On the same bush caterpillars of moths and of saw-flies may often be seen feeding together.  The lepidopterous caterpillar, in our countries at least, has never more than five pairs of pro-legs, situated on the third, fourth, fifth, sixth, and tenth abdominal segments; each of these pro-legs bears a number of minute hooklets, arranged in a circular or crescentic pattern, which assist the caterpillar in clinging to its food-plant.  The saw-fly caterpillar, on the other hand, may have as many as eight pairs of pro-legs, the series beginning on the second abdominal segment; here, however, the pro-legs have no hooklets.  Among the Lepidoptera, we notice a reduction in the number of pro-legs in the ‘looper’ caterpillars of Geometrid moths.  Here only two pairs are present, those on the sixth and tenth abdominal segments.  Consequently, as the caterpillar can cling only by the thorax and by the hinder region of the abdomen, the middle region of the body is first straightened out and then bent into an arch-like form, as the insect makes its progress by alternate movements of stretching and ‘looping.’

[Illustration:  Fig. 17. c, Ruby Tiger Moth (Phragmatobia fuliginosa); a, caterpillar; b, cocoon.  After Lugger, Insect Life, vol.  II.]

Caterpillars, with their relatively soft bodies, feeding openly on the leaves of plants, are exposed to the attacks of many enemies, and the various ways in which they obtain protection are well worth studying.  A clothing of hairs[7] or spines is often present, and it is interesting to find that many species of our native Tiger and Eggar Moths (Arctiadae and Lasiocampidae) which pass the winter in the larval stage, have caterpillars with an especially dense hairy covering (fig. 17).  Experiments have shown that hairy and spiny insects are distasteful to birds and other creatures that prey readily on smooth-skinned species, a conclusion that might well have been expected.  Certain smooth caterpillars however appear to be protected by producing some nauseous secretion, which renders them unpalatable.  Many of these, as the familiar cream yellow and black larva of the Magpie Moth (Abraxas grossulariata), are very conspicuously adorned, and furnish examples of what is known as ‘warning coloration,’ on the supposition that the gaudy aspect of such insects serves as an advertisement that they are not fit to eat, and that birds and other possible devourers thus learn to leave them alone.  On the other hand, smooth caterpillars which are readily eaten by birds are usually ‘protectively’ coloured, so as to resemble their surroundings and remain hidden except to careful seekers.  Many such caterpillars are green, the upper surface, which is naturally exposed to the light, being darker than the lower which is in shadow.  When the caterpillar is large, the green area is often broken up by pale lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or oblique, as on the great caterpillars of most Hawk