HINTS ON THE ANCESTRY OF INSECTS.
[Illustration: 177. Pentastoma.]
[Illustration: 178. Centipede.]
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.
[Illustration: 179. Young Pauropus.]
[Illustration: 180. Spring-tail.]
[Illustration: 181. Young Julus.]
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).
[Illustration: 182. Leptus.]
[Illustration: 183. Tardigrade.]
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.
[Illustration: 184. Male Stylops.]
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.
[Illustration: 185. Female Stylops.]
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.
[Illustration: 186. Embryo of Diplax.]
[Illustration: 187. Embryo of Louse.]
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.
[Illustration: 188. Embryo of Spider.]
[Illustration: 189. Embryo of Podura.]
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.
[Illustration: 190. Zoëa.]
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.
[Illustration: 191. Nauplius.]
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.
[Illustration: 192. Platygaster error.]
We may, however, as bearing upon this difficult question, cite some
remarkable discoveries of Professor Ganin, a Russian naturalist, on the
early stages of certain ichneumon parasites, which show some worm
features in their embryonic development. In a species of Platygaster
(Fig. 192, P. error of Fitch), which is a parasite on a two-winged gall
fly, the earliest stage observed after the egg is laid is that in which
the egg contains a single cell with a nucleus and nucleolus. Out of this
cell (Fig. 193 _A_, _a_) arise two other cells. The central cell (_a_)
gives origin to the embryo. The two outer ones multiply by subdivision
and form the embryonal membrane, or "amnion," which is a provisional
envelope and does not assist in building up the body of the germ. The
central single cell, however, multiplies by the subdivision of its
nucleus, thus building up the body of the germ. Figure 193 _B_, _g_,
shows the yolk or germ just forming out of the nuclei (_a_) and _b_, the
peripheral cells of the blastoderm skin, or "amnion." Figure 193 _C_
shows the yolk transformed into the embryo (_g_), with the outer layer
of blastodermic cells (_b_). The body of the germ is infolded, so that
the embryo appears bent on itself. Figure 193 _D_ shows the embryo much
farther advanced, with the two pairs of lobes (_md_, rudimentary
mandibles; _d_, rudimentary pad-like organs, seen in a more advanced
stage in _E_), and the bilobate tail (_st_). Figure 194 (_m_, mouth;
_at_, rudimentary antennæ; _md_, mandibles; _d_, tongue-like appendages;
_st_, anal stylets; the subject of this figure is of a different species
from the insect previously figured, which, however, it closely
resembles) shows the first larva stage after leaving the egg. This
strange form, the author remarks, would scarcely be thought an insect,
were not its origin and farther development known, but rather a
parasitic Copepodous crustacean, whence he calls this the Cyclops-like
stage. In this condition it clings to the inside of its
host by means of its hook-like jaws (_md_), moving about like a Cestodes
embryo with its well known six hooks. The tail moves up and down, and is
of but little assistance in its efforts to change its place. Singularly
enough, the nervous, vascular, and respiratory systems (tracheæ) are
wanting, and the alimentary canal is a blind sac, remaining in an
indifferent, or unorganized state. How long it remains in this state
could not be ascertained.
[Illustration: 193. Development of Platygaster.]
[Illustration: 194. First Larva of Platygaster.]
[Illustration: 195. Second Larva of Platygaster.]
The second larval stage (Fig. 195; _oe_, oesophagus; _ng_,
supra-oesophageal ganglion; _n_, nervous cord; _ga_, and _g_, genital
organs; _ms_, band of muscles) is attained by means of a moult, as usual
in the metamorphoses of insects. With the change of skin the larva
entirely changes its form. So-called hypodermic cells are developed. The
singular tail is dropped, the segments of the body disappear, and the
body grows oval, while within begins a series of remarkable changes,
like the ordinary development of the embryo of most other insects within
the egg. The cells of the hypodermis multiply greatly, and lie one above
the other in numerous layers. They give rise to a special primitive
organ closely resembling the "primitive band" of all insect embryos. The
alimentary canal is made anew, and the nervous and vascular systems now
appear, but the tracheæ are not yet formed. It remains in this state for
a much longer period than in the previous stage.
[Illustration: 196. Third Larva of Polynema.]
The third larval form only a few live to reach. This is of the usual
long, oval form of the larvæ of the ichneumons, and the body has
thirteen segments exclusive of the head. The muscular system has greatly
developed and the larva is much more lively in its motions than before.
The new organs that develop are the air tubes and fat bodies. The
"imaginal disks" or rudimentary portions destined to develop and form
the skin of the adult, or imago, arise in the pupa state, which
resembles that of other ichneumons. These disks are only engaged, in
Platygaster, in building up the rudimentary appendages, while in the
flies (Muscidæ and Corethra) they build up the whole body, according to
the remarkable discovery of Weismann.
Not less interesting is the history of the development of a species of
Polynema, another egg-parasite, which lays its eggs (one, seldom two) in
the eggs of a small dragon fly, Agrion virgo, which oviposits in the
parenchyma of the leaves of waterlilies. The eggs develop as in
Platygaster. The earliest stage of the embryo is very remarkable. It
leaves the egg when very small and immovable, and with scarcely a trace
of organization, being a mere flask-shaped sac of cells.[23] It remains
in this state five or six days.
In the second stage, or Histriobdella-like form, the larva is, in its
general appearance, like the low worm to which Ganin compares it. It may
be described as bearing a general resemblance to the third and fully
developed larval form (Fig. 196, _tg_, three pairs of abdominal
tubercles destined to form the sting; _l_, rudiments of the legs; _fk_,
portion of the fatty body; _at_, rudiments of the antennæ; _fl_,
imaginal disks, or rudiments of the wings). No tracheæ are developed in
the larva, nor do any exist in the imago. (Ganin thinks, that as these
insects are somewhat aquatic, the adult insects flying over the surface
of the water, the wings may act as respiratory organs, like gills.) It
lives six to seven days before pupating, and remains from ten to twelve
days in the pupa state.
The origin of the sting is clearly ascertained. Ganin shows that it
consists of three pairs of tubercles, situated respectively on the
seventh, eighth, and ninth segments of the abdomen (Fig. 196, _tg_). The
labium is not developed from a pair of tubercles, as is usual, but at
once appears as an unpaired, or single organ. The pupa state lasts for
five or six days, and when the imago appears it eats its way through a
small round opening in the end of the skin of its host, the Agrion
larva.
[Illustration: 197. Development of Egg-parasites.]
The development of Ophloneurus, another egg-parasite, agrees with that
of Platygaster and Polynema. This egg-parasite passes its early life in
the eggs of Pieris brassicæ, and two or three live to reach the imago
state, though about six eggs are deposited by the female. The eggs are
oval, and not stalked. The larva is at first of the form indicated by
figure 197 _E_, and when fully grown becomes of a broad oval form, the
body not being divided into segments. It differs from the genera already
mentioned, in remaining within its egg membrane, and not assuming their
strange forms. From the non-segmented, sac-like larva, it passes
directly into the pupa state.
The last egg-parasite noticed by Ganin, is Teleas, whose development
resembles that of Platygaster. It is a parasite in the eggs of Gerris,
the Water Boatman. Figure 197 _A_ represents the egg; _B_, _C_, and _D_,
the first stage of the larva, the abdomen (or posterior division of the
body) being furnished with a series of bristles on each side. (_B_
represents the ventral, _C_ the dorsal, and _D_ the profile view; _at_,
antennæ; _md_, hook-like mandibles; _mo_, mouth; _b_, bristles; _m_,
intestine; _sw_, the tail; _ul_, under lip or labium.) In the second
larval stage, which is oval in form, and not segmented, the primitive
band is formed.
In concluding the account of his remarkable discoveries, Ganin draws
attention to the great differences in the formation of the eggs and the
germs of these parasites from what occurs in other insects. The egg has
no nutritive cells; the formation of the primitive band, usually the
first indication of the germ, is retarded till the second larval stage
is attained; and the embryonal membrane is not homologous with the
so-called "amnion" of other insects, but may possibly be compared with
the skin developed on the upper side of the low, worm-like acarian,
Pentastomum, and the "larval skin" of the embryos of many low Crustacea.
He says, also, that we cannot, perhaps, find the homologues of the
provisional organs of the larvæ, such as the singularly shaped antennæ,
the claw-like mandibles, the tongue-or ear-like appendages, in other
Arthropoda (insects and Crustacea); but that they may be found in the
parasitic Lernæan crustaceans, and in the leeches, such as Histriobella.
He is also struck by the similarity in the development of these
egg-parasites to that of a kind of leech (Nephelis), the embryo of which
is provided with ciliæ, recalling the larva of Teleas (Fig. 197 _B_,
_C_), while in the true leeches (Hirudo) the primitive band is not
developed until after they have passed through a provisional larval
stage.
This complicated metamorphosis of the egg-parasites, Ganin also compares
to the so-called "hyper-metamorphosis" of certain insects (Meloë,
Sitaris, and the Stylopidæ) made known by Siebold, Newport and Fabre,
and he considers it to be of the same nature.
He also, in closing, compares such early larval forms as those given in
figures 193 _E_ and 194, to the free swimming Copepoda. Finally, he says
a few words on the theory of evolution, and remarks "there is no doubt
that, if a solution of the questions arising concerning the genealogical
relations of different animals among themselves is possible, comparative
embryology will afford the first and truest principles." He modestly
suggests that the facts presented in his paper will widen our views on
the genetic relations of the insects to other animals, and refers to the
opinion first expressed by Fritz Müller (Für Darwin, p. 91), and
endorsed by Hæckel in his "Generelle Morphologie," that we must seek for
the ancestors of insects and Arachnida in the Zoëa form of Crustacea. He
cautiously remarks, however, that "the embryos and larvæ observed by me
in the egg-parasites open up a new and wide field for a whole series of
such considerations; but I will suppress them, since I am firmly
convinced that a theory, which I build up to-day, can easily be
destroyed with some few facts which I learn to-morrow. Since comparative
embryology as a science does not yet exist, so do I think that all
genetic theories are too premature, and without a strong scientific
foundation."
The writer is perhaps less cautious, but he cannot refrain from making
some reflections suggested by the remarkable discoveries of Ganin. In
the first place, these facts bear strongly on the theory of evolution by
"acceleration and retardation." In the history of these early larval
stages we see a remarkable acceleration in the growth of the embryo. A
simple sac of unorganized cells, with a half-made intestine, so to
speak, is hatched, and made to perform the duty of an ordinary, quite
highly organized larva. Even the formation of the "primitive band,"
usually the first indication of the organization of the germ, is
postponed to a comparatively late period in larval life. The different
anatomical systems, _i.e._, the heart with its vessels, the nervous
system and the respiratory system (tracheæ), appear at longer or shorter
intervals, while in one genus the tracheæ are not developed at all. Thus
some portions of the animal are accelerated in their development more
than others, while others are retarded, and in some species certain
organs are not developed at all. Meanwhile all live in a fluid medium,
with much the same habits, and surrounded with quite similar physical
conditions.
The highest degree of acceleration is seen in the reproductive organs
of the Cecidomyian larva of Miastor, which produces a summer brood of
young, alive, and living free in the body of the child-parent; and in
the pupa of Chironomus, which has been recently shown by Von Grimm, a
fellow countryman of Ganin, to produce young in the spring, while the
adult fly lays eggs in the autumn in the usual manner. This is in fact a
true virgin reproduction, and directly comparable to the alternation of
generations observed in the jelly fishes, in Salpa, and certain
intestinal worms. We can now, in the light of the researches of Siebold,
Leuckart, Ganin and others, trace more closely than ever the connection
between simple growth and metamorphosis, and metamorphosis and
parthenogenesis, and perceive that they are but the terms of a single
series. By the acceleration in the development of a single set of organs
(the reproductive), no more wonderful than the acceleration and
retardation of the other systems of organs, so clearly pointed out in
the embryos of Platygaster and its allies, we see how parthenogenesis
under certain conditions may result. The barren Platygaster larva, the
fertile Cecidomyia larva, the fertile Aphis larva, the fertile
Chironomus pupa, the fertile hydroid polype, and the fertile adult queen
bee are simply animals in different degrees of organization, and with
reproductive systems differing not in quality, but in the greater or
less rapidity of their development as compared with the rest of the
body.
Another interesting point is, that while the larvæ vary so remarkably in
form, the adult ichneumon flies are remarkably similar to one another.
Do the differences in their larval history seem to point back to certain
still more divergent ancestral forms?
These remarkable hyper-metamorphoses remind us of the metamorphosis of
the embryo of Echinoderms into the Pluteus-and Bipinnaria-forms of the
starfish, sea urchins and Holothurians;[24] of the Actinotrocha-form
larva of the Sipunculoid worms; of the Tornaria into Balanoglossus, the
worm; of the Cercaria-form larva of Distoma; of the Pilidium-form larva
of Nemertes; and the larval forms of the leeches;[25] as well as the
mite Pentastomum, and certain other aberrant mites, such as Myobia.
While Fritz Müller and Dohrn have considered the insects as having
descended from the Crustacea (some primitive zoëa-form), and Dohrn has
adduced the supposed zoëa-form larva of these egg-parasites as a proof,
we cannot but think, in a subject so purely speculative as the ancestry
of animals, that the facts brought out by Ganin tend to confirm our
theory, that the ancestry of all the insects (including the Arachnids
and Myriopods) should be traced directly to the worms. The development
of the degraded, aberrant Arachnidan Pentastomum accords, in some
important respects, with that of the intestinal worms. The Leptus-form
larva of Julus, with its strange embryological development, in some
respects so like that of some worms, points in that direction, as
certainly as does the embryological development of the egg-parasite
Ophioneurus. The Nauplius form of the embryo or larva of nearly all
Crustacea, also points back to the worms as their ancestors, the
divergence having perhaps originated, as we have suggested, in the
Rotatoria.
While the Crustacea may have resulted from a series of prototypes
leading up from the Rotifers (Fig. 198), it is barely possible that one
of these creatures may have given rise to a form resulting in two series
of beings, one leading to the Leptus form, the other to the Nauplius.
For the true Annelides (Chætopods) are too circumscribed and homogeneous
a group to allow us to look to them for the ancestral forms of insects.
But that the insects may have descended from some low worms is not
improbable when we reflect that the Syllis and allied genera of
Annelides bear appendages consisting of numerous joints; indeed, the
strange Dujardinia rotifers, figured by Quatrefages, in its general form
is remarkably like the larva of Chloëon. It has a quite distinct head,
bearing five long, slender, jointed antennæ, and but eight or nine rings
to the body, which ends in two long, many jointed appendages exactly
like the tentacles. Quatrefages adds, that its movements are usually
slow, but "when it wishes to move more rapidly, it moves its body
alternately up and down with much vivacity, and shoots forwards by
bounds, so to speak, a little after the manner of the larvæ of the
mosquito" (Histoire Naturelle des Annelés, Tome 2, p. 69). The gills of
aquatic insects only differ from those of worms in possessing tracheæ,
though the gills of the Crustacea may be directly compared with those of
insects.
[Illustration: 198. A Rotifer.]
But when once inside the circle of the class of insects the ground is
firmer, as our knowledge is surer. Granting now that the Leptus-like
ancestor of the six-footed insects has become established, it is not so
difficult to see how the Poduræ and finally a form like Campodea
appeared. Aquatic forms resembling the larva of the Ephemeræ, Perlæ and,
more remotely, the Forficulæ and white ants of to-day were probably
evolved with comparative suddenness. Given the evolution of forms like
the earwigs (Forficula), cockroaches and white ants (Termes), the latter
of which abounded in the coal period, and it was not a great step
forward to the evolution of the Dragonflies, the Psocus, the Chrysopa,
the lice or parasitic Hemiptera, together with Thrips, thus forming the
establishment of lines of development leading up to those Neuroptera
with a complete metamorphosis, and finally to the grasshoppers and other
forms of Orthoptera, together with the Hemiptera.
[Illustration: 199. Chrysopa.]
[Illustration: 200. Panorpa.]
We have thus advanced from wingless to winged forms, _i. e._, from
insects without a metamorphosis to those with a partial metamorphosis
like the Perlas; to the May flies and Dragon flies, in which the adult
is still more unlike the larva; to the Chrysopa (Fig. 199) and Forceps
Tails (Panorpa, Fig. 200) and Caddis flies, in which, especially the
latter, the metamorphosis is complete, the pupa being inactive and
enclosed in a cocoon.
[Illustration: 201. Embryo of Diplax.]
Having assumed the creation of our Leptus by evolutional laws, we must
now account for the appearance of tracheæ and those organs so dependent
on them, the wings, which, by their presence and consequent changes in
the structure of the crust of the body, afford such distinctive
characters to the flying insects, and raise them so far above the
creeping spiders and centipedes. Our Leptus at first undoubtedly
breathed through the skin, as do most of the Poduras, since we have been
unable to find tracheæ in them, nor even in the prolarva of a genus of
minute ichneumon egg parasites, nor in the Linguatulæ and Tardigrades,
and some mites, such as the Itch insect and the Demodex, and other
Acari. In the Myriopod, Pauropus, Lubbock was unable to find any traces
of tracheæ. If we examine the embryo of an insect shortly before birth,
as in the young Dragon fly (figure 201, the dotted line _t_ crosses the
rudimentary tracheæ), we find it to consist of two simple tubes with
few branches, while there are no stigmata, or breathing holes, to be
seen in the sides of the body. This fact sustains the view of
Gegenbaur[26] that at first the tracheæ formed two simple tubes in the
body-cavity, and that the primary office of these tubes was for
lightening the body, and that their function as respiratory tubes was a
secondary one. The aquatic Protoleptus, as we may term the ancestor of
Leptus, may have had such tubes as these, which acted like the swimming
bladder of fishes for lightening the body, as suggested by Gegenbaur. It
is known that the swimming bladder of fishes becomes developed into the
lungs of air-breathing vertebrates and man himself. As our Leptus
adopted a terrestrial life and needed more air, a connection was
probably formed by a minute branch on each side of the body with some
minute pore (for such exist, whose uses are as yet unknown) through the
skin, which finally became specialized into a stigma, or breathing pore;
and from the tracheal system being closed, we now have the open tracheal
system of land insects.
The next inquiry is as to the origin of the wings. Here the question
arises if wingless forms are exceptional among the winged insects, and
the loss of wings is obviously dependent on the habits (as in the lice),
and environment of the species (as in beetles living on islands, which
are apt to lose the hinder pair of wings), why may not their acquisition
in the first place have been due to external agencies; and, as they are
suddenly discarded, why may they not have suddenly appeared in the first
place? In aquatic larvæ there are often external gill-like organs, being
simple sacs permeated by tracheæ (as in Agrion, Fig. 129, or the May
flies). These organs are virtually aquatic wings, aiding the insect in
progression as well as in aërating the blood, as in the true wings. They
are very variable in position, some being developed at the extremity of
the abdomen, as in Agrion, or along the sides, as in the May flies, or
filiform and arranged in tufts on the under side of the body, as in
Perla; and the naturalist is not surprised to find them absent or
present in accordance with the varying habits of the animal. For
example, in the larvæ of the larger Dragon flies (Libellula, etc.) they
are wanting, while in Agrion and its allies they are present.
Now we conceive that wings formed in much the same way, and with no more
disturbance, so to speak, to the insect's organization, appeared during
a certain critical period in the metamorphosis of some early insect. As
soon as this novel mode of locomotion became established we can easily
see how surrounding circumstances would favor their farther development
until the presence of wings became universal. If space permitted us to
pursue this interesting subject farther, we could show how invariably
correlated in form and structure are the wings of insects to the varied
conditions by which they are surrounded, and which we are forced to
believe stand in the relation of cause to effect. Again, why should the
wings always appear on the thorax and on the upper instead of the under
side? As this is the seat of the centre of gravity, it is evident that
cosmical laws as well as the more immediate laws of biology determine
the position and nature of the wings of an insect.
Correlated with the presence of wings is the wonderful differentiation
of the crust, especially of the thorax, where each segment consists of a
number of distinct pieces; while in the spiders and Myriopods the
segments are as simple as in the abdominal segments of the winged
insect. It is not difficult here to trace a series leading up from the
Poduras, in which the segments are like those of spiders, to the
wonderful complexity of the parts in the thoracic segments of the
Lepidoptera and Hymenoptera.
In his remarks "On the Origin of Insects,"[27] Sir John Lubbock says, "I
feel great difficulty in conceiving by what natural process an insect
with a suctorial mouth like that of a gnat or butterfly could be
developed from a powerfully mandibulate type like the Orthoptera, or
even from the Neuroptera." Is it not more difficult to account for the
origin of the mouth-parts at all? They are developed as tubercles or
folds in the tegument, and are homologous with the legs. Figure 186
shows that the two sorts of limbs are at one time identical in form and
relative position. The thought suggests itself that these long, soft,
finger-like appendages may have been derived from the tentacles of the
higher worms, but the grounds for this opinion are uncertain. At any
rate, the earliest form of limb must have been that of a soft tubercle
armed with one, or two, or many terminal claws, as seen in aquatic
larvæ, such as Chironomus (Fig. 202), Ephydra (Fig. 203 _a_, _b_, _c_,
pupa) and many others. As the Protoleptus assumed a terrestrial life and
needed to walk, the rudimentary feet would tend to elongate, and in
consequence need the presence of chitine to harden the integument, until
the habit of walking becoming fixed, the necessity of a jointed
structure arose. After this the different needs of the offspring of such
an insect, with their different modes of taking food, vegetable or
animal, would induce the diverse forms of simple, or raptorial, or
leaping or digging limbs. A peculiar use of the anterior members, as
seen in grasping the food and conveying it to the mouth (perhaps
originally a simple orifice with soft lips, as in Peripatus), would tend
to cause such limbs to be grouped together, to concentrate around the
mouth-opening, and to be directed constantly forwards. With use, as in
the case of legs, these originally soft mouth-feet would gradually
harden at the extremities, until serviceable in biting, when they would
become jaws and palpi. Given a mouth and limbs surrounding it, and we at
once have a rude head set off from the rest of the body. And in fact
such is the history of the development of these parts in the embryo. At
first the head is indicated by the buds forming the rudiments of limbs;
the segments to which they are attached do not form a true head until
after the mouth-parts have attained their jaw-like characters, and it is
not until the insect is about to be hatched, that the head is definitely
walled in.
[Illustration: 202. Foot of Chironomus.]
[Illustration: 203. Ephydra.]
We have arrived, then, at our Leptus, with a head bearing two pairs of
jaws. The spiders and mites do not advance beyond this stage. But in the
true insects and Myriopods, we have the addition of special sense
organs, the antennæ, and another pair of appendages, the labial palpi.
It is evident that in the ancestor of these two groups the first pair of
appendages became early adapted for purely sensory purposes, and were
naturally projected far in advance of the mouth, forming the antennæ.
Before considering the changes from the mandibulate form of insects to
those with mouth parts adapted for piercing and sucking, we must
endeavor to learn how far it was possible for the caterpillar or maggot
to become evolved from the Leptus-like larvæ of the Neuroptera,
Orthoptera, Hemiptera and most Coleoptera. I may quote from a previous
article[28] a few words in relation to two kinds of larvæ most prevalent
among insects. "There are two forms of insectean larvæ which are pretty
constant. One we call _leptiform_, from its general resemblance to the
larvæ of the mites (Leptus). The larvæ of all the Neuroptera, except
those of the Phryganeidæ and Panorpidæ (which are cylindrical and
resemble caterpillars), are more or less leptiform, _i. e._, have a
flattened or oval body, with large thoracic legs. Such are the larvæ of
the Orthoptera and Hemiptera, and the Coleoptera (except the
Curculionidæ; possibly the Cerambycidæ and Buprestidæ, which approach
the maggot-like form of the larvæ of weevils). On the other hand, taking
the caterpillar or bee larva, with their cylindrical, fleshy bodies, in
most respects typical of larval forms of the Hymenoptera, Lepidoptera
and Diptera, as the type of the _cruciform_ larva, etc. * * * The larvæ
of the earliest insects were probably leptiform, and the cruciform
condition is consequently an acquired one, as suggested by Fritz
Müller."[29] It seems that these two sorts of larvæ had also been
distinguished by Dr. Brauer in the article already referred to, with
which, however, the writer was unacquainted at the time of writing the
above quoted article. The similar views presented may seem to indicate
that they are founded in nature. Dr. Brauer, after remarking that the
Podurids seemed to fulfil Hæckel's idea of what were the most primitive
insects, and noticing how closely they resemble the larvæ of Myriopods,
says, "specially interesting are those forms among the Poduridæ which
are described as Campodea and Japyx, since the larvæ of a great number
of insects may be traced back to them"; but he adds, and with this view
we are unable to agree, "while others, the caterpillar-like forms
(Raupenform), resulted from them by a retrograde process, and also
the still lower maggot-like forms. While on the one hand Campodea, with
its abdominal feet, and the larva of Lithobius are related, so on the
other the Lepismatidæ, which are very near the Blattariæ, are nearly
related to the Myriopods, since their abdominal segments often bear
appendages (Machilis). The Campodea-form appears in most of the
Pseudoneuroptera [Libellulids, Ephemerids, Perlids, Psocids and Termes],
Orthoptera, Coleoptera, Neuroptera, perhaps modified in the Strepsiptera
[Stylops and Xenos] and Coccidæ in their first stage of development, and
indeed in many of these at their first moult." Farther on he says, "A
larger part of the most highly developed insects assume another
larva-form, which appears not only as a later acquisition, through
accommodation with certain definite relations, but also arises as such
before our eyes. The larvæ of butterflies and moths, of saw flies and
Panorpæ, show the form most distinctly, and I call this the caterpillar
form (Raupenform). That this is not the primitive form, but one later
acquired, we see in the beetles. The larvæ of Meloë and Sitaris in their
fully grown condition possess the caterpillar form, but the new born
larvæ of these genera show the Campodea form. The last form is lost as
soon as the larva begins its parasitic mode of life. * * * The larger
part of the beetles, the Neuroptera in part, the bees and flies (the
last with the most degraded maggot form) possess larvæ of this second
form." He considers that the caterpillar form is a degraded Campodea
form, the result of its stationary life in plants or in wood.
[Illustration: Pl 2. EXAMPLES OF LEPTIFORM LARVÆ.
EXPLANATION OF PLATE 2. Figure 1, different forms of Leptus; 2, Diplax;
3, Coccinella larva; 4, Cicada larva; 5, Cicindela larva; 6, Ant Lion;
7, Calligrapha larva; 8, Aphis larva; 9, Hemerobius larva; 10, Glyrinua
larva; 11, Carabid larva; 12, Meloë larva.]
[Illustration: Pl 3. EXAMPLES OF ERUCIFORM LARVÆ.
EXPLANATION OF PLATE 3. Figure 1. Panorpa larva; 2, Phryganea larva; 3,
Weevil larva; 4, third larva of Meloë; 5, Chionea larva; 6, Carpet Worm;
7, Phora larva; 8, Wheat Caterpillar; 9, Sphinx Caterpillar; 10,
Acronycta? larva; 11, Saw Fly larva; 12, Abia Saw Fly larva; 13,
Halictus larva; 14, Andrena larva.]
[Illustration: 204. Tipula Larva.]
For reasons which we will not pause here to discuss, we have always
regarded the eruciform type of larva as the highest. That it is the
result of degradation from the Leptus or Campodea form, we should be
unwilling to admit, though the maggots of flies have perhaps retrograded
from such forms as the larvæ of the mosquitoes and crane flies
(Tipulids, Fig. 204).
That the cylindrical form of the bee grub and caterpillar is the result
of modification through descent is evident in the caterpillar-like form
of the immature Caddis fly (Pl. 3, fig. 2). Here the fundamental
characters of the larva are those of the Corydalus and Sialis and
Panorpa, types of closely allied groups. The features that remind us of
caterpillars are superadded, evidently the result of the peculiar
tube-inhabiting habits of the young Caddis fly. In like manner the
caterpillar-form is probably the result of the leaf-eating life of a
primitive Leptiform larva. In like manner the soft-bodied maggot of the
weevil is evidently the result of its living habitually in cavities in
nuts and fruits. Did the soft, baggy female Stylops live exposed, like
its allies in other families, to an out-of-doors life, its skin would
inevitably become hard and chitinous. In these and multitudes of other
cases the adaptation of the form of the insect to its mode of life is
one of cause and effect, and not a bit less wonderful after we know what
induced the change of form.
Having endeavored to show that the caterpillar is a later production
than the young, wingless cockroach, with which geological facts
harmonize, we have next to account for the origin of a metamorphosis in
insects. Here it is necessary to disabuse the reader's mind of the
prevalent belief that the terms larva, pupa and imago are fixed and
absolute. If we examine at a certain season the nest of a humble bee, we
shall find the occupants in every stage of growth from the egg to the
pupa, and even to the perfectly formed bee ready to break out of its
larval cell. So slight are the differences between the different stages
that it is difficult to say where the larval stage ends and the pupa
begins, so also where the pupal state ends and the imago begins. The
following figures (205-208) will show four of the most characteristic
stages of growth, but it should be remembered that there are
intermediate stages between. Now we have noticed similar stages in the
growth of a moth, though a portion of them are concealed beneath the
hard, dense chrysalis skin. The external differences between the larval
and pupal states are fixed for a large part of the year in most
butterflies and moths, though even in this respect there is every
possible variation, some moths or butterflies passing through their
transformations in a few weeks, others requiring several months, while
still others take a year, the majority of the moths living under ground
in the pupa state for eight or nine months. The stages of metamorphosis
in the Diptera are no more suddenly acquired than in the bee or
butterfly. In all these insects the rudiments of the wings, legs, and
even of the ovipositor of the adult exist in the young larva. We have
found somewhat similar intermediate stages in the metamorphoses of the
beetles. The insects we have mentioned are those with a "complete
metamorphosis." We have seen that even in them the term "complete" is a
relative and not absolute expression, and that the terms larva and pupa
are convenient designations for states varying in duration, and assumed
to fulfil certain ends of existence, and even then dependent on length
of seasons, variation in climate, and even on the locality. When we
descend to the insects with an "incomplete" metamorphosis, as in the May
fly, we find that, as in the case of Chloëon, Sir John Lubbock has
described twenty-one stages of existence, and let him who can say where
the larval ends and the pupal or imaginal stages begin. So in a stronger
sense with the grasshopper and cockroach. The adult state in these
insects is attained after a number of moults of the skin, during each of
which the insect gradually draws nearer to the final winged form. But
even the so-called pupæ, or half winged individuals known not to be
adult, in some cases feel the sexual impulse, while a number of species
in each of the families represented by these two insects never acquire
wings.
[Illustration: 205. Larva. 206. Semi-pupa.
207. Advanced Semi-pupa. 208. Pupa.
EARLY STAGES OF THE HUMBLE BEE.]
Still how did the perfect metamorphosis arise? We can only answer this
indirectly by pointing to the Panorpa and Caddis flies, with their
nearly perfect metamorphosis, though more nearly allied otherwise to
those Neuroptera with an incomplete metamorphosis, as the lace-winged
fly, than the insects of any other suborder. If, among a group of
insects such as the Neuroptera, we find different families with all
grades of perfection in metamorphosis, it is possible that larger and
higher groups may exist in which these modes of metamorphosis may be
fixed and characteristic of each. Had we more space for the exposition
of many known facts, the sceptic might perceive that by observing how
arbitrary and dependent on the habits of the insects are the
metamorphoses of some groups, the fixed modes of other and more general
groups may be seen to be probably due to biological causes, or in other
words have been acquired through changes of habits or of the temperature
of the seasons and of climates. Many facts crowd upon us, which might
serve as illustrations and proofs of the position we have taken. For
instance, though we have in tropics rainy and dry seasons when, in the
latter, insects remain quiescent in the chrysalis state as in the
temperate and frigid zones, yet did not the change from the earlier ages
of the globe, when the temperature of the earth was nearly the same the
world over, to the times of the present distribution of heat and cold in
zones, possibly have its influence on the metamorphoses of insects and
other animals? It is a fact that the remains of those insects with a
complete metamorphosis (the bees, butterflies and moths, flies and
beetles) abound most in the later deposits, while those with an
incomplete metamorphosis are fewer in number and the earliest to appear.
Again, certain groups of insects are not found in the polar regions.
Their absence is evidently due to the adverse climatic conditions of
those regions. The development of the same groups is striking in the
tropics, where the sum of environing conditions all tend to favor the
multiplication of insect forms.
It should be observed that some insects, as the grasshopper, for
example, as Müller says, "quit the egg in a form which is distinguished
from that of the adult insect almost solely by the want of wings," while
the freshly hatched young of the bee, we may add, is farthest from the
form of the adult. It is evident that in the young grasshoppers, the
metamorphoses have been passed through, so to speak, in the egg, while
the bee larva is almost embryonic in its build. The helpless young
maggot of the wasp, which is fed solely by the parent, may be compared
to the human infant, while the lusty young grasshopper, which
immediately on hatching takes to the grass or clover field with all the
enthusiasm of a duckling to its native pond, may be likened to that
young feathered mariner. The lowest animals, as a rule, are at birth
most like the adult. So with the earliest known crustacea. The king
crabs, and in all probability the primeval trilobites, passed through
their metamorphoses chiefly in the egg. So in the ancient Nebaliads
(Peltocaris, Discinocaris and Ceratiocaris), if we may follow the
analogy of the recent Nebalia, the young probably closely resembled the
adult, while the living crabs and shrimps usually pass through the most
marked metamorphoses. Among the worms, the highest, and perhaps the most
recent forms, pass through the most remarkable metamorphoses.
[Illustration: 209. Jaws of Ant Lion.]
Another puzzle for the evolutionist to solve is how to account for the
change from the caterpillar with its powerful jaws, to the butterfly
with its sucking or haustellate mouth-parts. We shall best approach the
solution of this difficult problem by a study of a wide range of facts,
but a few of which can be here noticed. The older entomologists divided
insects into haustellate or suctorial, and mandibulate or biting
insects, the butterfly being an example of one, and the beetle serving
to illustrate the other category. But we shall find in studying the
different groups that these are relative and not absolute terms. We find
mandibulate insects with enormous jaws, like the Dytiscus, or Chrysopa
larva or ant lion, perforated, as in the former, or enclosing, as in the
latter two insects, the maxillæ (_b_), which slide backward and forward
within the hollowed mandibles (_a_, Fig. 209, jaws of the ant lion),
along which the blood of their victims flows. They suck the blood, and
do not tear the flesh of their prey. The enormous mandibles of the adult
Corydalus are too large for use and, as Walsh observed, are converted
in the male into simple clasping organs. And to omit a number of
instances, in the suctorial Hemiptera or bugs we have different grades
of structure in the mouth-parts. In the biting lice (Mallophaga) the
mouth is mandibulate; in the Thrips it is mandibulate, the jaws being
free, and the maxillæ bearing palpi, while the Pediculi are suctorial,
and the true bugs are eminently so. But in the bed bug it is easy to see
that the beak is made up of the two pairs of jaws, which are simply
elongated and adapted for piercing and sucking. Among the so-called
haustellate insects the mouth-parts vary so much in different groups,
and such different organs separately or combined perform the function of
sucking, that the term haustellate loses its significance and even
misleads the student. For example, in the house fly the tongue (Fig. 210
_l_, the mandibles, _m_, and maxillæ, _mp_, are useless), a fleshy
prolongation of the labium or second maxillæ, is the sucker, while the
mandibles and maxillæ are used as lancets by the horse fly (Fig. 211,
_m_, mandibles, _mx_, maxillæ). The maxillæ in the butterfly are united
to form the sucking tube, while in the bee the end of the labium (Fig.
212) is specially adapted for lapping, not sucking, the nectar of
flowers. But even in the butterfly, or more especially the moth, there
is a good deal of misapprehension about the structure of the so-called
"tongue." The mouth-parts of the caterpillar exist in the moth. The
mandibles of the caterpillar occur in the head of the moth as two small
tubercles (Fig. 213, _m_). They are aborted in the adult. While the
maxillæ are as a rule greatly developed in the moth, in the caterpillar
they are minute and almost useless. The labium or second maxillæ, so
large in the moth, serves simply as a spinneret in the caterpillar. But
we find a great amount of variation in the tongue or sucker of moths,
and in the silk moths the maxillæ are rudimentary, and there is no
tongue, these organs being but little more developed than in the
caterpillar. Figure 213, B, shows the minute blade-like maxilla of the
magnificent Luna moth, an approximation to the originally blade-like
form in beetles and Neuroptera. The maxillæ in this insect are minute,
rudimentary, and of no service to the creature, which does not take
food. In other moths of the same family we have found the maxillæ
longer, and touching at their tips, though too widely separate at base
to form a sucking tube, while in others the maxillæ are curved, and meet
to form a true tube.
[Illustration: 210. Mouth-parts of the House fly.]
[Illustration: 211. Mouth-parts of Horse fly.]
[Illustration: 212. Head of Humble bee.]
[Illustration: 213. Mouth-parts of Moths.]
In the Cecropia moth it is difficult to trace the rudiments of the
maxillæ at all, and thus we have in the whole range of the moths, every
gradation from the wholly aborted maxillæ of the Platysamia Cecropia, to
those of Macrosila cluentius of Madagascar, which form a tongue,
according to Mr. Wallace, nine and a quarter inches in length, probably
to enable their owner to probe the deep nectaries of certain orchids.
These changes in form and size are certainly correlated with important
differences in habits, and the evolutionist can as rightly say that the
structural changes were induced by use and disuse and change of habits
and the environment of the animal, as on the other hand the advocate of
special creation claims that the two are simply correlated, and that is
all we know about it.
[Illustration: 214. Ichneumon Fly.]
Another set of organs, placed on quite another region of the body, unite
to form the sting of the bee, or its equivalent the ovipositor of other
hymenopterous insects, such as the Ichneumon fly (Fig. 214), the "saw"
of the saw fly, and the augur of the Cicada. These are all formed on the
same plan, arising early in the larval stage as three pairs of little
tubercles, which ultimately form long blades, the innermost constituting
the true ovipositor. We have found that one pair of these organs forms
the "spring" of the Podura, and that in these insects it is three
jointed, and thus is morphologically a pair of legs soldered together at
their base. We would venture to regard the ovipositor of insects as
probably representing three pairs of abdominal legs, comparable with
those of the Myriopods, and even, as we have suggested in another place,
the three pairs of jointed spinnerets of spiders. Thus the ovipositor of
the bee has a history, and is not apparently a special creation, but a
structure gradually developed to subserve the use of a defensive organ.
So the organs of special sense in insects are in most cases simply
altered hairs. The hairs themselves are modified epithelial cells. The
eyes of insects, simple and compound, are at first simply epithelial
cells, modified for a special purpose, and even the egg is but a
modified epithelial cell attached to the walls of the ovary, which in
turn is morphologically but a gland. Thus Nature deals in simples, and
with her units of structure elaborates as her crowning work a temple in
which the mind of man, formed in the image of God, may dwell. Her
results are not the less marvellous because we are beginning to dimly
trace the process by which they arise. It should not lessen our awe and
reverence for Deity, if with minds made to adore, we also essay to trace
the movements of His hand in the origin of the forms of life.
Some writers of the evolution school are strenuous in the belief that
the evolution hypothesis overthrows the idea of archetypes, and plans of
structure. But a true genealogy of animals and plants represents a
natural system, and the types of animals, be they four, as Cuvier
taught, or five, or more, are recognized by naturalists through the
study of dry, hard, anatomical facts. Accepting, then, the type of
articulates as founded in nature from the similar modes of development
and points of structure perceived between the worms and the crustacea on
the one hand, and the worms and insects on the other, have we not a
strong genetic bond uniting these three great groups into one grand
subkingdom, and can we not in imagination perceive the successive steps
by which the Creator, acting through the laws of evolution, has built up
the great articulate division of the animal kingdom?
FOOTNOTES:
[Footnote 14: Memoirs of the Peabody Academy of Science, II.
Embryological Studies on Diplax, Perithemis, and the Thysanurus genus
Isotoma. Salem, 1871.]
[Footnote 15: Translated in 1859 by Mr. Dallas under the title "Facts
for Darwin."]
[Footnote 16: "Whether that common stem-form of all the Tracheata
[Insects, Myriopods and Spiders] which I have called Protracheata in my
'General Morphology' has developed directly from the true Annelides
(Coelminthes), or, the next thing to this (_zunachst_), out of
Zoea-form Crustacea (Zoepoda), will be hereafter established only
through a sufficient knowledge and comparison of the structure and mode
of growth of the Tracheata, Crustacea and Annelides. In either case is
the root of the Tracheata, as also of the Crustacea, to be sought in the
group of the true jointed worms (Annelides, Gephyrea and Rotatoria." He
considers the first insect to have appeared after the Silurian period,
viz., in the Devonian.]
[Footnote 17: The Zoëa is born with eight pairs of jointed appendages
belonging to the head, and with no thoracic limbs, while in insects
there are but four pairs of cephalic appendages and three pairs of legs.
Correlated with this difference is the entirely different mode of
grouping the body segments, the head and thorax being united into one
region in the crab, but separate in the insects, the body being as a
rule divided into a head, thorax and abdomen, while these regions are
much less distinctly marked in the crabs, and liable in the different
orders to great variations. The great differences between the Crustacea
and insects are noticeable at an early period in the egg.]
[Footnote 18: Considerations on the Transmutation of Insects in the
Sense of the Theory of Descent. Read before the Imperial
Zoological-botanical Society in Vienna, April 3, 1869.]
[Footnote 19: American Naturalist, vol. 3, p. 45, March, 1869.]
[Footnote 20: See Prof. Torell's discovery of Eophyton Linnæanum, a
supposed land plant allied to the rushes and grasses of our day, in
certain Swedish rocks of Lower Cambrian age. The writer has, through the
kindness of Prof. Torell, seen specimens of these plants in the Museum
of the Geological Survey at Stockholm. Mr. Murray, of the Canadian
Geological Survey, was the first to discover in America (Labrador,
Straits of Belle Isle) this same genus of plants. They are described and
figured by Mr. Billings, who speaks of them as "slender, cylindrical,
straight, reed-like plants," in the "Canadian Naturalist" for August,
1872.
Should the terrestrial nature of these plants be established on farther
evidence, then we are warranted in supposing that there were isolated
patches of land in the Cambrian or Primordial period, and if there was
land there must have been bodies of fresh water, hence there may have
been both terrestrial and aquatic insects, possibly of forms like the
Podurids, May flies, Perlæ, mites and Pauropus of the present day. There
was at any rate land in the Upper Silurian period, as Dr. J. W. Dawson
describes land plants (Psilophyton) from the Lower Heiderberg Rocks of
Gaspe, New Brunswick, corresponding in age with the Ludlow rocks of
England.
We might also state in this connection that Dr. Dawson, the eminent
fossil botanist of Montreal, concludes from the immense masses of carbon
in the form of graphite in the Laurentian rocks of Canada, that "the
Laurentian period was probably an age of most prolific vegetable growth.
* * * Whether the vegetation of the Laurentian was wholly aquatic or in
part terrestrial we have no means of knowing." In 1855, Dr. T. Sterry
Hunt asserted "that the presence of iron ores, not less than that of
graphite, points to the existence of organic life even during the
Laurentian or so-called Azoic period." In 1861 he went farther and
stated his belief in "the existence of an abundant vegetation during the
Laurentian period." The Eophyton in Labrador occurs above the Trilobite
(Paradoxides) beds, while in Sweden they occur below.]
[Footnote 21: In a communication made to the Boston Society of Natural
History, Oct. 17, 1870 (see also "American Naturalist" for Feb. and
Sept., 1871).]
[Footnote 22: On the Origin of Insects, a paper read before the Linnæan
Society of London Nov. 2, 1871, and reported in abstract in "Nature,"
Nov. 9, 1871.]
[Footnote 23: This reminds us (though Ganin does not mention it) of the
development of the embryo of Julus, the Thousand legs, which, according
to Newport, hatches the 25th day after the egg is laid. At this period
the embryo is partially organized, having faint traces of segments, and
is still enveloped in its embryonal membranes and retains its connection
with the shell. In this condition it remains for seventeen days, when it
throws off its embryonal membrane, and becomes detached from the shell.]
[Footnote 24: It is a suggestive fact that these deciduous forms give
way through histolysis to true larval forms, just as in some flies
(Musca vomitoria) the true larval form goes under, and the adult form is
built up from the imaginal disks of the larva. In an analogous manner
the deciduous, pluteus-condition of the young Echinoderm perishes and is
absorbed by the growing body of the permanent adult stage. This
deciduous stage of the ichneumon may accordingly be termed the prelarval
stage. Now as we find insects with and without this prelarval stage, and
in the radiates quite different degrees of metamorphoses, the inquiry
arises how far these differences are correlated with, and consequently
dependent upon, the physical surroundings of these animals in the free
swimming condition. Merely to point out the differences in the mode of
development of animals is an interesting matter, and one could do worse
things, but the philosophical naturalist cannot rest here. He must seek
how these differences were brought about.]
[Footnote 25: Leuckart, in his great work, "Die Menschlichen Parasiten,"
p. 700, after the analogy of Hirudo, which develops a primitive streak
late in larval life, ventures to consider the first indications of the
germ of Nemertes in its larval, Pilidium form as a primitive streak. He
also suggests that the development of the later larval forms of the
Echinoderms is the same in kind.
Moreover, nearly twenty years ago (1854) Zaddach, a German naturalist,
contended that the worms are closely allied in their mode of development
to the insects and crustaceans. He compares the mode of development of a
leech (Clepsine) and certain bristle-bearing worms (Sænuris, Lumbricatus
and Uaxes); and we may now from Kowalensky's researches (1871) add the
common earth worm (Lumbricus), in which there is no such metamorphosis
as in the sea Nereids, to that of insects; the mode of formation of the
primitive band in the leeches and earth worms being much like that of
insects. This confirms the view of Leuckart and Ganin, who both seem to
have overlooked Zaddach's remarks. Moreover, the rings of the harder
bodied worms, as Zaddach says, contain chitine, as in the insects.
Zaddach also enters into farther details, which in his opinion ally the
worms nearer to the insects than many naturalists at his time were
disposed to allow. The singular Echinoderes has some remarkable
Arthropod characters.]
[Footnote 26: Vergleichende Anatomie, 2te Auflage, 1870, p. 437. I
should, however, here add that I am told by Mr. Putnam that some fishes
which have no swim-bladder, are surface-swimmers, and _vice versa_.]
[Footnote 27: Reported In "Nature" for Nov. 9, 1871.]
[Footnote 28: The Embryology of Chrysopa, and its bearings on the
Classification of the Neuroptera, "American Naturalist," vol. v. Sept.,
1871.]
[Footnote 29: "It is my opinion that the 'incomplete metamorphosis' of
the Orthoptera is the primitive one, _inherited_ from the original
parents of all insects, and the 'complete metamorphosis' of the
Coleoptera, Diptera, etc., a subsequently acquired one." _Fuer Darwin_,
English Trans., p. 121.]