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.
CHAPTER III.
ON THE NATURE OF METAMORPHOSES.
In the preceding chapters we have considered the life history of insects after they have quitted the egg; but it is obvious that to treat the subject in a satisfactory manner we must take the development as a whole, from the commencement of the changes in the egg, up to the maturity of the animal, and not suffer ourselves to be confused by the fact that insects leave the egg in very different stages of embryonal development. For though all young insects when they quit the egg are termed “larvæ,” whatever their form may be (the case of the so-called Pupipara not constituting a true exception), still it must be remembered that some of these larvæ are much more advanced than others. It is evident that the larva of a fly, as regards its stage of development, corresponds in reality neither with that of a moth nor with that of a grasshopper. The maggots of flies, in which the appendages of the head are rudimentary, belong to a lower grade than the grubs of bees, &c., which have antennæ, mandibles, maxillæ, labrum, labium, and, in fact, all the mouth parts of a perfect insect.