The imagines of the Diptera (_genuina_), with the exception of the
_Aphaniptera_ and _Pupipara_, agree in all their chief characters,
such as the number and structure of the wings, the number and joints
of the legs, the peculiar formation of the thorax (fusion of the three
segments);[202] and even the structure of the mouth organs varies only
within narrow limits. This is in accordance with their mode of life,
which is very uniform in its main features: all the true Diptera live
in the light, moving chiefly by means of flight, but having also the
power of running; all those which take food in the imago condition
feed upon fluids. Their larvæ, on the other hand, are formed on two
essentially different types, the one--which I shall designate as the
gnat-type--possessing a horny head with eyes, three pairs of jaws,
and long or short antennæ, together with a 12- or 13-segmented body,
which is never provided with the three typical pairs of thoracic legs,
but frequently has the so-called abdominal legs on the first and last
segments. The other Dipterous larvæ are maggot-shaped and without a
horny head, or in fact without any head, since the first segment, the
homologue of the head, can in no case be distinguished through its
being larger than the others; it is on the contrary much smaller. The
typical insect mouth-parts are entirely absent, being replaced by a
variously formed and quite peculiar arrangement of hooks situated on
the mouth and capable of protrusion. Never more than eleven segments
are present besides the first, which is destitute of eyes; neither are
abdominal legs ever developed.
The mode of life differs very considerably in the two groups of larvæ.
Although the Dipterous maggots are not as a rule quite incapable of
locomotion like the grubs of the Hymenoptera (bees, ichneumons), the
majority are nevertheless possessed of but little power of movement
in the food-substance on which they were deposited as eggs. They do
not go in search of food, either because they are parasitic in other
insects in the same manner as the ichneumons (_Tachina_), or else they
live on decaying animal or vegetable substances or amidst large swarms
of their prey, like the larvæ of the _Syrphidæ_ amongst _Aphides_.
They generally undergo pupation in the same place as that which they
inhabit as larvæ and indeed in their larval skin which hardens into an
oval pupa-case. Some few leave their feeding place and pupate after
traversing a short distance (_Eristalis_).
As in the case of the Hymenoptera the structure of the larvæ can here
also be explained by peculiarities in their mode of life. Creatures
which live in a mass of food neither require special organs of
locomotion nor specially developed organs of sense (eyes). They have
no use for the three pairs of jaws since they only feed on liquid
substances, and the hooks within the mouth do not serve for the
reduction of food but only for fastening the whole body. With the jaws
and their muscular system there likewise disappears the necessity for a
hard surface of attachment, _i.e._ a corneous head.
The mode of life of the larvæ of the gnat-type is quite different
in most points. The majority, and indeed the most typically formed
of these, have to go in search of their food, whether they are
predaceous, such as the _Culicidæ_ and many of the other _Nemocera_
(_Corethra_, _Simulium_), or whether they feed on plants, which they
in some cases weave into a protective dwelling tube (certain species
of _Chironomus_). Many live in water and move with great rapidity;
others bury in the earth or in vegetable substances; and even those
species which live on fungi sometimes wander great distances, as in
the well-known case of the “army worm” where thousands of the larvæ of
_Sciara Thomæ_ thus migrate.
Now the two types of larvæ correspond generally with the two large
groups into which, as it appears to me correctly, the Diptera
(_genuina_) are as a rule divided. In this respect there is therefore
an equality of form-relationship--the grouping is the same, and
the incongruence depends only upon the form-divergence between the
two kinds of larvæ being greater than between the two kinds of
imagines.[203]
That the form-divergence is greater in the larvæ than in the imagines
cannot be doubted; that this distant form-relationship cannot,
however, be referred to a very remote common origin, _i.e._ to a very
remote blood-relationship, not only appears from the existence of
transition-forms between the two sub-orders, but can be demonstrated
here, as in the case of the Hymenoptera, by the embryonic development
of the maggot-like larvæ.
Seventeen years ago I showed[204] that the grub-formed larvæ of the
_Muscidæ_ in the embryonic state possessed a well-developed head with
antennæ and three pairs of jaws, but that later in the course of the
embryonic development a marked reduction and transformation of these
parts takes place, so that finally the four head segments appear as a
single small ring formed from the coalesced pairs of maxillæ, whilst
the so-called “fore-head” (the first head segment), together with the
mandibles, becomes transformed into a suctorial-head armed with hooks
and lying within the body. At the time of writing I drew no conclusion
from these facts with reference to the phyletic development of these
larval forms; nor did Bütschli, six years later, in the precisely
analogous case of the larvæ of the bees. The inference is, however, so
obvious that it is astonishing that it should not have been drawn till
the present time.[205]
There can be no doubt that the maggot-like larvæ of insects are not by
any means ancient forms, but are, on the contrary, quite recent, as
first pointed out by Fritz Müller,[206] and afterwards by Packard[207]
and Brauer,[208] and as is maintained in the latest work by Paul
Mayer[209] on the phylogeny of insects.
The Dipterous maggots have evidently descended from a larval form
which possessed a horny head with antennæ and three pairs of jaws, but
which had no appendages to the abdominal segments; they are therefore
ordinary Dipterous larvæ of the gnat-type which have become modified
in a quite peculiar manner and adapted to a new mode of life, just as
the grubs of the Hymenoptera are larvæ of the saw-fly type, which have
become similarly transformed, although by no means in the same manner.
The resemblance between the two types of larva is to a great extent
purely external, and depends upon the process designated “convergence”
by Oscar Schmidt, _i.e._ upon the adaptation of heterogeneous animal
forms to similar conditions of life. By adaptation to a life within
a mass of fluid nutriment, the caterpillar-formed larvæ of the
Hymenoptera and the _Tipula_-like larvæ of the Diptera have acquired
a similar external appearance, and many similarities in internal
structure, or, in brief, have attained to a considerable degree of
form-relationship, which would certainly have tended to conceal the
wide divergence in blood-relationship did not the embryological forms
on the one side and the imagines on the other provide us with an
explanation.
It is certainly of great interest that in another order of insects--the
Coleoptera--grub-formed larvæ occur quite irregularly, and their origin
can be here traced to precisely the same conditions of life as those
which have produced the grubs of bees. I refer to the honey-devouring
larvæ of the _Meloïdæ_ (_Meloë_, _Sitaris_, _Cantharis_). The case is
the more instructive, inasmuch that the six-legged larval form is not
yet relegated to the development within the egg, but is retained in the
first larval stage. In the _second larval stage_ the maggot-form is
first assumed, although this is certainly not so well pronounced as in
the Diptera or Hymenoptera, as neither the head nor the thoracic legs
are so completely suppressed as in these orders. Nevertheless, these
parts have made a great advance in the process of transformation.
The grub-like larvæ of the Hymenoptera and Diptera appear to me
especially instructive with reference to the main question of the
causes of transformation. The reply to the questions: what gives the
impetus to change? is this impetus internal or external? can scarcely
be given with greater clearness than here. If these larvæ have
abandoned their ancestral form and have acquired a widely divergent
structure, arising not only from suppression but partly also from an
essentially new differentiation (suctorial head of the _Muscidæ_), and
if these structural changes show a close adaptation to the existing
conditions of life, from these considerations alone it is difficult
to conceive how such transformations can depend upon the action of a
phyletic force. The latter must have foreseen that at precisely this
or that fixed period of time the ancestors of these larvæ would have
been placed under conditions of life which would make it desirable
for them to be modified into the maggot-type. But if at the same
time the imagines are removed in a less degree from those of the
caterpillar-like larvæ, this divergence being in exact relation with
the deviations in the conditions of life, I at least fail to see how we
can escape the consequence that it is the external conditions of life
which produce the transformations and induce the organism to change.
It is to me incomprehensible how one and the same vital force can in
the same individual induce one stage to become transformed feebly and
the other stage strongly, these transformations corresponding in extent
with the stronger or weaker deviations in the conditions of life to
which the organism is exposed in the two stages; to say nothing of the
fact that by such unequal divergences the idea of a perfect system
(creative thought) is completely upset.
Nor can the objection be raised that we are here only concerned with
insignificant changes--with nothing more than the arrested development
of single organs and so forth, in brief, only with those changes which
can be ascribed to the action of the environment.
We are here as little concerned with a mere suppression of organs
through arrested development as in the case of the Cirripedia;
the transformation and reconstruction of the whole body goes even
much further than in these Crustacea, although not so conspicuous
externally. Where do we elsewhere find insects having the head inside
a cavity of the body (sectorial head of the _Muscidæ_), and of
which the foremost segment--the physiological representative of the
head--consists entirely of the coalesced antennæ and pairs of maxillæ?
The incongruences in the form-relationships are, however, exceedingly
numerous in the case of the Diptera, and a special treatise would
be necessary to discuss them thoroughly. I may here mention only one
case, because the inequality shows itself in this instance in a quite
opposite sense.
Gerstäcker, who is certainly a competent entomologist, divides the
Diptera into three tribes, viz. the _Diptera genuina_, the _Pupipara_,
and the _Aphaniptera_. The latter, the fleas, possess in their divided
thoracic segments and in their jointed labial appendages characters so
widely divergent from those of the true Diptera and of the _Pupipara_
that Latreille and the English zoologists have separated them entirely
from the Diptera and have raised them into a separate order.[210] Those
who do not agree in this arrangement, but with Gerstäcker include the
fleas under the Diptera, will nevertheless admit that the morphological
divergence between the _Aphaniptera_ and the two other tribes is far
greater than that which exists between the latter. Now the larvæ of the
fleas are completely similar in structure to those of the gnat-type,
since they possess a corneous head with the typical mouth parts and
antennæ and a 13-segmented body devoid of legs. Were we only acquainted
with the larvæ of the fleas we should rank them with the true Diptera
under the sub-order _Nemocera_. On first finding such a larva we should
expect to see emerge from the pupa a small gnat.
While the imagines of the _Nemocera_ and _Aphaniptera_ thus show but a
very remote form-relationship their larvæ are very closely allied. Can
any one doubt that in this case it is not the larva but the imago which
has diverged to the greatest extent? Have not the fleas moreover become
adapted to conditions of life widely different from those of all other
Diptera, whilst their larvæ do not differ in this respect from many
other Dipterous larvæ?
We have here, therefore, another case of unequal phyletic development,
which manifests itself in the entirely different form-relationship
of the larvæ and the imagines. Thus in this case, as in that of the
Lepidoptera, it is sometimes the larval and at other times the imaginal
stage which has experienced the greatest transformation, and, as in the
order mentioned, the objection that a phyletic vital force produces
greater and more important differentiations in the higher imaginal
stage than in the lower or less developed larval stage, is equally
ineffectual.
If, however, it be asked whether the unequal phyletic development
depends in this case upon an unequal number of transforming impulses
which the two stages may have experienced during an equal period of
time, this must be decidedly answered in the negative. The unequal
development obviously depends in this case, as in the higher systematic
groups of the Lepidoptera, upon the unequal value of the parts affected
by the changes. These parts are on the one side of small importance,
and on the other side of great importance, to the whole structure of
the insect. This is shown in the last-mentioned case of the fleas,
where, of the typical parts of the body, only the wings have become
rudimentary, whilst the antennæ, mouth-parts, and legs, and even the
form and mode of segmentation (free thoracic segments), must have
suffered most important modifications; their larvæ, on the other hand,
can have experienced only unimportant changes, since they still agree
in all typical parts with those of the gnat-type.
Although therefore in this and in similar cases a greater number of
transforming impulses may well have occurred on the one side than on
the other--and it is indeed highly probable that this number has not
been absolutely the same--nevertheless the chief cause of the striking
incongruence is not to be found therein, but rather in the _strength_
of the transforming impulses, if I may be permitted to employ this
figure, or, more precisely expressed, in the importance of the parts
which become changed and at the same time in the amount of change.
In this conclusion there is implied as it appears to me an important
theoretical result which tells further against the efficacy of a
phyletic force.
If the so-called “typical parts” of an animal disappear completely
through the action of the environment only, and still further, if these
parts can become so entirely modified as to give rise to quite new and
again typical structures (suctorial head of the _Muscidæ_) without
the typical parts of the other stage of the same individual being
thereby modified and transformed into a new type of structure, how can
we maintain a distinction between typical and non-typical parts with
respect to their origin? But if a difference exists with respect only
to the physiological importance of such parts, _i.e._ their importance
for the equilibrium of the whole organization, while, with reference to
transformation and suppression, exactly the same influences appear to
be effective as those which bring about a change in or a disappearance
of the so-called adventitious parts, where is there left any scope
for the operation of the supposed phyletic force? What right have we
to assume that the typical structures arise by the action of a vital
force? Nevertheless this is the final refuge of those who are bound
to admit that a great number of parts or characters of an animal can
become changed, suppressed, or even produced by the action of the
environment.