PUPÆ.
1. Adminicula (Adminicula). Semicoronets of minute teeth which arm the back of the abdomen of subterraneous pupæ, by which they are enabled to emerge from under the earth. Plate XVI. Fig. 13. e.
2. Cremastræ (Cremastræ). The anal hooks by which many pupæ suspend themselves. Plate XXIII. Fig. 8. a.
3. Cocoon (Folliculus). The silken case in which the pupæ of many insects are inclosed. Plate XVII. Fig. 5-8.
N.B. Other terms for Pupæ are explained Vol. III. p. 249.
Having considered insects as to their History, Anatomy and Physiology, we must next enter a new and ample field, in which, like most of our predecessors, we shall often be perplexed and bewildered by the infinite variety of devious paths which traverse it, and by the mazy labyrinths in which the more we wander the less ground we seem to gain.—You will easily perceive I am speaking of the System of Insects. System is a subject which has engaged the attention of Naturalists from the time of Aristotle to the present day; and even now that it has been so much and so ably discussed, they are far from being agreed concerning it. In our own country a clue has, however, of late been furnished, which upon the whole seems better calculated to enable us to thread the intricate labyrinth of nature, than any thing previously excogitated.
There are two words relating to this subject concerning which Naturalists seem not to have very precise ideas—Method and System. They have often been confounded and used indifferently to signify the same thing. Thus we hear of a Natural Method and a Natural System. Linné seems to have regarded the former of these terms as representing the actual disposition of objects in nature[1126], while by System he understands their classification and arrangement by Naturalists[1127]. But if we consider their real meaning,—a Method should signify an Artificial, and a System a Natural arrangement of objects[1128]. As many systematists, however, have aimed at giving a natural arrangement, though with various success,—some, as the French school, (to which we are principally indebted for the progress already made,) approximating nearer to the true idea than others,—and none having a perfect conception of it, of which probably in our present state, our minds, from its intricacy, are incapable,—it might perhaps be as well to call every arrangement whose object is confessedly artificial, a Method; and that which aims at the plan of nature, a System. Under this view system-makers would be divided into two classes,—the Methodists and Systematists.
The system of nature, which we are now to consider, may be viewed under a double aspect; for with regard to all created objects there is a System of Distribution, and a System of Correlation, which appear to be quite independent of each other. The former will best fall under our notice when we are treating of the Geography of insects: I shall therefore now confine myself to the latter.
When the Almighty Creator willed to bring into existence this mundane system, he formed it according to a preconcerted plan, with all its parts beautifully linked together and mutually corresponding. All things were ordered in measure, and number, and weight[1129]. There was nothing deficient, nothing superfluous; but the whole in the strictest sense "was very good[1130]," and calculated in the highest degree to answer the purpose of its Great Author. I call it a system of Correlation, because there is discernible in it, in the first place, a concatenation of its parts, by which, as to their forms and uses, objects are linked together in groups by a chain of affinities; so that we pass from one to the other by gentle gradations, without having to overleap any wide interval. We see also a gradual ascent from low to high, from less to more excellent. And this leads us to another kind of relationship between natural objects, by which, though placed in distinct groups or in a different series, they in some sort represent and symbolize each other. Examples of this relationship by analogy are to be found in every kingdom of nature, and often form an ascending series from the lowest to the highest; for, as we shall see hereafter, these resemblances appear to maintain a certain correspondence with each other as to their relative situations; so that, for instance, in the animal kingdom they ascend step by step, without being linked by affinity or having any real juxtaposition, from the lowest groups, towards man, who stands alone at the head, or in the centre of all.—I shall say something on each of these kinds of relationship.
I. The relation of affinity may be considered as to its series and groups. A series, of course, consists of parts either concatenated like a chain, or placed separately at small intervals from each other. It may run either in a right line, or deviate from it in various ways. It appears to be the opinion of most modern Physiologists, that the series of affinities in nature is a concatenated or continuous series; and that though an hiatus is here and there observable, this has been caused either by the annihilation of some original group or species in consequence of some great convulsion of nature, or that the objects required to fill it up are still in existence but have not yet been discovered[1131]: and this opinion is founded on a dictum of Linné, Natura ... saltus non facit[1132]. If this dictum be liberally interpreted, according to the evident meaning of the word saltus, few will be disposed to object to it; since both observation and analogy combine to prove that there must be a regular approximation of things to each other in the works of God; and that could we see the whole according to his original plan, we should find no violent interval to break up that approximation: but if it be contended, that in this plan there is no difference in the juxtaposition of the nearest groups or individuals, and never any interval between them, I think we are going further than either observation or analogy will warrant. Were this really and strictly the case, it seems to follow that every group or individual species must on one side borrow half its characters from the preceding group or species, and on the other impart half to the succeeding[1133]. But one of the most evident laws of creation is variety; and if we survey all the works of the Most High, we shall no where discover that kind of order and symmetry that this strict interpretation implies. The general march of nature therefore seems to say, that there must be varying though not violent intervals in the series of beings: or in other words, some conterminous species or groups have more characters in common than others.
It was the opinion of Bonnet (in this field himself a host) and many other Naturalists, that the series of beings was not only continuous, but undeviating, ascending in a direct line from the lowest to the highest[1134]. Others, finding that this theory could not be made to accord with the actual state of things in nature, thought that a scale of the kingdoms of nature must represent a map or net[1135]; thus abandoning a continuous series: and Lamarck, as was before observed[1136], for the solution of the difficulty, arranged Invertebrate animals in a double subramose one. Mr. W. S. MacLeay and (without consultation nearly at the same time) Professor Agardh, Mr. Fries, &c. have given to the learned world an opinion which approximates more nearly to what we see in nature: viz. That the arrangement of objects is indeed in a continuous series, but which in its progress forms various convolutions, each of which may be represented by a circle, or a series that returns into itself[1137]. According to this opinion,—which seems the most consistent of any yet advanced, and which reconciles facts which upon no other plan can be reconciled,—the series of beings is involved in the highest degree, rolling wheel within wheel ad infinitum, and revolving, if I may so speak, round its centre and summit—man[1138]: who, though not including in himself all that distinguishes them, is still the great Archetype in which they terminate, and from which they degrade on all sides.
It is by this convolving series that the various groups into which the kingdoms of nature seem resolvable are formed. We are instructed by the highest authority that every thing was created "after its kind;" and the common sense of mankind in all ages has imposed classic, generic, and other names implying sections, as well as specific ones, upon natural objects: and though many modern Physiologists have asserted that species form the only absolute division in nature; yet as all seem to allow that there are groups, and many that these are represented by a circle or group returning into itself[1139], the most absolute division in nature, we will not contend for a term[1140]. We now come to consider these groups themselves, and may notice them under various denominations.
It is customary to consider all the substances of which our globe consists as divided into three kingdoms,—the Mineral, Vegetable, and Animal; but strictly speaking the primary division is into organized and inorganized matter; the former resolving itself into the two kingdoms last mentioned. These, like England and Scotland of old, have their "Land Debateable;" occupied by those Productions moyennes, (to use a term of Bonnet's[1141],) which are as it were partly animal and partly vegetable. From this territory common to both, the two kingdoms are extended in a nearly parallel direction till they reach their extreme limits, without any incursion from either side upon their mutual boundaries, but each showing its kindred with the other by certain resemblances observable between opposite points; so that valley corresponds with valley, mountain with mountain, river with river, sea with sea[1142]; not, however, so as to form an exact counterpart, but only in some general features. But to leave metaphor;—as the vegetable kingdom is distinguished from the mineral by its organization and life, by its circulation of sap, and by its powers of reproduction by seed or otherwise; so is the animal from the vegetable by its powers of volition and locomotion[1143], by its nervous systems and organs of sensation, and the senses to which they minister, by its muscular irritability, and by its instinctive endowments.
Having made these observations with regard to the primary division of natural objects in general,—what I have further to say will be confined to the animal kingdom, and ultimately to the branch of which we are treating.
i. Lamarck divided the animal kingdom into two provinces, or subkingdoms as they are now called; the one consisting of all those animals whose skeleton is internal and built upon a vertebral column, which are denominated Vertebrates; and the second, of those whose skeleton or its representative is for the most part external, including the muscles,—these are called Invertebrates[1144]. Though this distinction is so marked as in general to form a most striking characteristic, yet when these two provinces approach each other, it begins to disappear. Thus the vertebral column, forming one piece with the shell[1145], becomes almost external in the Chelonian reptiles, or tortoises and turtles, and almost disappears in the cyclostomous fishes; and there is the beginning of an internal one in the Cephalopoda, or cuttle-fish belonging to the Invertebrates. Dr. Virey, assuming the nervous system as his basis, long since divided the animal kingdom, without assigning names to them, into three subkingdoms[1146]; M. Cuvier has four—Vertebrata; Mollusca; Articulata; Radiata[1147]: and Mr. MacLeay, finding five variations of that system, divides animals into five provinces or subkingdoms, of which I formerly gave you some account[1148];—viz. Vertebrata, in which the nervous system has only one principal centre; Annulosa, in which it is ganglionic, with the ganglions arranged in a series, with a double spinal chord; Mollusca, in which it is ganglionic, with the ganglions dispersed irregularly but connected by nervous threads; Radiata, in which it is filamentous, with the nervous threads radiating from the mouth; and Acrita, in which this system is molecular[1149]. And to this division of the kingdom, as founded on a satisfactory basis, I should recommend you to adhere: still however we may speak of vertebrate and invertebrate animals, as forming the primary subdivision of them, taken from a striking character and obvious to every one who sees them.
If you inquire into the rank of each of these subkingdoms, of course you will assign the principal station to the Vertebrates, which are the most perfectly organized, to which man belongs, and over which he immediately presides. If we form the scale according to the nervous system of each province, that in which the organ of sensation and intellect is most concentrated will stand first; and in proportion as this organ is multiplied and dispersed will be the station of the rest, which will place them in the order in which I have mentioned them; and the Annulosa, to which insects belong, will precede the Mollusca, which Cuvier and Lamarck had placed before them on account of their system of circulation. But when we reflect that a heart and circulation occur in some of the conglomerate Polypi[1150], animals that approach the vegetable kingdom; that some of the acephalous Mollusca have no visible organs of sense, except that of taste, whose substance is little better than a homogeneous gelatinous pulp, and who seem from their inert nature to have very slight powers of voluntary motion[1151], we shall be convinced that a heart and circulation alone, unaccompanied by a more concentrated nervous system and more perfect structure, cannot place an animal above those which in every other respect so obviously excel them. With regard to insects particularly, we may further ask—Who that considers how man employs his powers and organs even in his most degraded state, or that contemplates the wonderful works that he is enabled to accomplish when his faculties receive their due cultivation and direction, can avoid regarding him as superior to the rest of the animal creation? And what unsophisticated mind, not entangled in the trammels of system, when it surveys the industry, the various proceedings, and almost miraculous works that have been laid before you, the waxen palaces of the bee,—the paper cottages of the wasp and hornet,—the crowded metropolis of the white ants,—the arts, the manufactures, and stratagems of other insects,—the associations and labours for the common good of those that are gregarious;—will not at once conclude that they must be a superior race to the slug, the snail, and others, which live only to eat and propagate their kind?
Or who, that considers the wonderful structure of the animals whose cause I advocate,—the analogy that exists between their organs of manducation, of motion, and of sensation, and between various other parts of it[1152], with those of the higher animals,—the acuteness of their senses, their wonderful strength of muscle[1153], and powers of locomotion[1154],—but will think them superior to the headless and almost inanimate oyster or muscle, or the conglomerate Alcyonia, though they have a heart and circulation?
Who again, that observes that in proportion as pedate animals approach to the human type, their motions are accomplished by fewer organs,—that man walks ore sublimi upon two legs; the majority of quadrupeds upon four; insects upon six: the Arachnida apparently upon eight; most Crustacea upon ten; and the Myriapods and others upon many,—but will thence conclude that insects must precede the Arachnida and Crustacea?
Who, once more, that reflects that if any of the superior animals are deprived of a limb it can never be reproduced, and that in insects the same circumstance occurs; while spiders and Crustacea if they lose a leg have the power of reproducing it, and the Mollusca if they are decapitated can gain a new head,—will consent to their being placed after any of these animals[1155]?
Lastly, who that recollects that the Mollusca are hermaphrodites, like most plants, bearing both male and female organs in the same body,—but will allow that insects, in which the sexes are separate as in the Vertebrates, must be more perfect, and of a higher grade[1156]?
ii. We now come to the Classes into which the Annulosa are divided. This term appears first to have been employed by Tournefort, and was adopted by Linné[1157]. As the nervous system of animals furnishes the most prominent distinction of a subkingdom, so the circulation of their fluids, and their respiration necessarily connected with it, seems best to point out the classes into which it may next be resolved. But having fully explained my ideas on this subject in a former letter, I need not here repeat what I then said[1158].
iii. As we have subkingdoms, so we may also have subclasses, or such large divisions of a class—not founded upon internal organization or any of the primary vital functions, but upon different modes of taking their food, or such other secondary characters—as include more than one Order. To this description Clairville's Mandibulata and Haustellata appear to me to belong, which I think are by no means entitled to the rank of Classes; for whoever compares these two tribes together will at the first glance be convinced, by the numerous characters they possess in common, notwithstanding the different mode in which they take their food, that they form one connected primary group. This circumstance, therefore, only furnishes a clue for their further subdivision into two secondary groups, separated by distinctions certainly of a lower value than those which separate the Crustacea and Arachnida from Insecta. This is further confirmed by the variations that take place in their mode of feeding in their different states; some from masticators becoming suctorious (Lepidoptera), and others from being suctorious becoming masticators (Myrmeleon, Dytiscus, &c.),—which shows that this character does not enter the essential idea of the animal.
iv. Next to Classes and Subclasses we are to consider those groups of insects that are denominated Orders. The characters of these at first were taken principally from the instruments of flight or the absence of them; and the name appropriated to each Order by Linné, after Aristotle, had reference to this circumstance. But this alone does not afford characters sufficiently discriminating: for though to an accurate observer a difference in these organs appears to be characteristic of most of the Orders, yet in some it is not easily detected or defined. In the Neuroptera there are as many different types of wings as there are of tribes or suborders. So that it seems not possible so to construct the definition of every Order, as to take its character from the organs of flight alone. Linné was sensible of this, and was compelled to have recourse to subsidiary characters in the majority of his: his observation therefore with regard to Genera,—that the character does not give the genus, but the genus the character[1159],—applies equally to Orders; and the characters included in the definition of an Order, should be the result of a careful examination of its component groups.
On a former occasion I named to you the Orders into which it appeared to me the Class Insecta might be divided[1160]; they were these. Coleoptera; Strepsiptera; Dermaptera; Orthoptera; Hemiptera; Trichoptera; Lepidoptera; Neuroptera; Hymenoptera; Diptera: Aphaniptera; Aptera. I then briefly explained them merely for the sake of illustration, and that you might know what description of insects were meant when these Orders were mentioned in my letters, without intending to affirm that I had arranged them in a natural series, or that all of them were perfectly natural. I shall now consider them separately, and conclude with giving my sentiments as to which should be placed first.
* orders in which the ordinary Trophi all occur, or the Mouth is perfect[1161]. (Mandibulata.)
1. Coleoptera[1162] (Eleutherata F.). Aristotle may be called the founder of this Order, since he both named and defined it[1163]. Both his name and definition were adopted by Linné; and the former (with the exception of Fabricius and his school) by all succeeding Entomologists. To his definition Wings in a sheath[1164], other characters have been added; as the folding of the wings, and the straight suture by which the elytra are united[1165]. Aristotle's character, though to be found in the great majority of the Order, is not universal, since there are some beetles that have neither wings nor sheath, as the female glow-worm; and many that though they have the sheath have no wings, as Meloe, many Carabi, &c. To the transverse folding of the wings there are also exceptions; as in Buprestis, Molorchus, &c. The straight suture by which one elytrum exactly coincides with the other without lapping over, fails in Meloe: so that no one of these characters can be called universal in the Order; but as an exception or two does not invalidate a rule, and these are sufficiently universal for the purpose of pointing it out, they may be retained. Perhaps it will be an improvement to add the kind of the metamorphosis, which, as far as known, prevails universally.
Def. Metamorphosis incomplete[1166].
Legs inosculating, posterior coxæ usually transverse.
Elytra corneous, or coriaceous, without veins, united by a straight suture, so as mostly to cover the wings completely[1167].
Wings longitudinally and transversely folded[1168]: neuration simple[1169].
2. Strepsiptera[1170] (Rhiphiptera Latr.) The characters of this Order were first given in the Linnean Transactions, and it has been adopted by Latreille, who however, without sufficient reason, has changed the name originally imposed to Rhiphiptera[1171]. Rossi, who was the first that discovered an insect of this Order, concluded that because it was parasitic it must be Hymenopterous; and it is certainly more nearly related to that Order than to the Diptera, amongst which M. Lamarck has arranged it, and with which it has no character in common, except having two wings. This is one of those Orders, consisting of few genera and species, which, from their connecting two circles, Mr. MacLeay has called osculant, who places it between the Hymenoptera and Coleoptera[1172].
Def. Metamorphosis subincomplete[1173]?
Pseudelytra twisted, attached to the anterior leg[1174].
Wings not covered by the elytra, longitudinally folded, forming nearly the quadrant of a circle[1175]: neuration simple.
Anus styliferous[1176].
3. Dermaptera[1177] (Ulonota F. Orthoptera Oliv.). This is another osculant Order, evidently connecting the Coleoptera with the Orthoptera. The elytra are of a coriaceous substance, have a straight suture, and are not veined, and the wings are folded longitudinally as well as transversely,—circumstances which connect it with the former Order,—while the shape of its wings, its oral organs, and its metamorphosis, show its affinity to the latter. It was established at the same time and in the same work with the preceding Order, in pursuance of a suggestion of Dr. Leach, and consists solely of the Linnean genus Forficula.
Def. Metamorphosis semicomplete.
Elytra coriaceous, without veins, united by a straight suture, so as partly to cover the wings.
Wings longitudinally and transversely folded, each forming nearly the quadrant of a circle: neuration radiating[1178].
Anus forcipate.
4. Orthoptera[1179] (Ulonota F.). This Order, which Linné at first regarded as belonging to the Coleoptera[1180], and afterwards improperly added to the suctorious Hemiptera, was very judiciously separated from both by De Geer, under the name of Dermaptera, a name not improper, and which ought to have been retained. Its present name was, I believe, assigned to it by Olivier; and as this is generally in use, I shall not attempt to disturb it. Dr. Leach divided the Order into two, separating the Blattina from it, under the name of Dictyoptera[1181]. He was led to this by the tegmina decussating or lapping obliquely over each other, whereas in the rest the horizontal portion of one tegmen lies longitudinally over that of the other; he also probably took their depressed body into consideration;—these circumstances, however, rather indicate a tribe or suborder; and as such Mr. MacLeay regards it.
Def. Metamorphosis semicomplete.
Legs suspended.
Tegmina generally pergameneous[1182], reticulated with nervures, more or less incumbent, covering the wings.
Wings longitudinally folded, ample: neuration reticulated.
5. Neuroptera[1183] (Synistata, Odonota F.). Of all the Linnean Orders this appears to consist of the most discordant tribes; so that it seems next to impossible to construct a definition that will include them all, unless indeed we admit M. Latreille's idea, adopted by Mr. MacLeay[1184], that a varied metamorphosis is its essential character; or, to speak more largely, variety itself seems the characteristic of the insects composing it, in every state; and there is scarcely a common distinctive character in their perfect state, upon detecting which in any individual you may exclaim—This is a Neuropterous insect. The only one that I have been enabled to seize is, that their scapulæ and parapleuræ are parallel and placed obliquely[1185]. Whether, with all this puzzling variation and dissonance between the different tribes of which it is now composed, this Order can be considered as a natural group, in the present state of our knowledge it would be rash to decide. I shall observe, however, that the Libellulina,—whether we regard their metamorphosis and the singular character before described that distinguishes their larva and pupa[1186], their oral instruments[1187], the remarkable position of their legs[1188], their general form, the wonderful and peculiar machinery by which their wings are moved[1189], and other circumstances of their internal anatomy,—if any are to be regarded as forming a separate Order, are the first entitled to that distinction. At present, with our friend Mr. MacLeay, I shall consider it as not further divisible, and as consisting of five principal forms. I must not omit to observe, that in the Ephemerina the parts of the mouth, except the labrum and palpi, appear to be mere rudiments[1190].
Def. Metamorphosis varying. Larva a hexapod.
Wings four in most, and reticulated with numerous areolets.
Prothorax distinct.
Scapulæ and Parapleuræ parallel and oblique.
Tail of the female without a terebrant, or pungent multivalve ovipositor[1191].
6. Hymenoptera[1192] (Piezata F.). Mr. MacLeay considers Sirex L. as being osculant between the Order we are now entering upon and the Trichoptera, and Tenthredo, L. as belonging to the latter. He appears to ground this opinion chiefly upon a consideration of their larvæ and a slight difference in their ovipositor. As the Order, as settled by Linné, has always been deemed one of the most natural ones, and all the great Entomologists of the present æra have agreed with him in thinking it so; it seems to me that to prove them mistaken in this opinion, the question should have been discussed at more length, and that it requires arguments of more weight than any Mr. MacLeay has at present produced to set it aside. He appears in general to lay great stress upon an agreement in larvæ and the kind of metamorphosis; and I am ready to acknowledge that it forms a strong presumption in favour of any hypothesis of affinity between certain tribes. But when it is had recourse to as fundamental and infallible, I think it is pushed far beyond what it will bear, or is warrantable. I may be wrong; but in my apprehension, a striking agreement in their general structure in the perfect state, which is the acme of their nature, affords a much more satisfactory reason for keeping two tribes together, than any difference observable in their larvæ or metamorphosis, for separating them. Let any one compare the structure of these two tribes with the Trichoptera on one side, and the Hymenoptera on the other, and it will require but a glance to convince him of their greater affinity to the latter; and the simple inspection only of Jurine's plates of the wings of Hymenoptera is calculated to produce the same effect. With regard to their larvæ, the resemblance between the case-worms and the pseudo-caterpillars of the saw-flies seems to me very distant, and the numerous prolegs of the latter have scarcely a legitimate representative in the former. The larvæ of the genus Lyda lose the prolegs intirely, and in one species, which much resembles the vermiform larvæ of Hymenoptera, the real legs are so extremely short as to be scarcely discernible[1193]; so that it requires no great stretch of faith to believe that saw-flies or Sirices may exist in whose larvæ the legs disappear[1194]. But it is this very tribe, whose larvæ thus approach to those of the other Hymenoptera, in which Mr. MacLeay finds the greatest external resemblance to the Trichoptera[1195]. In fact the difference between the saw-flies and Siricidæ, and the remainder of the Hymenoptera, amounts to little more than what takes place in the Diptera Order between the Tipulidæ, Asilidæ, Muscidæ, &c., in which also the metamorphosis differs.
Another argument upon which Mr. MacLeay seems to lay some stress, is taken from the number of parts into which the ovipositor of the saw-flies is resolvable, which he finds to consist of four pieces; while in what he considers as the genuine Hymenoptera, it is formed only of three[1196]: but in fact, in these last there are two spiculæ, answering to the two saws of Tenthredo, so that the vagina in which these move may be considered as a double sheath: only, as these were to be pushed out at the same time, and the others alternately, it was necessary that in the latter each sheath should be separate, to admit of this motion; but as to its composition, the weapon in both is essentially the same. At any rate this structure could furnish a reason only for the formation of a separate group in the same Order, but none for the transfer of such group to another, which had no such instrument at all; since, as we have seen, the Trichoptera extrude their eggs at once in a mass[1197]. I do not mean, however, that it should be inferred from what I have here said, that there is no tendency in the saw-flies towards a Trichopterous type, for in them nature seems pointing that way, but the distance is too great, and the number of types of form necessary to fill up the interval too many, to warrant in my opinion their removal from the one Order to the other.
Def. Metamorphosis incomplete[1198].
Trophi in most not used for mastication[1199].
Wings four: neuration generally areolate[1200].
Prothorax obsolete, giving place to an ample collar.
Tarsi pentamerous.
Ovipositor 5-6-valved, the vagina darting forth two retroserrulate spiculæ.
** orders in which all the ordinary Trophi do not occur, or the Mouth is imperfect[1201] (Haustellata).
7. Hemiptera[1202] (Ryngota F.). Linné at first confined this Order to those insects which have a promuscis, which he denominated a rostrum[1203]; but afterwards, convinced that the Orthoptera of the moderns could not be associated properly with the Coleoptera; instead of forming them into a distinct Order, as nature would have dictated—perhaps to avoid the multiplication of Orders and without altering his definition—with equal infelicity he added them to this. Subsequent Entomologists, who saw the impropriety of masticating insects thus herding with suctorious ones, restricted the Order to its old limits; but Latreille very judiciously altered its arrangement, and divided it into two Sections, separating those whose hemelytra terminate in membrane, from those in which they are mostly tegmina, or of a substance intermediate between that of the elytra of Coleoptera and that of the wings of the Tetrapterous Orders. He denominated the first of these sections, or rather suborders, Heteroptera, and the last Homoptera[1204]. Dr. Leach, observing that very considerable differences take place both in the economy and structure of Heteropterous and Homopterous insects, followed De Geer in considering them as separate Orders, which he has called Hemiptera and Omoptera, and in which he has been followed by Mr. MacLeay; who, however, with his usual accuracy and judgment, has restored the aspirate to the latter name[1205]. Their agreement in having a promuscis, or instrument of suction, with a jointed sheath, at present induces me to hesitate as to the propriety of their separation, and to consider them as forming secondary rather than primary sections of the Class. That you may be enabled to judge for yourself upon this subject, I will state the principal features in which they differ. In the first place, the Heteropterous section usually sucks the juices of animals, and the Homopterous, those of plants; in the former, the Hemelytra, besides their different substance, as well as the wings, cross each other; while in the latter, the organs of flight are deflexed, and do not lap over each other at all. The antennæ also of the one are often long, and do not terminate in a bristle; while in the other, with few exceptions, they are very short and setigerous. In the Heteroptera the body is depressed and flat, in the Homoptera convex and thick. In the former, the scutellum is one of the principal features of the trunk; in the latter, not at all remarkable[1206]. Other differences in the structure, both of head, trunk, and abdomen, might be pointed out; but these you will chiefly find noticed in my letters on the External Anatomy of Insects, where I treated of those parts. I shall here, therefore, only further mention the ovipositor also as forming a most striking distinction[1207].
Def. Metamorphosis semicomplete in almost all.
Mouth promuscidate[1208].
Wings covered by Hemelytra or Tegmina[1209].
Tarsi mostly trimerous, rarely dimerous or monomerous[1210].
8. Trichoptera[1211] Kirby (Synistata F. Neuroptera Latr.). MM. Latreille and MacLeay are of opinion that Semblis F. and Phryganea L. ought to be associated in the same group; and the latter gentleman has backed his opinion by some apparently cogent arguments[1212]: there are others, however, that seem to me more cogent, for considering them as belonging to different Orders. Whoever examines the several tribes into which Mr. MacLeay has divided the Neuroptera, will observe in all of them a distinct prothorax, a circumstance which they possess in common with those Orders that use their mandibles for mastication; whereas in those that do not use them for mastication, as the Hymenoptera, or that take their food by suction, this part is replaced by a mostly narrow collar, forming a part of the alitrunk[1213]. The existence then of the prothorax in the Perlidæ, and of the collar in the Trichoptera, affords no slight presumptive evidence that they belong to different Orders. Another circumstance that weighs much with me is, that the type of the neuration of the wings in Perla is taken from the Neuroptera, in the Trichoptera from the Lepidoptera; the same observation extends to the legs of both[1214], and likewise to the abdomen. Even in their oral organs, as far at least as relates to their mandibles, those of Perla, though membranaceous—a circumstance occurring even in Coleoptera—are of a Neuropterous type; while the angular termination of the cheeks in the Phryganeæ approaches to the Lepidopterous mandibular rudiments. The principal argument on which Mr. MacLeay's opinion seems to rest, is, that the larvæ of both are aquatic, and clothe themselves in cases formed of various materials: but though this circumstance shows that they approximate in the system, it does not prove that they belong to the same order, since the general habit and appearance of the two animals when arrived at perfection contravenes it. The larvæ of Myrmeleon and of Leptis Vermileo form pitfalls of sand for their prey, and when they become pupæ, cover themselves with it[1215]; but this in them does not even prove an affinity, but only an analogy. The larva of Perla is carnivorous[1216], that of Phryganea mostly herbivorous[1217]: so that they are not precisely similar in their habits. Whether they resemble each other altogether, in their form, does not clearly appear. The above reasons will, I trust, justify me for considering them at present as belonging to different Orders; but if further discoveries should confirm the opinion Mr. MacLeay espouses, I shall have no hesitation in yielding to it.
Def. Metamorphosis incomplete[1218].
Mouth emandibulate.
Prothorax replaced by a collar.
Wings four, upper pair mostly hairy, lower ample, folded: neuration branching.
Anus without setæ. Eggs extruded in a gelatinous mass[1219].
9. Lepidoptera[1220] (Glossata F.). Concerning this Order, no difference of opinion exists amongst Entomologists. Besides the scales that cover their wings, they are distinguished by the peculiar instrument of suction formerly described: neither of these characters, however, is perfectly universal; some of the Order (Nudaria) having no scales upon their wings, and others being without any antlia (Aglossa). Other peculiar characters are to be found in them; for instance, the patagia, or tippets, that adorn their evanescent thorax[1221], and the tegulæ, or base-covers, of a shape quite dissimilar to those of Hymenoptera, which cover and defend the base of their wings[1222]. As in the last Order, their legs are located all together with scarcely any space intervening between them; and they often agree also in their spurs.
Def. Metamorphosis obtected[1223].
Mouth antliate[1224].
Prothorax very short, covered by a pair of tippets.
Wings four, covered partially or generally with minute scales: neuration branching, often with a central areolet.
10. Diptera[1225] (Antliata F.). This Order likewise appears indebted for its name to the philosopher of Stagyra, who distinguishes the members of it from their counter-parts—the Hymenoptera—by their having an oral, while these have an anal sting[1226]: and we may add, that while the last, on account of their wonderful economy and the benefits which by them Providence confers upon mankind, have been justly regarded as the princes of the winged insect world,—the former, when we consider the filthy and disgusting habits of their grubs, and the annoyance, both from their numbers and incessant assaults, of them, in their fly-state, may very properly be considered as its canaille. Almost all the tribes of Hymenoptera, from the saw-flies to the ants, have their representatives in this Order. Though the number of wings is its prominent feature, yet there are two-winged insects in other Orders, as some Ephemeræ: and the Eproboscidea of Latreille seem rather a kind of winged Aptera, if we consider their trophi, than real Diptera; or they may form an osculant group, partly winged and partly apterous, between the two. I have before remarked, that though, apparently, the insects of this Order have only two wings, yet the under or secondary wings of the other Orders have in them their representative[1227]. Their poisers also, I formerly observed to you, are probably more connected with their respiration than with their flight[1228].
Def. Metamorphosis incomplete, or coarctate.
Mouth proboscidate[1229].
Prothorax replaced by a collar. Sutures of the trunk mostly spurious[1230].
Wings two, with winglets attached to them: neuration various[1231]. Poisers.
Tarsi pentamerous.
Ovipositor various[1232].
11. Aphaniptera[1233] (Aptera L. Lamarck. Rhyngota F. Suctoria Latr.) This is an osculant Order, and is distinguished from the other Aptera L. in undergoing a regular metamorphosis. The larva is vermiform, the pupa incomplete, and inclosed in a cocoon. Probably the common flea and the chigoe would form distinct genera. The number of species of fleas is greater than has been supposed. I have been informed that Dr. Leach is acquainted with fourteen British species alone. Besides their metamorphosis, they are distinguished from the Aptera by the number of segments into which their body is divided, and by their pentamerous tarsi. Something like elytra and a scutellum appear to distinguish these insects.
Def. Metamorphosis incomplete.
Body apterous, compressed.
Mouth rostrulate[1234].
Tarsi pentamerous.
We are now come to those insects which, though they change their skin in their progress to their state of perfection, and some of them, as we have seen[1235], gain additional segments and pairs of legs, yet none of them acquire wings or wing-cases: these I have considered as forming one Order, under the denomination of
12. Aptera[1236] (Synistata, Antliata, Unogata, Mitosata F.). I do not give this as a natural Order. Our knowledge, however, of the internal organization of its groups, is not at present sufficiently matured to warrant the formation of them into new Classes[1237]: till that is more fully ascertained, it seems to me therefore best to consider these groups as forming three Suborders: the first consisting of the Hexapods; the second of the Octopods; and the third of the Polypods. It will be better, I think, instead of giving a general character of the Order,—which principally consists in the insects composing it being Apterous, or never acquiring organs of flight,—to define each of these groups.
Hexapods (Ametabolia Leach, Ametabola McL.). Six legs may be regarded as the natural number in all the insect tribes[1238]: but our business now is with those Aptera whose body consists of three greater segments, and which in none of their states have ever more or less than six legs, and consist of the three Linnean genera Pediculus, Lepisma, and Podura (Thysanura and Anoplura). Some of the mites (Acarus L.) are hexapods, but their body has no distinction of head, trunk, and abdomen. The metamorphosis of most female Blattæ, and of some other Orthoptera that are apterous, cannot be regarded as materially different from that of the Hexapods. Amongst the Anoplura,—the Pediculi, or lice, are suctorious, and the Nirmi, or bird-lice, masticators,—a circumstance which in them does not appear to indicate even a different Order, and proves that undue stress ought not to be laid, independently of general characters, on the mode in which insects take their food.
Def. Metamorphosis complete.
Body consisting of three principal segments.
Mouth perfect, or rostellate[1239].
Antennæ distinct.
Legs six, in every state.
Octopods. This suborder consists of the Trachean Arachnida of Latreille, excluding the Pycnogonida; of the Acaridea, Sironidea, Phalangidea, and part of the Scorpionidea of Mr. MacLeay, and, with some exceptions, of the Linnean genera Acarus and Phalangium. This last tribe (for with Linné, I include Chelifer and Obsidium in the Phalangidea,) on one side approaches Scorpio by Thelyphonus, and on the other the Aranidea by Gonyleptes; or, according to Mr. MacLeay, the transit is to both by Galeodes[1240]. But as there is reason for thinking that this last belongs to the Pulmonary Arachnida[1241], and forms a peculiar type in that Class, I consider the transit from the one to the other as above stated. The folded abdomen of Gonyleptes seems much to correspond with that of the cancriform spiders (Carkinodes cancriformis, &c.).
Def. Metamorphosis complete.
Body consisting of one or two principal segments.
Mouth various[1242].
Antennæ obsolete, or represented by mandibles.
Legs mostly eight, but in a few six only[1243].
Polypods. This suborder consists of Dr. Leach's Class Myriapoda, or the Chilognatha and Chilopoda of Latreille, corresponding with the Linnean genera Iulus and Scolopendra. Mr. MacLeay has arranged them in the same Class with the Hexapods, and connects them with the Anoplura by means of certain intestinal worms of an indistinct annulose structure[1244] (Entozoa Nematoidea Rud.), in which the sexes are diœcious, and some of which are furnished with lateral spinulæ,—thus, as he supposes, connected with the Polypods; and with the Anoplura by others (Epizoaria Lam.) in which appendages appear somewhat analogous to the legs of Hexapods, as in Cecrops Leach, and which like them are parasitic animals[1245]. But the right of these worms to be considered as members of the same Class with the Hexapods and Polypods at present appears rather problematical, and requires further examination.
Def. Metamorphosis subcomplete[1246].
Body consisting of numerous segments.
Mouth perfect[1247].
Eyes compound or aggregate.
Antennæ distinct.
Legs six on the trunk, many on the abdomen.
I must next say something on the Orders of the Arachnida. Every one, at first sight, sees that spiders and scorpions are separated by characters so strongly marked, that they look rather like animals belonging to different Classes than to the same: these form the two primary Orders of the Arachnida, and they appear to be connected by two secondary or osculant ones,—on the one side by Galeodes, and on the other by Thelyphonus and Phrynus[1248]. This Class, although there is an appearance of eight legs, is, strictly speaking, of a Hexapod type; for the anterior pair, ordinarily regarded as legs and performing their function, are really the analogues of the maxillary palpi of perfect insects. This will be evident to you if you examine any species of Galeodes. These animals, if we look at them cursorily, we should regard as Decapods; but when we trace the two anterior pairs of apparent legs to their insertion, we find that both proceed from the head, which in that genus is distinct from the trunk; while the three last pairs, which alone are furnished with claws, are planted, as legs usually are, in the latter part. The first pair represent the ordinary palpi of Arachnida, are analogous to the labial ones of Hexapods, and, as likewise in Phrynus and Thelyphonus, are more robust than what are usually taken for the first pair of legs; but they differ in being considerably longer, and instead of terminating in a chela are furnished with a retractile sucker[1249]. The second pair are more slender and shorter than the first; they correspond precisely with what are deemed the first pair of legs of Octopods and Arachnida, and are clearly analogous to the maxillary palpi of perfect insects. Whether the base of the first pair of these palpi is in any respect analogous to the labium of insects, (as that of the second seems to be to their maxillæ,) I am not prepared to assert: it will therefore be most advisable to name these palpi anterior and posterior: but as they evidently proceed from the head in Galeodes, and in that genus are clearly analogous to those of the Phrynidea, (which in their turn as clearly represent those of the Aranidea,) it follows that in all they are organs of the part representing the head, and therefore not in a primary sense legs; although in a secondary, as M. Savigny has proved, they may be so called[1250].