XX INTRODUCTION. 



s. These organs are constantly found in trie 

 Jhead of insects. Their usual number is two, 

 placed laterally ; their surface is cut into so many 

 facets that Leeuwenhoek has counted seventeen 

 thousand two hundred and thirty-five in the cornea 

 ,of a butterfly ; each facet may be considered as a 

 crystalline lens, concave within and convex without : 

 they have no lids. In some orders of insects, be- 

 sides these two lateral eyes, there are, for the most 

 part, three others placed between the antennge ; 

 their surface is smooth, they are called stemmata, 

 their use is unknown, although it is supposed they 

 assist in vision, since the eyes of spiders are nearly 

 of the same form, and in them these are the sole 

 organs of that faculty. 



The Thorax. The next division of the body 

 of the insect, placed immediately behind the head, 

 is the thorax ; this supports the members, namely 

 the wings and legs. The wings are composed of 

 two membranes, an upper one, in which nervures 

 or cords are traced ; a lower one, separable from 

 the upper. These nervures or cords contain a spi- 

 ral vessel ; " whence they appear," says Kirby, " to 

 be air vessels communicating with the trachea in 

 the trunk. The expansion of the wing at the 

 will of the insect is a problem .that can only be 

 solved, by supposing that a subtle fluid is intro- 

 duced into these vessels, which seems perfectly 

 analogous to those in the wings of birds ; and that 

 thus an impulse is communicated to eveiy part of 

 fthe organ sufficient to keep it in proper tension: 



