CATERPILLARS.
Finally, to take the case of caterpillars, Weismann has admirably worked out the life history of many forms, and shows how the complex markings have arisen by development. Broadly, a caterpillar consists of 13 segments, the head being one. The head is often marked with darker colour, and the last segment with its clasping feet is also very frequently emphasized, as in [Figs. 1 & 3, Plate VII]. The spiracles are generally marked by a series of spots, and often connected by a line. Here the tendency to repetition shows itself strongly, for not only the spiracles themselves, but the corresponding points in the segments without spiracles are frequently spotted, and, moreover, these spots are frequently repeated in rows above the spiracular line. Of this, Deilephila galii and D. Euphorbiæ, [Figs. 1-5, Plate VII.], are good examples.
The segmentation is also generally emphasized, as shown in all the examples on the plate, but in its simplicity in [Fig. 10].
Running down the centre of the back a more or less distinct line is often seen, as shown in the figures. This corresponds with the great dorsal alimentary canal lying just below the skin, and Weismann has shown that in young larvæ this line is transparent, and the green food can be seen through the skin. We have here, perhaps, a relic of the direct colouration noticed in the transparent cœlenterata.
Where larvæ possess horns either upon the head, as in Apatura iris and Papilio machaon, or on the tail, as in many of the sphyngidæ, like [Figs. 1-5, Plate VII.], these appendages are always emphasized in colour. As they are frequently oblique, we often find that this obliquity is continued as a slanting spot, as in D. galii and euphorbiæ, and sometimes repeated as a series of oblique stripes, as in [Fig. 4].
It must be admitted that in insects we have strong evidence of structural decoration.
