MODE OF GROWTH OF THE HAIRS 



193 



importance, it is evidently of great biological importance, the welfare or even 

 the life of the insect depending upon it ; and it varies in each species of insect, 

 especially in Diptera, where the position of even a single seta characterizes the 

 species. 



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 



A 



FIG. 218. Section through FIG. 219. Flattened hairs from the lateral tufts of larva 



an antennal pectination of Sa- of Gastropacha americana : A, three from the lateral tuft of 

 turnia carpini : , hypo- Heteropacha rileyana. 

 derinis, formative cells of the 

 hairs (o) ; d, cuticula ; e, 

 trachea. After Semper. 



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 setse arising over nerve cells or nerve termi- 

 nations and will be discussed under the organs of sense. 



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 Syn- 

 aptera (Thysanura and Colembola), as well as Lepidoptera and 

 Trichoptera, they also occur in the Psocidee (Amphientomum), in 



FIG. 220. The same in G. quercifolia : a, a small 

 hair ending in two minute processes. 



