When I say that we have here a case of two parts adapted to each other, that is, strictly speaking, understating the case, for, in the crickets and locusts, for instance, there is a whole series of peg-like chitinous papillæ ([Fig. 86]), the so-called 'bridge,' each of which must have arisen by itself through variation of the corresponding spot of skin. At least I can see no ground for the assumption that the chitinous surfaces on which the 'bridge' is now placed would necessarily, from internal reasons, have varied precisely in the line of the bridge as it has done.
Fig. 102. Brush and comb on the
leg of a Bee (Nomada). tib, end of
the tibia. t1, first tarsal joint with the
brush and its comb (tak). Between
these and the tibial spine (tisp) with
its lappet (L) the cross-section of an
antenna (At) is indicated. Drawn from
a preparation by Dr. Petrunkewitsch.
Instructive examples of the co-adaptation of several parts to a common action in organs which are not subject to the Lamarckian principle are afforded by the diverse arrangements for cleaning the antennæ the bearers of the smelling-organ which are so important to the life of insects (Fig. 102). Here even the adaptation of an indented area on the tibia of the anterior leg to the cylindrical form of the antenna which passes through it, is sometimes so striking (Fig. 102, tak) that it might be thought that it must have arisen through a gradual wearing out; yet this is impossible, since we have to do with hard dead chitinous surfaces, and moreover not with a solid mass, like a hone, which is worn down by the knife, but with a hollow, thin-walled tube. In ants, bees, and ichneumon-flies this minute, semi-circular indentation contains small, pointed, triangular saw-teeth, closely set like those of a comb (tak), and the apparatus is made usable by the fact that a firm spine (tisp), fused to the end of the tibia, overhangs the notch and presses the antenna towards it. In many species this spine is double, or it is furnished with a thin comb or lappet (Fig. 102, L), or with rows of teeth, or with short bristles; in short, it may be equipped in the most different ways. Not infrequently, as in wasps of the genera Sphex, Scolia, Ammophila, the spine itself is also bent in a semicircle on the surface directed towards the notch, and this may be effected in very different ways, either by a bending of the whole thickness of the spine, or by the presence of a comb which is concave on its inner surface. I should never come to an end if I were to enumerate all the remarkable details which may be found in the two main parts of this apparatus, and which show very clearly how essential a co-operation of the two is in fulfilling the function of cleaning the antennæ. This fitting together of the two main parts cannot have been brought about in accordance with the Lamarckian principle; the adaptation must therefore have come about in some other way.
The same thing is shown by the legs and other appendages of insects and crustaceans, which are adapted for the most diverse functions, and the individual sections of which must be correlated if the function is to be possible. Let us consider only the claw structures in crustaceans and scorpions. Here, too, it seems as if the outgrowth of the last joint of the leg, which functions as the arm of the claw, must have arisen as a direct effect of use, through the pressure of an object held fast by the last joint, the movable half of the claw. Frequently, moreover, tooth-like protuberances occur on the fixed blade of the claw (Fig. 103). But how could these have arisen as a direct effect of pressure, since they are always preformed during the soft state of the appendage before use, and are only made use of after it is fully hardened. The soft crustaceans, the so-called 'butter-crabs' which have just cast their shells, creep carefully away and avoid using their limbs until they have become hard again. Here, too, we have the co-adaptation of two parts which vary independently, and which cannot be affected by the Lamarckian principle.
Fig. 103. Claw (Sch) on the leg of a 'Beach-fly,' an
Amphipod Crustacean (Orchestia). I, II, the two first
joints. uA, the lower blade of the claw, a non-mobile
prolongation of the penultimate joint. oA, the upper
blade of the claw, the movable last joint; the tubercles
and indentations of the two blades fit one another. After
F. Müller.
But the appendages furnish more complex examples of mutual adaptation. Thus the individual sections of the anterior leg of the mole-cricket (Gryllotalpa) have varied greatly, yet quite differently, and the whole together forms a most effective digging-tool. With it the animal digs out the earth before it to right and to left, and to do this it makes with both legs simultaneous outward movements, which are otherwise quite unusual among insects, and does so with such strength that Rösel von Rosenhof saw two bodies each weighing three pounds pushed away in this manner. In this case four chief parts of the leg (Fig. 104), the coxa (cox), the femur (fe), the tibia (tib), and the tarsi (tars) are so adapted to each other in form, joints, thickness of skeleton, and size, that they cannot have varied otherwise than in relation to each other, but each piece has done so in an individual manner. Most remarkable of all is the short broad tibia, equipped with four large, hard teeth, which has to perform the digging in the ground after the manner of a spade, while the disproportionately thin and weak tarsal joints, the last of which bears two perfectly straight spines instead of claws, are directed upwards, and do not touch the ground, being no longer used for walking. Rösel supposed, probably correctly, that they are used for cleaning the spade when it becomes clogged up with earth, since the animal cannot clean it with its mouth. These quite unusually formed parts of the limb cannot have become what they are as the direct results of use, because, for one thing, it would have been not their broad surfaces, but their narrow edges, which would most easily cut through the earth, that would have been directed outwards. The peculiar curving, first concave, then convex, of the outer surface of the digging foot is exactly what is best adapted for cutting into the earth and for the pushing aside which follows, but it is not what it would have become if the chitin-wall had yielded to the pressure of the earth and adapted itself to it. But, as we are again dealing with the chitinous skeleton, there can be no question of the direct effect of use, and, it seems to me, it must be admitted that here we have a case of co-adaptation of at least seven different parts, which have varied independently of each other, without any assistance from the Lamarckian principle.
Fig. 104. Digging leg
of the Mole-cricket
(Gryllotalpa). cox, coxa
attaching the limb to
the thorax. fe, the short
broad femur. tib, the
tibia forming a broad
spade with six large
sharp teeth. tars, the
tarsal joints, which are
turned upwards and
cannot be used in locomotion.
After Rösel.