_THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE OF
LARVÆ._
The facts recapitulated briefly in the preceding chapter show, that the
forms of insect larvæ depend greatly on the group to which they belong.
Thus the same tree may harbour larvæ of Diptera, Hymenoptera,
Coleoptera, and Lepidoptera; each presenting the form typical of the
family to which it belongs.
If, again, we take a group, such, for instance, as the Lamellicorn
beetles, we shall find larvæ extremely similar in form, yet very
different in habits. Those, for instance, of the common cockchafer (Fig.
1) feed on the roots of grass; those of _Cetonia aurata_ (Fig. 2)
inhabit ants' nests; the larvæ of the genus _Trox_ (Fig. 3) are found on
dry animal substances; of _Oryctes_ (Fig. 4) in tan-pits; of _Aphodius_
(Fig. 5) in dung; of _Lucanus_ (the stag-beetle, Fig. 6) in wood.
[Illustration: FIG. 1, Larva of the Cockchafer (_Melolontha_).
(Westwood, Int. to the Modern Classification of Insects, vol. i. p.
194.). 2, Larva of _Cetonia_. 3, Larva of _Trox_. 4, Larva of _Oryctes_.
5, Larva of _Aphodius_ (Chapuis and Candèze, Mém. Soc. Roy. Liège,
1853). 6, Larva of _Lucanus_. (Packard, Guide to the Study of Insects,
Fig. 403).]
On the other hand, in the present chapter it will be my object to show
that the form of the larva depends very much on the conditions of its
life. Thus, those larvæ which are internal parasites, whether in
animals or plants, are vermiform, as are those which live in cells, and
depend on their parents for food. On the other hand, larvæ which burrow
in wood have strong jaws and generally somewhat weak thoracic legs;
whilst those which feed on leaves have the thoracic legs more developed,
but less so than the carnivorous species. Now, the Hymenoptera, as a
general rule, belong to the first category: the larvæ of the Ichneumons,
&c., which live in animals,--those of the Cynipidæ, inhabiting
galls,--and those of ants, bees, wasps, &c., which are fed by their
parents, are fleshy, apodal grubs; though the remarkable fact that the
embryos of bees in one stage of their development possess rudiments of
thoracic legs which subsequently disappear, seems to show, not indeed
that the larvæ of bees were ever hexapod, but that bees are descended
from ancestors which had hexapod larvæ, and that the present apod
condition of these larvæ is not original, but results from their mode of
life.
On the other hand, the larvæ of _Sirex_ (Fig. 14) being wood-burrowers,
possess well-developed thoracic legs. Again, the larvæ of the
Tenthredinidæ, which feed upon leaves, closely resemble the caterpillars
of Lepidoptera, even to the presence of abdominal pro-legs.
[Illustration: FIG. 7, Larva of _Brachytarsus_ (Ratzeburg, Forst.
Insecten). 8, Larva of _Crioceris_ (Westwood, loc. cit.).]
The larvæ of most Coleoptera (Beetles) are active, hexapod, and more or
less flattened: but those which live inside vegetable tissues, such as
the weevils, are apod fleshy grubs, like those of Hymenoptera. Pl. II.,
Fig. 6, represents the larva of the nut-weevil, _Balaninus_ (Pl. I., Fig.
6), and it will be seen that it closely resembles Pl. II., Fig. 5, which
represents that of a fly (_Anthrax_), Pl. I., Fig. 5, and Pl. II., Figs.
7, 8, and 9, which represent respectively those of a _Cynips_ or gall-fly
(Pl. I., Fig. 7), an ant (Pl. I., Fig. 8), and wasp (Pl. I., Fig. 9).
Nor is _Balaninus_ the only genus of Coleoptera which affords us examples
of this fact. Thus in the genus _Scolytus_ (Pl. I., Fig. 4), the larvæ
(Pl. II., Fig. 4), which, as already mentioned, feed on the bark of the
elm, closely resemble those just described, as also do those of
_Brachytarsus_ (Fig. 7). On the other hand, the larvæ of certain beetles
feed on leaves, like the caterpillars of Lepidoptera; thus that of
_Crioceris Asparagi_ (Fig. 8)--which, as its name denotes, feeds on the
asparagus--closely resembles the larvæ of certain Lepidoptera, as for
instance of _Thecla spini_. From this point of view the transformations
of the genus _Sitaris_ (Pl. III., Fig. 4), which have been very carefully
investigated by M. Fabre, are peculiarly interesting.[12]
[Illustration: FIG. 9, Larva of _Sitaris numeralis_ (Fabre, Ann. des Sci.
Nat., sér. 4, tome vii.). 10, Larva of _Sitaris humeralis_, in the second
stage. 11, Larva of _Sitaris humeralis_, in the third stage. 12, Larva of
_Sitaris humeralis_, in the fourth stage. 13, Pupa of _Sitaris_.]
The genus _Sitaris_ (a small beetle allied to Cantharis, the blister-fly,
and to _Meloë_, the oil-beetle) is parasitic on a kind of Bee
(Anthophora), which excavates subterranean galleries, each leading to a
cell. The eggs of the _Sitaris_, which are deposited at the entrance of
these galleries, are hatched at the end of September or beginning of
October; and M. Fabre not unnaturally expected that the young larvæ,
which are active little creatures with six serviceable legs (Fig. 9),
would at once eat their way into the cells of the Anthophora. No such
thing: till the month of April following they remain without leaving
their birthplace, and consequently without food; nor do they in this
long time change either in form or size. M. Fabre ascertained this, not
only by examining the burrows of the _Anthophoras_, but also by direct
observation of some young larvæ kept in captivity. In April, however,
his captives at last awoke from their long lethargy, and hurried
anxiously about their prisons. Naturally inferring that they were in
search of food, M. Fabre supposed that this would consist either of the
larvæ or pupæ of the Anthophora, or of the honey with which it stores
its cell. All three were tried without success. The first two were
neglected, and the larvæ, when placed on the latter, either hurried
away, or perished in the attempt, being evidently unable to deal with
the sticky substance. M. Fabre was in despair: "Jamais expérience," he
says, "n'a éprouvé pareille déconfiture. Larves, nymphes, cellules,
miel, je vous ai tous offert; que voulez-vous donc, bestioles maudites?"
The first ray of light came to him from our countryman, Newport, who
ascertained that a small parasite found by Léon Dufour on one of the
wild bees, and named by him Triungulinus, was, in fact, the larva of
_Meloë_. The larvæ of _Sitaris_ much resembled Dufour's Triungulinus;
and acting on this hint, M. Fabre examined many specimens of Anthophora,
and found on them at last the larvæ of his _Sitaris_. The males of
Anthophora emerge from the pupæ sooner than the females, and M. Fabre
ascertained that, as they come out of their galleries, the little
_Sitaris_ larvæ fasten upon them. Not, however, for long: instinct
teaches them that they are not yet in the straight path of development;
and, watching their opportunity, they pass from the male to the female
bee. Guided by these indications, M. Fabre examined several cells of the
Anthophora: in some, the egg of the Anthophora floated by itself on the
surface of the honey; in others, on the egg, as on a raft, sat the still
more minute larva of the _Sitaris_. The mystery was solved. At the
moment when the egg is laid the _Sitaris_ larva springs upon it. Even
while the poor mother is carefully fastening up her cell, her mortal
enemy is beginning to devour her offspring: for the egg of the
Anthophora serves not only as a raft, but as a repast. The honey which
is enough for either, would be too little for both; and the _Sitaris_,
therefore, at its first meal, relieves itself from its only rival. After
eight days the egg is consumed, and on the empty shell the _Sitaris_
undergoes its first transformation, and makes its appearance in a very
different form, as shown in Fig. 10.
The honey which was fatal before is now necessary; the activity which
before was necessary is now useless; consequently, with the change of
skin, the active, slim larva changes into a white, fleshy grub, so
organized as to float on the surface of the honey, with the mouth
beneath, and the spiracles above the surface: "grâce à l'embonpoint du
ventre," says M. Fabre, "la larve est à l'abri de l'asphyxie." In this
state it remains until the honey is consumed; then the animal
contracts, and detaches itself from its skin, within which the further
transformations take place. In the next stage, which M. Fabre calls the
pseudo-chrysalis (Fig. 11), the larva has a solid corneous envelope and
an oval shape; and in its colour, consistency, and immobility reminds
one of a Dipterous pupa. The time passed in this condition varies much.
When it has elapsed, the animal moults again, again changes its form,
and assumes that shown in Fig. 12; after this it becomes a pupa (Fig.
13) without any remarkable peculiarities. Finally, after these wonderful
changes and adventures, in the month of August the perfect _Sitaris_ (Pl.
III., Fig. 4) makes its appearance.
On the other hand, there are cases in which larvæ diverge remarkably
from the ordinary type of the group to which they belong, without, as it
seems in our present imperfect state of information, any sufficient
reason.
Thus the ordinary type of Hymenopterous larva, as we have already seen,
is a fleshy apod grub; although those of the leaf-eating and wood-boring
groups, Tenthredinidæ and Siricidæ (Fig. 14), are caterpillars, more or
less closely resembling those of Lepidoptera. There is, however, a group
of minute Hymenoptera, the larvæ of which reside within the eggs or
larvæ of other insects. It is difficult to understand why these larvæ
should differ from those of Ichneumons, which are also parasitic
Hymenoptera, and should be, as will be seen by the accompanying figures,
of such remarkable and grotesque forms. The first known of these curious
larvæ was observed by De Filippi,[13] who, having collected some of the
transparent eggs of a small Beetle (_Rhynchites betuleti_), to his great
surprise found more than half of them attacked by a parasite, which
proved to be the larva of a minute Hymenopterous insect belonging to the
Pteromalidæ. Fig. 15 shows the egg of the Beetle, with the parasitic
larva, which is represented on a larger scale in Fig. 16.
[Illustration: FIG. 14, Larva of _Sirex_ (Westwood, loc. cit.). 15, Egg of
_Rhynchites_, showing the parasitic Larva in the interior. 16, the
parasitic Larva more magnified.]
More recently this group has been studied by M. Ganin,[14] who thus
describes the development of _Platygaster_. The egg, as in allied
Hymenopterous families, for instance in _Cynips_, is elongated and
club-shaped (Fig. 17). After a while a large nucleated cell appears in
the centre (Fig. 18). This nucleated cell divides (Fig. 19) and
subdivides. The outermost cells continue the same process, thus forming
an outer investing layer. The central, on the contrary, enlarges
considerably, and develops within itself a number of daughter cells
(Figs. 20 and 21), which gradually form a mulberry-like mass, thus
giving rise to the embryo (Fig. 22).
[Illustration: FIG. 17, Egg of _Platygaster_ (after Ganin). 18, Egg of
_Platygaster_ showing the central cell. 19, Egg of _Platygaster_ after the
division of the central wall. 20, Egg of Platygaster more advanced. 21,
Egg of _Platygaster_ more advanced. 22, Egg of Platygaster showing the
rudiment of the embryo.]
Ganin met with the larvæ of _Platygaster_ in those of a small gnat,
_Cecidomyia_. Sometimes as many as fifteen parasites occurred in one gnat,
but as a rule only one of these attained maturity. The three species of
_Platygaster _differ considerably in form, as shown in Figs. 23-25. They
creep about within the larva of _Cecidomyia_ by means of the strong hooked
feet, _kf_, somewhat aided by movements of the tail. They possess a
mouth, stomach, and muscles, but the nervous, vascular, and respiratory
systems do not make their appearance until later. After some time the
larva (Fig. 23) changes its skin, assuming the form represented in Fig.
26. In this moult the last abdominal segment of the first larva is
entirely thrown off: not merely the outer skin, as in the case of the
other segments, but also the hypodermis and the muscles. This larva, as
will be seen by the figure, resembles a barrel or egg in form, and is
.870 mm. in length, the external appendages having disappeared, and the
segments being indicated only by the arrangement of the muscles. _slkf_
is the oesophagus leading into a wide stomach which occupies nearly
the whole body, _gsae_ is the rudiment of the supra-oesophageal
ganglia, _bsm_ the ventral nervous cords. The ventral nervous mass has
the form of a broad band, with straight sides; it consists of embryonal
cells, and remains in this undeveloped condition during the whole larval
state.
[Illustration: FIG. 23, Larva of _Platygaster_ (after Ganin)--_mo_, mouth;
_a_, antenna; _kf_, hooked feet; _z_, toothed process; _lfg_, lateral
process; _f_, branches of the tail. 24, Larva of another species of
_Platygaster_. The letters indicate the same parts as in the preceding
figure. 25, Larva of a third species of _Platygaster_. The letters
indicate the same parts as in the preceding figures. 26, Larva of
_Platygaster_ in the second stage--_mo_, mouth; _slkf_, oesophagus;
_gsae_, supra-oesophageal ganglion; _lm_, muscles; _bsm_, nervous
system; _ga_, _gh_, rudiments of the reproductive glands. 27, Larva of
_Platygaster_ in the third stage--_mo_, mouth; _md_, mandibles; _gsae_,
supra-oesophageal ganglion; _slk_, oesophagus; _ag_, ducts of the
salivary glands; _bnm_, ventral nervous system; _sp_, salivary glands;
_msl_, stomach; _im_, imaginal discs; _tr_, tracheæ; _fk_, fatty tissue;
_ed_, intestine; _ga_, rudiments of reproductive organs; _ew_, wider
portion of intestine; _ao_, posterior opening.]
At the next moult the larva enters its third state, which, as far as the
external form (Fig. 27) is concerned, differs from the second only in
being somewhat more elongated. The internal organs, however, are much
more complex and complete. The tracheæ have made their appearance, and
the mouth is provided with a pair of mandibles. From this point the
metamorphoses of _Platygaster_ do not appear to differ materially from
those of other parasitic Hymenoptera.
An allied genus, _Polynema_, has also very curious larvæ. The perfect
insect is aquatic in its habits, swimming by means of its wings; flying,
if we may say so, under water.[15] It lays its eggs inside those of
Dragon-flies; and the embryo, as shown in Fig. 28, has the form of a
bottle-shaped mass of undifferentiated embryonal cells, covered by a
thin cuticle, but without any trace of further organization. Protected
by the egg-shell of the Dragon-fly, and bathed in the nourishing fluid
of the Dragon-fly's egg, the young _Polynema_ imbibes nourishment through
its whole surface, and increases rapidly in size. The digestive canal
gradually makes its appearance; the cellular mass forms a new skin
beneath the original cuticle, distinctly divided into segments, and
provided with certain appendages. After a while the old cuticle is
thrown off, and the larva gradually assumes the form shown in Fig. 29.
The subsequent metamorphoses of _Polynema_ offer no special peculiarities.
[Illustration: FIG. 28, Embryo of _Polynema_ (after Ganin). 29, Larva of
_Polynema_--_asch_, rudiments of the antenna; _flsch_, rudiments of the
wings; _bsch_, rudiments of the legs; _vfg_. lateral projections;
_gsch_, rudiments of the ovipositor; _fk_, fatty tissue.]
From these facts--and, if necessary, many more of the same nature might
have been brought forward--it seems to me evident that while the form of
any given larva depends to a certain extent on the group of insects to
which it belongs, it is also greatly influenced by the external
conditions to which it is subjected; that it is a function of the life
which the larva leads and of the group to which it belongs.
The larvæ of insects are generally regarded as being nothing more than
immature states--as stages in the development of the egg into the
imago; and this might more especially appear to be the case with those
insects in which the larvæ offer a general resemblance in form and
structure (excepting of course so far as relates to the wings) to the
perfect insect. Nevertheless we see that this would be a very incomplete
view of the case. The larva and pupa undergo changes which have no
relation to the form which the insect will ultimately assume. With a
general tendency to this goal, as regards size and the development of
the wings, there are coincident other changes having reference only to
existing wants and condition. Nor is there in this, I think, anything
which need surprise us. External circumstances act on the insect in its
preparatory states, as well as in its perfect condition. Those who
believe that animals are susceptible of great, though gradual, change
through the influence of external conditions, whether acting, as Mr.
Darwin has suggested, through natural selection, or in any other manner,
will see no reason why these changes should be confined to the mature
animal. And it is evident that creatures which, like the majority of
insects, live during the successive periods of their existence in very
different circumstances, may undergo considerable changes in their
larval organization, in consequence of forces acting on them while in
that condition; not, indeed, without affecting, but certainly without
affecting to any corresponding extent, their ultimate form.
I conclude, therefore, that the form of the larva in insects, whenever
it departs from the hexapod _Campodea_ type, has been modified by the
conditions under which it lives. The external forces acting upon it are
different from those which affect the mature form; and thus changes are
produced in the young which have reference to its immediate wants,
rather than to its final form.
And, lastly, as a consequence, that metamorphoses may be divided into
two kinds, developmental and adaptional or adaptive.