On the Origin and Metamorphoses of Insects Part 6

For my own part, I am satisfied that Natural Selection is a true cause, and, whatever may be the final result of our present inquiries-whether animated nature be derived from one ancestral source, or from many-the publication of the Origin of Species will none the less have const.i.tuted an epoch in the History of Biology. But, how far the present condition of living beings is due to that cause; how far, on the other hand, the action of Natural Selection has been modified and checked by other natural laws-by the unalterability of types, by atavism, &c.; how many types of life originally came into being; and whether they arose simultaneously or successively,-these and many other similar questions remain unsolved, even admitting the theory of Natural Selection. All this has indeed been clearly pointed out by Mr. Darwin himself, and would not need repet.i.tion but for the careless criticism by which in too many cases the true question has been obscured. Without, however, discussing the argument for and against Mr. Darwin's conclusions, so often do we meet with travesties of it like that which I have just quoted, that it is well worth while to consider the stages through which some group, say for instance that of insects, have probably come to be what they are, a.s.suming them to have developed under natural laws from simpler organisms. The question is one of great difficulty. It is hardly necessary to say that insects cannot have pa.s.sed through all the lower forms of animal life, and naturalists do not at present agree as to the actual line of their development.

In the case of insects, the gradual course of evolution through which the present condition of the group has probably been reached, has been discussed by Mr. Darwin, by Fritz Muller, Haeckel, Brauer, myself and others.

In other instances Palaeontology throws much light on this question.

Leidy has shown that the milk-teeth of the genus _Equus_ resemble the permanent teeth of the ancient _Anchitherium_, while the milk-teeth of _Anchitherium_ again approximate to the dental system of the still earlier _Merychippus_. Rutimeyer, while calling attention to this interesting observation, adds that the milk-teeth of _Equus caballus_ in the same way, and still more those of _E. fossilis_, resemble the permanent teeth of _Hipparion_.

"If we were not acquainted with the horse," says Flower,[54] "we could scarcely conceive of an animal whose only support was the tip of a single toe on each extremity, to say nothing of the singular conformation of its teeth and other organs. So striking have these characters appeared to many zoologists, that the animals possessing them have been reckoned as an order apart, called Solidungula; but palaeontology has revealed that in the structure of its skull, its teeth, its limbs, the horse is nothing more than a modified _Palaeotherium_; and though still with gaps in certain places, many of the intermediate stages of these modifications are already known to us, being the _Palaeotherium_, _Anchitherium_, _Merychippus_, and _Hipparion_."

"All Echinoids," says A. Aga.s.siz,[55] "pa.s.s, in their early stages, through a condition which recalls to us the first Echinoids which made their appearance in geological ages." On embryological grounds, he observes, we should "place true Echini lowest, then the Clypeastroids, next the Echinolamps, and finally the Spatangoids." Now among the Echinoids of the Trias there are no Clypeastroids, Echinolamps, or Spatangoids. The Clypeastroids make their appearance in the Lias, the Echinolamps in the Jura.s.sic, while the Spatangoids commence in the Cretaceous period.

Again[56] "in the Radiates, the Acalephs in their first stages of growth, that is, in their Hydroid condition, remind us of the adult forms among Polyps, showing the structural rank of the Acalephs to be the highest, since they pa.s.s beyond a stage which is permanent with the Polyps; while the Adult forms of the Acalephs have in their turn a certain resemblance to the embryonic phases of the cla.s.s next above them, the Echinoderms; within the limits of the cla.s.ses, the same correspondence exists as between the different orders; the embryonic forms of the highest Polyps recall the adult forms of the lower ones, and the same is true of the Acalephs as far as these phenomena have been followed and compared among them." Indeed, the accomplished authors from whom I have taken the above quotation, do not hesitate to say[57] that "whenever such comparisons have been successfully carried out, the result is always the same; the present representatives of the fossil types recall in their embryonic condition the ancient forms, and often explain their true position in the animal kingdom."

Fossil insects are unfortunately rare, there being but few strata in which the remains of this group are well preserved. Moreover, well-characterized Orthoptera and Neuroptera occur as early as the Devonian strata; Coleoptera and Hemiptera in the Coal-measures; Hymenoptera and Diptera in the Jura.s.sic; Lepidoptera, on the contrary, not until the Tertiary. But although it appears from these facts that, as far as our present information goes, the Orthoptera and Neuroptera are the most ancient orders, it is not, I think, conceivable that the latter should have been derived from any known species of the former; on the other hand, the earliest known Neuroptera and Orthoptera, though in some respects less specialized than existing forms, are as truly, and as well characterized, Insects, as any now existing; nor are we acquainted with any earlier forms, which in any way tend to bridge over the gap between them and lower groups, though, as we shall see, there are types yet existing which throw much light on the subject.

In the consideration then of this question, we must rely princ.i.p.ally on Embryology and Development. I have already referred to the cases in which species, very unlike in their mature condition, are very similar one to another when young. Haeckel, in his "Naturliche Schopfungsgeschichte," gives a diagram which ill.u.s.trates this very well as regards Crustacea. Pls. 1-4 show the same to be the case with Insects.

The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat, the Gra.s.shopper,-these and other less familiar types seem at first to have little in common. They differ in size, in form, in colour, in habits, and modes of life. Yet the researches of entomologists, following the clue supplied by the ill.u.s.trious Savigny, have proved, not only that while differing greatly in details, they are constructed on one common plan; but also that other groups, as for instance, Crustacea (Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown to be fundamentally similar. In Pl. 4 I have figured the larvae of an _Ephemera_ (Fig. 1), of a _Meloe_ (Fig. 2), of a Dragon-fly (Fig. 3), of a Sitaris (Fig. 4), of a _Campodea_ (Fig. 5), of a _Dyticus_ (Fig. 6), of a Termite (Fig. 7), of a _Stylops_ (Fig. 8), and of a _Thrips_ (Fig. 9). All these larvae possess many characters in common. The mature forms are represented in the corresponding figures of Plate 3, and it will at once be seen how considerably they differ from one another. The same fact is also ill.u.s.trated in Figs. 48-55, where Figs. 48-51 represent the larval states of the mature forms represented in Figs. 52-55. Fig. 48 is the larva of a moth, _Agrotis suffusa_ (Fig. 52); Fig. 49 of a beetle, _Haltica_ (Fig. 53); Fig. 50 of a Saw-fly, _Cimbex_ (Fig. 54); and Fig.

51 of a Centipede, _Julus_ (Fig. 55).

[Ill.u.s.tration: FIG. 48, Larva of Moth (_Agrotis suffusa_), after Packard. 49, Larva of Beetle (_Haltica_), after Westwood. 50, Larva of Saw-fly (_Cimbex_), Brischke and Zaddach. Beob. ub d. arten. der Blatt und Holzwespen, Fig. 8. 51, Larva of _Julus_. Newport, Philos.

Transactions, 1841.]

Thus, then, although it can be demonstrated that perfect insects, however much they differ in appearance, are yet reducible to one type, the fact becomes much more evident if we compare the larvae. M.

Brauer[58] and I[59] have pointed out that two types of larvae, which I have proposed to call _Campodea_-form and _Lindia_-form, and which Packard has named Leptiform and Eruciform, run through the princ.i.p.al groups of insects. This is obviously a fact of great importance: as all individual _Meloes_ are derived from a form resembling Pl. 2, Fig. 2, it is surely no rash hypothesis to suggest that the genus itself may have been so.

[Ill.u.s.tration: FIG. 52, _Agrotis suffusa_ (after Packard). 53, _Haltica_ (after Westwood).]

[Ill.u.s.tration: FIG. 54, _Cimbex_, Brischae and Zaddach. l.c. T. 2, Fig.

9.]

[Ill.u.s.tration: FIG. 55. _Julus_ (after Gervais).]

Firstly, however, let me say a word as to the general Insect type. It may be described shortly as consisting of animals possessing a head, with mouth parts, eyes and antennae; a many segmented body, with three pairs of legs on the segments immediately following the head; with, when mature, either one or two pairs of wings, generally with caudal appendages I will not now enter into a description of their internal anatomy. It will be seen that, except as regards the wings, Pl. 4, Fig.

4, representing the larva of a small beetle named _Sitaris_, answers very well to this description. Many other Beetles are developed from larvae closely resembling those of _Meloe_ (Pl. 4, Fig. 2), and Sitaris (Pl. 4, Fig. 4); in fact-except those species the larvae of which, as, for instance of the Weevils (Pl. 2, Fig. 6), are internal feeders, and do not require legs-we may say that the Coleoptera generally are derived from larvae of this type.

I will now pa.s.s to a second order, the Neuroptera. Pl. 4, Fig. 1, represents the larva of _Chloeon_, a species the metamorphoses of which I described some years ago in the Linnean Transactions,[60] and it is obvious that in essential points it closely resembles the form to which I have just alluded.

The Orthoptera, again, the order to which Gra.s.shoppers, Crickets, Locusts, &c. belong, commence life in a similar condition; and the same may also be said of the Trichoptera.

The larvae of Bees when they quit the egg are entirely legless, but in an earlier stage they possess well-marked rudiments of thoracic legs, showing, as it seems to me, that their apodal condition is an adaptation to their circ.u.mstances. Other Hymenopterous larvae, those for example of _Sirex_ (Fig. 9), and of the Saw-flies (Fig. 50) have well-developed thoracic legs.

From the difference in external form, and especially from the large comparative size of the abdomen, these larvae, as well as those of Lepidoptera (Fig. 48), have generally been cla.s.sed with the maggots of Flies, Weevils, &c., rather than with the more active form of larva just adverted to. This seems to me, as I have already pointed out,[61] to be a mistake. The caterpillar type differs, no doubt, in its general appearance, owing to its greater clumsiness, but still essentially agrees with that already described.

No Dipterous larva, so far as I know, belongs truly to this type; in fact, the early stages of the pupa in the Diptera seem in some respects to correspond to the larvae of other Insect orders. The Development of the Diptera is, however, as Weissman[62] has shown, very abnormal in other respects.

Thus, then, we find in many of the princ.i.p.al groups of insects that, greatly as they differ from one another in their mature condition, when they leave the egg they more nearly resemble the typical insect type; consisting of a head; a three-segmented thorax, with three pairs of legs; and a many-jointed abdomen, often with a.n.a.l appendages. Now, is there any mature animal which answers to this description? We need not have been surprised if this type, through which it would appear that insects must have pa.s.sed so many ages since (for winged Neuroptera have been found in the carboniferous strata) had long ago become extinct. Yet it is not so. The interesting genus _Campodea_ (Pl. 3, Fig. 5) still lives; it inhabits damp earth, and closely resembles the larva of _Chloeon_ (Pl. 2, Fig. 1), const.i.tuting, indeed, a type which, as shown in Pl. 4, occurs in many orders of insects. It is true that the mouth-parts of _Campodea_ do not resemble either the strongly mandibulate form which prevails among the larvae of Coleoptera, Orthoptera, Neuroptera, Hymenoptera, Lepidoptera; or the suctorial type of the h.o.m.optera and Heteroptera. It is, however, not the less interesting or significant on that account, since, as I have elsewhere[63] pointed out, its mouth-parts are intermediate between the mandibulate and haustellate types; a fact which seems to me most suggestive.

It appears, then, that there are good grounds for considering that the various types of insects are descended from ancestors more or less resembling the genus _Campodea_, with a body divided into head, thorax, and abdomen: the head provided with mouth-parts, eyes, and one pair of antennae; the thorax with three pairs of legs; and the abdomen, in all probability, with caudal appendages.

If these views are correct, the genus _Campodea_ must be regarded as a form of remarkable interest since it is the living representative of a primaeval type, from which not only the Collembola and Thysanura, but the other great orders of insects have derived their origin.

From what lower group the _Campodea_ type was itself derived is a question of great difficulty. Fritz Muller indeed says,[64] "if all the cla.s.ses of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial cla.s.ses, with their tracheal respiration, have branched off." Haeckel, moreover, is of the opinion that the Tracheata are developed from the Crustacea, and probably from the Zoepoda. For my own part, though I feel very great diffidence in expressing an opinion at variance with that of such high authorities, I am rather disposed to suggest that the _Campodea_ type may possibly have been derived from a less highly developed one, resembling the modern Tardigrade,[65] a (Fig.

56) smaller and much less highly organized being than _Campodea_. It possesses two eyes, three anterior pairs of legs, and one at the posterior end of the body, giving it a curious resemblance to some Lepidopterous larvae.

[Ill.u.s.tration: FIG. 56, Tardigrade (after Dujardin).]

These legs, however, as will be seen, are reduced to mere projections.

But for them, the Tardigrada would closely resemble the vermiform larva so common among insects. Among Trichoptera the larva early acquires three pairs of legs, but as Zaddach has shown,[66] there is a stage, though it is quickly pa.s.sed through, in which the divisions of the body are indicated, but no trace of legs is yet present. Indeed, there appear to be reasons for considering that while among Crustacea the appendages appear before the segments, in Insects the segments precede the appendages, although this stage of development is very transitory, and apparently, in some cases, altogether suppressed. I say "apparently,"

because, as I have already mentioned, I am not yet satisfied that it will not eventually be found to be so in all cases. Zaddach, in his careful observations of the embryology of _Phryganea_, only once found a specimen in this stage, which also, according to the researches of Huxley,[67] seems to be little more than indicated in _Aphis_. It is therefore possible that in other cases, when no such stage has been observed, it not really may be absent, but, from its transitoriness, may have hitherto escaped attention.

Fritz Muller has expressed the opinion[68] that this vermiform type is of comparatively recent origin. He says: "The ancient insects approached more nearly to the existing Orthoptera, and perhaps to the wingless Blattidae, than to any other order, and the complete metamorphosis of the Beetles, Lepidoptera, &c., is of later origin." "There were," he adds, "perfect insects before larvae and pupae." This opinion has been adopted by Mr. Packard[69] in his "Embryological Studies on Hexapodous Insects."

M. Brauer[70] also considers that the vermiform larva is a more recent type than the Hexapod form, and is to be regarded not as a developmental form, but as an adaptational modification of the earlier active hexapod type. In proof of this he quotes the case of _Sitaris_.

Considering, however, the peculiar habits of this genus, to which I have already referred, and also that the vermiform type is altogether lower in organization and less differentiated than the _Campodea_ form, I cannot but regard this case as exceptional; one in which the development has been, as it were, to use an expression of Fritz Muller's, "falsified" by the struggle for existence, and which therefore does not truly indicate the successive stages of evolution. On the whole, the facts seem to me to point to the conclusion that, though the grub-like larvae of Coleoptera and some other insects, owe their present form mainly to the influence of external circ.u.mstances, and partially also to atavism, still the _Campodea_ type is itself derived from earlier vermiform ancestors.

Nicolas Wagner has shown in the case of a small gnat, allied to _Cecidomyia_, that even now, in some instances, the vermiform larvae possess the power of reproduction. Such a larva (as, for instance, Fig.

57) very closely resembles some of the Rotatoria, such for instance as _Albertia_ or _Notommata_, which however possess vibratile cilia. There is, indeed, one genus-_Lindia_ (Fig. 58)-in which these ciliae are altogether absent, and which, though resembling _Macrobiotus_ in many respects, differs from that genus in being entirely dest.i.tute of legs. I have never met with it myself, but it is described by Dujardin, who found it in a ditch near Paris, as being oblong, vermiform, divided into rings, and terminating posteriorly in two short conical appendages. The jaws are not unlike those of the larvae of Flies, and indeed many naturalists meeting with such a creature would, I am sure, regard it as a small Dipterous larva; yet Dujardin figures a specimen containing an egg, and seems to have no doubt that it is a mature form.[71]

For the next descending stage we must, I think, look among the Infusoria, through such genera as _Chaetonotus_ or _Ichthydium_. Other forms of the Rotatoria, such for instance as _Rattulus_, and still more the very remarkable species discovered in 1871 by Mr. Hudson,[72] and described under the name of _Pedalion mira_, seem to lead to the Crustacea through the Nauplius form. Dr. Cobbold tells me that he regards the _Gordii_ as the lowest of the Scolecida; Mr. E. Ray Lankester considers some of the Turbellaria, such genera as _Mesostomum_, _Vortex_, &c., to be the lowest of existing worms; excluding the parasitic groups. Haeckel[73] also regards the Turbellaria as forming the nearest approach to the Infusoria. The true worms seem, however, to const.i.tute a separate branch of the animal kingdom.

We may take, as an ill.u.s.tration of the lower worms, the genus Prorhynchus (Fig. 59), which consists of a hollow cylindrical body, containing a straight simple tube, the digestive organ.

But however simple such a creature as this may be, there are others which are far less complex, far less differentiated; which therefore, on Mr. Darwin's principles, may be considered still more closely to represent the primaeval ancestor from which these more highly-developed types have been derived, and which, in spite of their great antiquity-in spite of, or perhaps in consequence of, their simplicity, still maintain themselves almost unaltered.

Thus the form which Haeckel has described[74] under the name _Protamba primitiva_, Pl. 5, Fig. 1-5, consists of a h.o.m.ogeneous and structureless substance, which continually alters its form; putting out and drawing in again more or less elongated processes, and creeping about like a true _Amba_, from which, however, _Protamba_ differs, in the absence of a nucleus. It seems difficult to imagine anything simpler; indeed, as described, it appears to be an ill.u.s.tration of properties without structure. It takes into itself any suitable particle with which it comes in contact, absorbs that which is nutritious, and rejects the rest. From time to time a constriction appears at the centre (Pl. 5, Fig. 2), its form approximates more and more to that of an hour-gla.s.s (Pl. 5, Fig. 3), and at length the two halves separate, and each commences an independent existence (Pl. 5, Fig. 5).

[Ill.u.s.tration: FIG. 59, _Prorhynchus stagnaus_.[75]]

[Ill.u.s.tration: PLATE V.

FIGS. 1-5, _Protamba_; 6-9, _Protamyxa Aurantiaca_, Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18, _Magosphaera planula_, Haeckel, loc. cit. pl. 5.]

In the true _Ambas_, on the contrary, we find a differentiation between the exterior and the interior: the body being more or less distinctly divisible into an outer layer and an inner parenchyme. In the _Ambas_, as in _Protamba_, multiplication takes place by self-division, and nothing corresponding to s.e.xual reproduction has yet been discovered.

Somewhat more advanced, but still of great simplicity, is the _Protomyxa aurantiaca_ (Pl. 5, Fig. 8), discovered by Haeckel[76] on dead sh.e.l.ls of _Spirula_, where it appears as a minute orange speck, which shows well against the clear white of the _Spirula_. Examined with a microscope, the speck is seen to be a spherical ma.s.s of orange-coloured, h.o.m.ogeneous, alb.u.minous matter, surrounded by a delicate, structureless membrane. It is obvious from this description that these bodies closely resemble eggs, for which indeed Haeckel at first mistook them. Gradually, however, the yellow sphere broke itself up into smaller spherules (Pl.

5, Fig. 9), after which the containing membrane burst, and the separate spherules, losing their globular form, crept out as small _Ambae_ (Pl.

5, Fig. 6), or amboid bodies. These little bodies moved about, a.s.similated the minute particles of organic matter, with which they came in contact, and gradually increased in size (Pl. 5, Fig. 7) with more or less rapidity according to the amount of nourishment they were able to obtain. They threw out arms in various directions, and if divided each section maintained its individual existence. After a while their movements ceased, they contracted into a ball, and again secreted round themselves a clear structureless envelope.

This completes their life history as observed by Haeckel, who found it easy to retain them in his gla.s.ses in perfect health, and who watched them closely.

As another ill.u.s.tration I may take the _Magosphaera planula_, discovered by Haeckel on the coast of Norway.

In one stage of its existence (Pl. 5, Fig. 10) it is a minute ma.s.s of gelatinous matter, which continually alters its form, moves about, feeds, and in fact behaves altogether like the _Amba_ just described.

It does not, however, remain always in this condition. After a while it contracts into a spherical form (Pl. 5, Fig. ii), and secretes round itself a structureless envelope, which, with the nucleus, gives it a very close resemblance to a minute egg.

Gradually the nucleus divides, and the protoplasm also separates into two spherules (Pl. 5, Fig. 12); these two subdivide into four (Pl. 5, Fig. 13), and so on (Pl. 5, Fig 14), until at length thirty-two are present, compressed into a more or less polygonal form (Pl. 5, Fig. 15).

Here this process ends. The separate spherules now begin to lose their smooth outline, to throw out processes, and to show amboid movements like those of the creatures just described. The processes or pseudopods grow gradually longer, thinner, and more pointed. Their movements become more active, until at length they take the form of ciliae. The spherical _Magosphaera_, the upper surface of which has thus become covered with ciliae, now begins to rotate within the cyst or envelope, which at length gives way and sets free the contained sphere, which then swims about freely in the water (Pl. 5, Fig. 16), thus closely resembling _Synura_, or one of the Volvocineae. After swimming about in this condition for a certain time, the sphere breaks up into the separate cells of which it is composed (Pl. 5, Fig. 17). As long as the individual cells remained together, they had undergone no changes of form, but after separating they show considerable contractility, and gradually alter their form, until they become undistinguishable from true _Ambae_ (Pl. 5, Fig. 18).

Finally, according to Haeckel, these amboid bodies, after living for a certain time in this condition, return to a state of rest, again contract into a spherical form, and secrete round themselves a structureless envelope. The life history of some other low organisms, as for instance _Gregarina_, is of a similar character.