The spring consists of a pair of three-jointed appendages, with the basal joints soldered together early in embryonic life, while the other two joints are free, forming a fork. It is longest in Smynthurus and Degeeria, and shortest in Achorutes (Fig. 172, b), where it forms a simple, forked tubercle; and is obsolete in Lipura and Anura, its place being indicated by an oval scar. The third joint varies in form, being hairy, serrate and knife-like in form, as in Tomocerus (Fig. 159, a), or minute, with a supplementary tooth, as in Achorutes (Fig. 172, c). This spring is in part homologous with the ovipositor of the higher insects, which originally consists of three pairs of tubercles, each pair arising apparently from the seventh, eighth, and ninth (the latter the penultimate) segments of the abdomen in the Hymenoptera. The spring of the Podura seems to be the homologue of the third pair of these tubercles, and is inserted on the penultimate segment. This comparison I have been able to make from a study of the embryology of Isotoma.
Another organ, and one which, so far as I am aware, has been overlooked by previous observers, I am disposed to consider as possibly an ovipositor. In the genus Achorutes, it may be found in the segment just before the spring-bearing segment, and situated on the median line of the body. It consists (Fig. 163) of two squarish valves, from between which projects a pair of minute tubercles, or blades, with four rounded teeth on the under side. This pair of infinitesimal saws reminds one of the blades of the saw-fly, and I am at a loss what their use can be unless to cut and pierce so as to scoop out a shallow place in which to deposit an egg. It is homologous in situation with the middle pair of blades which composes the ovipositor of higher insects, and if it should prove to be used by the creature in laying its eggs, we should then have, with the spring, an additional point of resemblance to the Neuroptera and higher insects, and instead of this spring being an important differential character, separating the Thysanura from other insects, it binds them still closer, though still differing greatly in representing only a part of the ovipositor of the higher insects. (This is a catch for holding the spring in place.)
But all the Poduras differ from other insects in possessing a remarkable organ situated on the basal segment of the abdomen. It is a small tubercle, with chitinous walls, forming two valves from between which is forced out a fleshy sucker, or, as in Smynthurus, a pair of long tubes, which are capable of being darted out on each side of the body, enabling the insect to attach itself to smooth surfaces, and rest in an inverted position.
The eggs are laid few in number, either singly or several together, on the under side of stones, chips or, as in the case of Isotoma Walkerii, under the bark of trees. They are round, transparent. The development of the embryo of Isotoma in general accords with that of the Phryganeidæ and suggests on embryological grounds the near relationship of the Thysanura to the Neuroptera.
The earliest stage observed was at the time of the appearance of the primitive band (Fig. 164, a, b, folding of the primitive band; c, the dotted line crosses the primitive band, and terminates in a large yolk granule) which surrounds the egg as in the Caddis flies. Soon after, the primitive segments appear (Fig. 165; 1, antennæ; 2, mandibles; 3, maxillæ; the labium was not seen; 5-7, legs; c, yolk surrounded by the primitive band) and seem to originate just as in the Caddis flies. Figure 166 is a front view of the embryo shortly before it is hatched; figure 167, side view of the same, the figures as in Fig. 165; sp, spring; l, labrum. The labrum or upper lip, and the clypeus are large and as distinct as in the embryos of other insects, a fact to which we shall allude again. The large three-jointed spring is now well developed, and the inference is drawn that it represents a pair of true abdominal legs. The embryo when about to hatch throws off the egg-shell and amnion in a few seconds. The larva is perfectly white and is very active in its movements, running over the damp, inner surface of the bark. It is a little over a hundredth of an inch in length, and differs from the adult in being shorter and thicker, with the spring very short and stout. In fact the larva assumes the form of the lower genera of the family, such as Achorutes and Lipura, the adult more closely resembling Degeeria. The larva after its first moult retains its early clumsy form, and is still white. After a second moult it becomes purplish, and much more slender, as in the adult. The eggs are laid and the young hatched apparently within a period of from six to ten days.
Returning to the stage indicated by figures 166 and 167, I am induced to quote some remarks published in the Memoirs of the Peabody Academy of Science, No. 2, p. 18, which seem to support the view that these insects are offshoots from the Neuroptera.
"The front of the head is so entirely different from what it is in the adult, that certain points demand our attention. It is evident that at this period the development of the insect has gone on in all important particulars much as in other insects, especially the Neuropterous Mystacides as described by Zaddach. The head is longer vertically than horizontally, the frontal, or clypeal region is broad, and greater in extent than the epicranio-occipital region. The antennæ are inserted high up on the head, next the ocelli, falling down over the clypeal region. The clypeus, however, is merged with the epicranium, and the usual suture between them does not appear distinctly in after life, though its place is seen in figure 167 to be indicated by a slight indentation. The labrum is distinctly defined by a well marked suture, and forms a squarish, knob-like protuberance, and in size is quite large compared to the clypeus. From this time begins the process of degradation, when the insect assumes its Thysanurous characters, which consist in an approach to the form of the Myriopodous head, the front, or clypeal region being reduced to a minimum, and the antennæ and eyes brought in closer proximity to the mouth than in any other insects."
Sir John Lubbock has given us an admirable account of the internal anatomy of these little creatures, his elaborate and patient dissections filling a great gap in our knowledge of their internal structure. The space at our disposal only permits us to speak briefly of the respiratory system. Lubbock found a simple system of tracheæ in Smynthurus which opens by "two spiracles in the head, opposite the insertion of the antennæ," i. e., on the back of the head. (Von Olfers says that they open on the prothorax.) Nicolet and Olfers claim to have found tracheæ in several lower genera (Orchesella, Tomocerus, and Achorutes and allied genera), but Lubbock was unable to detect them, and I may add that I have not yet been able after careful search to find them either in living specimens, or those rendered transparent by potash.
Having given a hasty sketch of the external aspect of the Poduras, I extract from Lubbock's work a synopsis of the families and genera for the convenience of the student, adding the names of known American species, or indications of undescribed native forms.
Smynthuridæ.—Body globular or ovoid; thorax and abdomen forming one mass; head vertical or inclined; antennæ of four or eight segments. Eyes eight on each side, on the top of the head. Legs long and slender. Saltatory appendage with a supplementary segment.
Smynthurus. Antennæ four-jointed, bent at the insertion of the fourth, which is nearly as long as the other three, and appears to consist of many small segments. No conspicuous dorsal tubercles. (In this country Fitch has described five species: S. arvalis, elegans, hortensis, Novæboracensis, and signifer. Figure 156 represents a species found in Maine.)
Dicyrtoma. Antennæ eight-jointed, five before, three after the bend. Two dorsal tubercles on the abdomen.
Papirius.[12] Antennæ four-jointed, without a well-marked elbow, and with a short terminal segment offering the appearance of being many-jointed.
Poduridæ.—This family comprises those species of the old genus Podura, in which the mouth has mandibles [also maxillæ and a labium], and the body is elongated, with a more or less developed saltatory appendage at the posterior extremity.
Orchesella. Segments of the body unequal in size, more or less thickly clothed with clubbed hairs. Antennæ long, six-jointed. Eyes six in number on each side, arranged in the form of an S. (One or two beautiful species live about Salem.)
Degeeria. Segments of the body unequal in size, more or less thickly clothed by clubbed hairs. Antennæ longer than the head and thorax, filiform, four-jointed. Eyes eight in number on each side of the head. (Two species, Degeeria decem-fasciata, Pl. 10, Figs. 2, 3, and D. purpurascens, Figs. 4, 5, are figured in the "Guide to the Study of Insects." Figure 168 represents a species found in Salem, Mass., closely allied to the European D. nivalis. Five species are already known in New England.)
Seira. Body covered with scales. Antennæ four-jointed; terminal segment not ringed. Eyes on a dark patch. Thorax not projecting over the head. Abdominal segments unequal.
Templetonia. Segments of the body subequal, clothed by clubbed hairs, and provided with scales. Antennæ longer than the head and thorax, five-jointed, with a small basal segment, and with the terminal portion ringed.
Isotoma. Four anterior abdominal segments subequal, two posterior ones small; body clothed with simple hairs and without scales. Antennæ four-jointed, longer than the head; segments subequal. Eyes seven in number on each side, arranged in the form of an S. (Three species are found in Massachusetts, one of which (I. plumbea) is figured on Pl. 10, Figs. 6, 7, of the "Guide to the Study of Insects," third edition.)
Tomocerus. Abdominal segments unequal, with simple hairs and scales. Antennæ very long, four-jointed, the two terminal segments ringed. Eyes seven in number on each side. (The European T. plumbea, Podura plumbea of authors, is our species, and is common. Fig. 160, greatly enlarged, copied from Templeton; Fig. 159, side view, see also Fig. 161, where the mouth-parts are greatly enlarged, the lettering being the same, md, mandibles; mx, maxillæ; mp, maxillary palpus; lb, labium; lp, labial palpus; lc, lacinia; g, portion ending in three teeth; l, lobe of labium; sp, ventral sucking disk; the dotted line's passing through the body represent the course of the intestine; b, end of tibia, showing the tarsus, with the claw, and two accessory spines; a, third joint of the spring. Fig. 162, lacinia of maxilla greatly enlarged. Fig. 169, different forms of scales, showing the great variation in size and form, the narrow ones running into a linear form, becoming hairs. The markings are also seen to vary, showing, their unreliable character as test objects, unless a single scale is kept for use.)
Lepidocyrtus. Abdominal segment unequal, with simple hairs and scales. Antennæ long, four-jointed. Eyes eight in number on each side. (Fig. 170, L. albinos, an European species, from Hardwicke's "Science Gossip." Fig. 171, a scale. Two species live in New England.)
Podura. Abdominal segments subequal. Hairs simple, no scales. Antennæ four-jointed, shorter than the head. Eyes eight in number on each side. Saltatory appendage of moderate length.
Achorutes. Abdominal segments subequal. Antennæ short, four-jointed. Eyes eight in number on each side. Saltatory appendage quite short.
Figure 172 represents a species of this genus very abundant under the bark of trees, etc., in New England. It is of a blackish lead color; a, end of tibia bearing a tenant hair, with the tarsal joint and large claw; b, spring; c, the third joint of the spring, with the little spine at the base; figure 163, the supposed ovipositor; a, the two blades spread apart; b, side view. The mouth-parts in this genus are much as in Tomocerus, the maxillæ ending in a lacinia and palpus.
The three remaining genera, Lipura, Anurida and Anura, are placed in the "family" Lipuridæ, which have no spring. Lubbock remarks that "this family contains as yet only two[13] genera, Lipura (Burmeister), in which the mouth is composed of the same parts as those in the preceding genera, and Anura (Gervais), in which the mandibles and maxillæ disappear." Our common white Lipura is the European L. fimetaria Linn. (Fig. 173, copied from Lubbock). The site of the spring is indicated by an oval scar.
Figure 174 represents Anurida maritima found under stones between tide marks at Nantucket. It is regarded the same as the European species by Lubbock, to whom I had sent specimens for comparison. This genus differs in the form of the head from Lipura and also wants the terminal upcurved spines, while the antennæ are much more pointed. The legs (Fig. 175) end in a large, long, curved claw. On examining specimens soaked in potash, I have found that the mouth-parts of this species (Fig. 176,) md, mandibles; mx, maxillæ; e, eyes, and a singular accessory group of small cells, are like those of Achorutes, as previously noticed by Laboulbène. The mandibles, like those of other Poduras, end in from three to six teeth, and have a broad, many-toothed molar surface below. The maxillæ; end in a tridentate lacinia as usual, though the palpi and galea I have not yet studied.
The genus Anura may be readily recognized by the mouth ending in an acutely conical beak, with its end quite free from the head and hanging down beneath it. The body is short and broad, much tuberculated, while the antennæ are short and pointed, and the legs are much shorter than in Lipura, not reaching more than a third of their length beyond the body. Our common form occurs under the bark of trees.
For the reason that I can find no valid characters for separating these three genera as a family from the other Poduras, I am inclined to think that they form, by the absence of the spring, only a subdivision (perhaps a subfamily) of the Poduridæ.
The best way to collect Poduras is, on turning up the stick or stone on the under side of which they live, to place a vial over them, allowing them to leap into it; they may be incited to leap by pushing a needle under the vial. They may also be collected by a bottle with a sponge saturated with ether or chloroform. They may be kept alive for weeks by keeping moist slips of blotting paper in the vial. In this way I have kept specimens of Degeeria, Tomocerus and Orchesella, from the middle of December till late in January. During this time they occasionally moulted, and Tomocerus plumbeus, after shedding its skin, ate it within a few hours. Poduras feed ordinarily on vegetable matter, such as dead leaves and growing cryptogamic vegetation. These little creatures can be easily preserved in a mixture of alcohol and glycerine, or pure alcohol, though without the glycerine the colors fade.
We have entered more fully in this chapter into the details of structure than heretofore, too much so, perhaps, for the patience of our readers. But the study of the Poduras possesses the liveliest interest, since these lowest of all the six-footed insects may have been among the earliest land animals, and hence to them we may look with more or less success for the primitive, ancestral forms of insect life.
Though our course through the different groups of insects may have seemed rambling and desultory enough, and pursued with slight reference to a natural classification of the insects of which we have spoken, yet beginning with the Hive bee, the highest intelligence in the vast world of insects, we have gradually, though with many a sudden step, descended to perhaps the most lowly organized forms among all the insects, the parasitic mites. While the Demodex is probably the humblest in its organization of any of the insects we have treated of, there is still another mite, which, some eminent naturalists continue to regard as a worm, which is yet lower in the scale. This is the Pentastoma (Fig. 177, P. tænioides), which lives in the manner of the tape worm a parasitic life in the higher animals, though instead of inhabiting the alimentary canal, the worm-like mite takes up its abode in the nostrils and frontal sinus of dogs and sheep, and sometimes of the horse. At first, however, it is found in the liver or lungs of various animals, sometimes in man. It is then in the earliest or larval state, and assumes its true mite form, being oval in shape, with minute horny jaws adapted for boring, and with two pairs of legs armed with sharp retractile claws. Such an animal as this is little higher than some worms, and indeed is lower than many of them.
We should also not pass over in silence the Centipedes (Fig. 178, Scolopocryptops sexspinosa) and Galley worms, or Thousand legs and their allies (Myriopods), which by their long slender bodies, and great number of segments and feet, vaguely recall the worms. But they, with the mites, are true insects, as they are born with only three pairs of feet, as are the mites and ticks, and breathe by tracheæ; and thus a common plan of structure underlies the entire class of insects.
A very strange Myriopod has been discovered by Sir John Lubbock in Europe, and we have been fortunate enough to find a species in this country. It is the Pauropus. It consists, when fully grown, of nine segments, exclusive of the head, bearing nine pairs of feet. The young of Pauropus (Fig. 179) is born with three pairs of feet, and in its general appearance reminds us of a spring-tail (Fig. 180) as may be seen by a glance at the cut. This six-legged form of Pauropus may also be compared with the young galley worm (Fig. 181).
Passing to the group of spiders and mites, we find that the young mites when first hatched have but three pairs of feet, while their parents have four, like the spiders. Figure 182 represents the larva (Leptus) of the red garden mites; while a figure of the "water bear," or Tardigrade (Fig. 183), is introduced to compare with it, as it bears a resemblance to the young of the mites, though their young are born with their full complement of legs, an exception to their nearest allies, the true mites. Now if we compare these early stages of mites and myriopods with those of the true six-footed insects, as in the larval Meloë, Cicada, Thrips and Dragon fly, we shall see quite plainly that they all share a common form. What does this mean? To the systematist who concerns himself with the classification of the myriads of different insects now living, it is a relief to find that all can be reduced to the comparatively simple forms sketched above. It is to him a proof of the unity of organization pervading the world of insects. He sees how nature, seizing upon this archetypal form has, by simple modifications of parts here and there, by the addition of wings and other organs wanting in these simple creatures, rung numberless changes in this elemental form. And starting from the simplest kinds, such as the Poduras, Spiders, Grasshoppers and May flies, allied creatures which we now know were the first to appear in the earlier geologic ages, we rise to the highest, the bees with their complex forms, their diversified economy and wonderful instincts. In ascending this scale of being, while there is a progress upwards, the beetles, for instance, being higher than the bugs and grasshoppers; and the butterflies and moths, on the whole, being more highly organized than the flies; and while we see the hymenopterous saw-flies, with their larvæ mimicking so closely the caterpillars of the butterflies, in the progress from the saw-flies up to the bees we behold a gradual loss of the lower saw-fly characters in the Cynips and Chalcid flies, and see in the sand-wasps and true wasps a constant and accelerating likeness to the bee form. Yet this continuity of improving organizations is often broken, and we often see insects which recall the earlier and more elementary forms.
Again, going back of the larval period, and studying the insect in the egg, we find that nearly all the insects yet observed agree most strikingly in their mode of growth, so that, for instance, the earlier stages of the germ of a bee, fly or beetle, bear a remarkable resemblance to each other, and suggest again, more forcibly than when we examine the larval condition, that a common design or pattern at first pervades all. In the light of the studies of Von Baer, of Lamarck and Darwin, should we be content to stop here, or does this ideal archetype become endowed with life and have a definite existence, becoming the ancestral form of all insects, the prototype which gave birth to the hundreds of thousands of insect forms which are now spread over our globe, just as we see daily happens where a single aphis may become the progenitor of a million offspring clustering on the same tree? Is there not something more than analogy in the two things, and is not the same life-giving force that evolves a million young Aphides from the germ stock of a single Aphis in a single season, the same in kind with the production of the living races of insects from a primeval ancestor? When we see the Aphis giving origin in one season to successive generations, the individuals of which may be counted by the million, it is no less mysterious than that other succession of forms of insect life which has peopled the globe during the successive chapters of its history. While we see in one case the origin of individual forms, and cannot explain what it is that starts the life in the germ and so unerringly guides the course of the growing embryo, it is illogical to deny that the same life-giving force is concerned in the production of specific and generic forms.
Who can explain the origin of the sexes? What is the cause that determines that one individual in a brood of Stylops, for example (Fig. 184, male; Fig. 185, grub-like female in the body of its host), shall be but a grub, living as a parasite in the body of its host, while its fellow shall be winged and as free in its actions as the most highly organized insect? It is no less mysterious, because it daily occurs before our eyes. So perhaps none the less mysterious, and no more discordant with known natural laws may the law that governs the origin of species seem to those who come after us. Certainly the present attempts to discover that law, however fatuitous they may seem to many, are neither illogical, nor, judging by the impetus already given to biology, or the science of life, labor altogether spent in vain. The theory of evolution is a powerful tool, when judiciously used, that must eventually wrest many a secret from the grasp of nature.
But whether true or unproved, the theory of evolution in some shape has actually been adopted by the large proportion of naturalists, who find it indispensable in their researches, and it will be used until found inadequate to explain facts. Notwithstanding the present distrust, and even fear, with which it is received by many, we doubt not but that in comparatively few years all will acknowledge that the theory of evolution will be to biology what the nebular hypothesis is to geology, or the atomic theory is to chemistry. While the evolution theory is as yet imperfect, and many objections, some seemingly insuperable, can be raised against it, it should be borne in mind that the nebular hypothesis is still comparatively crude and unsatisfactory, though indispensable as a working theory to the geologist; and in chemistry, though the atomic theory may not be satisfactorily demonstrated to some minds until an atom is actually brought to sight, it is yet invaluable in research.
Many short sighted persons complain that such a theory sets in the back-ground the idea of a personal Creator; but minds no less devout, and perhaps a trifle more thoughtful, see the hand of a Creator not less in the evolution of plants and animals from prëexistent forms, through natural laws, than in the evolution of a summer's shower, through the laws discovered by the meteorologist, who looks back through myriads of ages to the causes that led to the distribution of mountain chains, ocean currents and trade winds, which combine to produce the necessary conditions resulting in that shower.
Indeed, to the student of nature, the evolution theory in biology, with the nebular hypothesis, and the grand law in physics of the correlation of forces, all interdependent, and revealing to us the mode in which the Creator of the Universe works in the world of matter, together form an immeasurably grander conception of the order of creation and its Ordainer, than was possible for us to form before these laws were discovered and put to practical use. We may be allowed, then, in a reverent spirit of inquiry, to attempt to trace the ancestry of the insects, and without arriving, perhaps, at any certain result, for it is largely a matter of speculation, point out certain facts, the thoughtful consideration of which may throw light on this difficult and embarrassing question.
Without much doubt the Poduras are the lowest of the six-footed insects. They are more embryonic in their appearance than others, as seen in the large size of the head compared with the rest of the body, the large, clumsy legs, and the equality in the size of the several segments composing the body. In other characters, such as the want of compound eyes, the absence of wings, the absence of a complete ovipositor, and the occasional want of tracheæ, they stand at the base of the insect series. That they are true insects, however, we endeavored to show in the previous chapter, and that they are neuropterous, we think is most probable, since not only in the structure of the insect after birth do they agree with the larvæ of certain neuropters, but, as we have shown in another place[14] in comparing the development of Isotoma, a Poduran, with that of a species of Caddis fly, the correspondence throughout the different embryological stages, nearly up to the time of hatching, is very striking. And it is a remarkable fact, as we have previously noticed, that when it begins to differ from the Caddis fly embryo, it begins to assume the Poduran characters, and its development consequently in some degree retrogrades, just as in the lice previous to hatching, as we have shown in a previous chapter, so that I think we are warranted at present in regarding the Thysanura, and especially the family of Podarids as degraded neuropters. Consequently the Poduras did not have an independent origin and do not, perhaps, represent a distinct branch of the genealogical tree of articulates. While the Poduras may be said to form a specialized type, the Bristle-tails (Lepisma, Machilis, Nicoletia and Campodea) are, as we have seen, much more highly organized, and form a generalized or comprehensive type. They resemble in their general form the larva of Ephemerids, and perhaps more closely the immature Perla, and also the wingless cockroaches.
Now such forms as these Thysanura, together with the mites and the singular Pauropus, we cannot avoid suspecting to have been among the earliest to appear upon the earth, and putting together the facts, first, of their low organization; secondly, of their comprehensive structure, resembling the larvæ of other insects; and thirdly, of their probable great antiquity, we naturally look to them as being related in form to what we may conceive to have been the ancestor of the class of insects. Not that the animals mentioned above were the actual ancestors, but that certain insects bearing a greater resemblance to them than any others with which we are acquainted, and belonging possibly to families and orders now extinct, were the prototypes and progenitors of the insects now known.
Though the study of the embryology of insects is as yet in its infancy, still with the facts now in our possession we can state with tolerable certainty that at first the embryos of all insects are remarkably alike, and the process of development is much the same in all, as seen in the figure of Diplax (Fig. 186), the louse (Fig. 187), the spider (Fig. 188) and the Podura (Fig. 189), and we could give others bearing the same likeness. We notice that at a certain period in the life of the embryo all agree in having the head large, and bearing from two to four pairs of mouth organs, resembling the legs; the thorax is merged in with the abdomen, and the general form of the embryo is ovate. Now this general embryonic form characterizes the larva of the mites, of the myriopods and of the true insects. To such a generalized embryonic form to which the insects may be referred as the descendants, we would give the name of Leptus, as among Crustacea the ancestral form is referred to Nauplius, a larval form of the lower Crustacea, and through which the greater part of the Crabs, Shrimps, Barnacles, water fleas, etc., pass to attain their definite adult condition. A little water flea was described as a separate genus, Nauplius, before it was known to be the larva of a higher water flea, and so also Leptus was thought to be a mature mite. Accordingly, we follow the usage of certain naturalists in dealing with the Crustacea, and propose for this common primitive larval condition of insects the term Leptus.
The first to discuss this subject of the ancestry of insects was Fritz Müller, who in his "Für Darwin,"[15] published in 1863, says, at the end of his work, "Having reached the Nauplius, the extreme outpost of the class, retiring farthest into the gray mist of primitive time, we naturally look round us to see whether ways may not be descried thence towards other bordering regions. * * * But I can see nothing certain. Even towards the nearer provinces of the Myriopoda and Arachnida I can find no bridge. For the Insecta alone, the development of the Malacostraca [Crabs, Lobsters, Shrimps, etc.] may perhaps present a point of union. Like many Zoëæ, the Insecta possess three pairs of limbs serving for the reception of nourishment, and three pairs serving for locomotion; like the Zoëæ they have an abdomen without appendages; as in all Zoëæ the mandibles in Insecta are destitute of palpi. Certainly but little in common, compared with the much which distinguishes these two animal forms. Nevertheless, the supposition that the Insecta had for their common ancestor a Zoëa which raised itself into a life on land, may be recommended for further examination" (p. 140).
Afterwards Hæckel in his "Generelle Morphologie" (1866) and "History of Creation," published in 1868, reiterates the notion that the insects are derived from the larva (Zoëa, Fig. 190) of the crabs, though he is doubtful whether they did not originate directly from the worms.[16]
It may be said in opposition to the view that the insects came originally from the same early crustacean resembling the larva of a crab or shrimp, that the differences between the two types are too great, or, in other words, the homologies of the two classes too remote,[17] and the two types are each too specialized to lead us to suppose that one was derived from the other. Moreover, we find through the researches of Messrs. Hartt and Scudder that there were highly developed insects, such as May flies, grasshoppers, etc., in the Devonian rocks of New Brunswick, leading us to expect the discovery of low insects even in the Upper Silurian rocks. At any rate this discovery pushes back the origin of insects beyond a time when there were true Zoëæ, as the shrimps and their allies are not actually known to exist so far back as the Silurian, not having as yet been found below the coal measures.
The view that the insects were derived from a Zoëa was also sustained by Friedrich Brauer, the distinguished entomologist of Vienna, in a paper[18] read in March, 1869. Following the suggestion of Fritz Müller and Hæckel, he derives the ancestry of insects from the Zoëa of crabs and shrimps. However, he regards the Podurids as the more immediate ancestors of the true insects, selecting Campodea as the type of such an ancestral form, remarking that the "Campodea-stage has for the Insects and Myriopods the same value as the Zoëa for the Crustacea." He says nothing regarding the spiders and mites.
At the same time[19] the writer, in criticising Hæckel's views of the derivation of insects from the Crustacea (ignorant of the fact that he had also suggested that the insects were possibly derived directly from the worms, and also independently of Brauer's opinions) declared his belief that though it seemed premature, after the discovery of highly organized winged insects in rocks so ancient as the Devonian, and with the late discovery of a land plant in the Lower Silurian rocks of Sweden,[20] to even guess as to the ancestry of insects, yet he would suggest that, instead of being derived from some Zoëa, "the ancestors of the insects (including the six-footed insects, spiders and myriopods) must have been worm-like and aquatic, and when the type became terrestrial we would imagine a form somewhat like the young Pauropus, which combines in a remarkable degree the characters of the myriopods and the degraded wingless insects, such as the Smynthurus, Podura, etc. Some such forms may have been introduced late in the Silurian period, for the interesting discoveries of fossil insects in the Devonian of New Brunswick, by Messrs. Hartt and Scudder, and those discovered by Messrs. Meek and Worthen in the lower part of the Coal Measures at Morris, Illinois, and described by Mr. Scudder, reveal carboniferous myriopods (two species of Euphorberia) more highly organized than Pauropus, and a carboniferous scorpion (Buthus?) closely resembling a species now living in California, together with another scorpion-like animal, Mazonia Woodiana, while the Devonian insects described from St. John by Mr. Scudder, are nearly as highly organized as our grasshoppers and May flies. Dr. Dawson has also discovered a well developed milleped (Xylobius) in the Lower Coal Measures of Nova Scotia; so that we must go back to the Silurian period in our search for the earliest ancestor, or (if not of Darwinian proclivities) prototype, of insects."
Afterwards[21] the writer, carrying out the idea suggested above, "referred the ancestry of the Myriopods, Arachnids, and Hexapodous insects to a Leptus-like terrestrial animal, bearing a vague resemblance to the Nauplius form among Crustacea, inasmuch as the body is not differentiated into a head, thorax and abdomen [though the head may be free from the rest of the body] and there are three pairs of temporary locomotive appendages. Like Nauplius, which was first supposed to be an adult Entomostracan, the larval form of Trombidium had been described as a genus of mites under the name of Leptus (also Ocypete and Astoma) and was supposed to be adult."
In the same year Sir John Lubbock[22] agrees with Brauer that the groups represented by Podura and Campodea may have been the ancestors of the insects, remarking that "the genus Campodea must be regarded as a form of remarkable interest, since it is the living representative of a primæval type from which not only the Collembola (Podura, etc.) and Thysanura, but the other great orders of insects, have all derived their origin."
The comparison of the Leptus with the Nauplius, or pre-Zoëal stage of Crustacea, is much more natural. But here we are met with apparently insuperable difficulties. While the Nauplius (Fig. 191) has but three pairs of appendages, which become the two pairs of antennæ and succeeding pair of limbs of the adult, in the Leptus as the least number we have five pairs, two of which belong to the head (the maxillæ and mandibles) and three to the thorax; besides these is a true heed, distinct from the hinder region of the body. It is evident that the Leptus fundamentally differs from the Nauplius and begins life on a higher plane. We reject, therefore, the Crustacean origin of the insects. Our only refuge is in the worms, and how to account for the transmutation of any worm with which we are at present acquainted into a form like the Leptus, with its mandibulated mouth and jointed legs, seems at first well nigh impossible. We have the faintest possible indication in the structure of some mites, and of the Tardigrades and Pentastoma, where there is a striking recurrence, as we may term it, to a worm-like form, readily noticed by every observer, whatever his opinion may be on the developmental theory. In the Demodex we see a tendency of the mite to assume under peculiar circumstances an elongated, worm-like form. The mouth-parts are aborted (though from what we know of the embryology of other mites, they probably are indicated early in embryonic life), while the eight legs are not jointed, and form simple tubercles. In the Tardigrades, a long step lower, we have unjointed fleshy legs armed with from two to four claws, but the mouth-parts are essentially mite in character. A decided worm feature is the fact that they are hermaphrodites, each individual having ovaries and spermaries, as is the case with many worms.
When we come to the singular creatures of which Pentastoma and Linguatula are the type, we have the most striking approximation to the worms in external form, but these are induced evidently by their parasitic mode of life. They lose the rudimentary jointed limbs which some (Linguatula especially) have well marked in the embryo, and from being oval, rudely mite-like in form, they elongate, and only the claws or simple curved hooks, like those of young tape worms, remain to indicate the original presence of true jointed legs.
In seeking for the ancestry of our hypothetical Leptus among the worms, we are at best groping in the dark. We know of no ancestral form among the true Annelides, nor is it probable that it was derived from the intestinal worms. The only worm below the true Annelides that suggests any remote analogy to the insects is the singular and rare Peripatus, which lives on land in warm climates. Its body, not divided into rings, is provided with about thirty pairs of fleshy tubercles, each ending in two strong claws, and the head is adorned with a pair of fleshy tubercles. It is remotely possible that some Silurian land worm, if any such existed, allied to our living Peripatus, may have been the ancestor of a series of types now lost which resulted in an animal resembling the Leptus.