Fig. 218.—Section through an antennal pectination of Saturnia carpini: a, hypodermis, formative cells of the hairs (c); d, cuticula; e, trachea.—After Semper.
Fig. 219.—Flattened hairs from the lateral tufts of larva of Gastropacha americana: A, three from the lateral tuft of Heteropacha rileyana.
The mode of development of the hairs was first described by Semper. In the pectination of the antenna of Saturnia carpini he observed that the hairs arise, like the scales of the wings, from large round formative-cells lying in the cavity, which send out through the hypodermis and cuticle a long slender process which finally becomes the hair (Fig. 218).
Tactile hairs are those setæ arising over nerve cells or nerve terminations and will be discussed under the organs of sense.
Fig. 220.—The same in G. quercifolia: a, a small hair ending in two minute processes.
Scales.—In very rare cases the hairs of caterpillars (Fig. 219) are flattened and scale-like, and this passage in the same insect of cylindrical hairs into flattened scale-like ones, shows that the scales are only modified hairs. Also, as we shall see farther on, Semper has proved that their mode of origin is identical. While true scales are characteristic of Synaptera (Thysanura and Colembola), as well as Lepidoptera and Trichoptera, they also occur in the Psocidæ (Amphientomum), in many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ, Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in the Culicidæ, and a few other Diptera, though they are especially characteristic of the Lepidoptera, not a species of this great order being known to be entirely destitute of them.
