; and many minute ones may be taken coursing about in the ears of the latter: some to devour the fungilli that infest the grain, as Phalacrus corruscus in Reticularia Segetum; others to attack the grain itself, as Cecidomyia Tritici; others to destroy these destroyers, as three little parasites belonging to the Chalcidites[1527]. But I have already mentioned most of those insects that are to be expected in such situations[1528]: I shall therefore only further observe, that upon barley particularly you will meet with the species of Latreille's genus Cephus.
With respect to soils, those that are light appear to be most prolific in insects. Warm sandy banks are frequented by Cicindela campestris, Opatrum sabulosum, Helops quisquilius, &c.: in them (when of a southern aspect) Ammophilæ, Pompili, and numerous Hymenoptera nidificate. Chalk also attracts various insects. Latreille observes, that the Licini, Papilio Cleopatra, several species of Dasytes, and some Lamiæ, delight in this kind of soil[1529]:—in my own neighbourhood I have observed Polyommatus Corydon principally in chalk-pits. One of these pits, under a wood in an adjoining parish, has produced me several valuable insects. Here I took Apion ebeninum, Orobitis globosus, a new species of Evæsthetus, several of the rarer Pselaphidæ and Cholevæ, and Chætophorus cretifer before noticed[1530]. I do not mean, however, that all these are properly chalk insects; but they fall into these pits, where they are readily discerned, from the contrast of their colours with the whiteness of the chalk. By watching attentively the bottom of one, vast numbers in a warm day may be taken when they fall or are climbing upwards. Of all soils clay offers the fewest inducements to the Entomologist, who will lose both his time and labour in a clay-pit; while in one of sand, chalk, or marl, they will usually not be mispent. Vegetable earth also affords a harbour to various larvæ, and the pupæ of many nightfliers amongst the Lepidoptera, by digging in it, especially under trees, may be obtained. Even the bare rocks have their insect frequenters that take shelter in their fissures; and in the early part of your career especially you should always turn over large stones, as beneath them many of the Harpalidæ and other Eutrechina frequently lie hid: and in this situation, both in Suffolk and Sussex, Lomechusa emarginata, one of our scarcest Brachyptera, has been taken. Old trees also, and planks that have laid long without being moved, often afford a shelter to many of the minute Coleoptera; as Pselaphidæ, Aleocharidæ, Cryptophagidæ, Scymnidæ, &c. Live fences, especially when the hawthorn is in blossom, and where trees are also intermixed, are attended by innumerable insects of almost every description; and even the black-thorn will present you with one of our most splendid weevils (Rhynchites Bacchus). Dead fences are almost as fertile in insects as living ones. In gates, posts, rails, and other timber when felled, the timber-devouring tribes take their station:—between the bark and the wood are the Bostrichidæ; in the wood itself, the Anobidæ and the Capricorn beetles. Here also you may meet with many Hymenoptera, which either devour timber or nidificate in it,—as the Siricidæ, Chelostoma, Trypoxylon, Sapyga, and several Diptera. In the decaying hedgestakes and sticks, where the Sphæria decorticans has turned off the bark, you may meet with Anthribus brevirostris; with A. latirostris, and other beetles, in S. fraxinea; and A. albinus, which I have more than once captured as it was emerging from the fissure of a gate-post, probably feeds on some internal fungus. The grassy balks that separate open fields usually abound in umbelliferous plants, which are attended by numerous Hymenoptera and Diptera, particularly by the various species of the splendid tribe of Chrysidæ: and the grassy banks of fences, where the aspect is sunny, are generally bored by a variety of insects of the former Order, to prepare a nest for their young. Andrenidæ and Nomadidæ particularly select this situation, the latter probably depositing their eggs in the burrows of the former[1531]. By watching these places in the spring, you may perhaps have the good fortune to meet with a Stylops. It is singular, that some insects choose, for their own residence or that of their young, the hardest and most trodden pathways. Thus, some ants will build their subterranean apartments under gravel walks; and so do many species of the genus Halictus[1532], the habits and economy of which have been so ably detailed by M. Walckenaër[1533]: Cerceris also, and other Hymenoptera, will choose such places, however public, for the site of their nests or burrows. The ground is so consolidated by the constant foot, that they, probably find such situations spare them a world of labour, and therefore in their choice balance one inconvenience by another.