STRUCTURE OF INSECTS. 319 



which arise from the presence of this latter sub- 

 stance, are often very brilliant, and, as is the 

 case with many other classes of animals, the in- 

 tensity of the tints is heightened by the action of 

 light. The elytra of tropical insects display a 

 gorgeous metallic lustre depending on the re- 

 flexion of the prismatic colours ; and the same 

 variegated hues adorn the scales of butterflies 

 of those regions. 



Hair grows in various parts of the surface of 

 insects. Where the integument is membranous 

 and transparent, these hairs may be distinctly 

 perceived to originate from enlarged roots, or 

 bulbs, and to pass out through apertures in the 

 skin ; as is the case with the hair of the larger 

 animals. Their chemical composition, however, 

 is very different, for they are formed of the same 

 substance as the integuments, namely entomo- 

 line. The purposes served by the hairs are not 

 always obvious. In many cases they seem in- 

 tended to protect the integuments from the water, 

 which they repel from their surfaces. They also 

 tend to prevent injury arising from friction ; and 

 are found to be more abundant in those parts, as 

 the joints, which are liable to rub much against 

 one another. 



The divisions of the body are frequently 

 marked by deep incisions ; whence has origin- 

 ated the term insect, expressive of this separation 

 into sections. It is, however, a character which 



