CLASS INSECTS. 377 



first sight appears to be a single eye, is in reality formed by 

 the agglomeration of a number of small eyes, having each a 

 cornea, vitreous humour of a conical form, a layer of colour- 

 ing matter, and a nervous filament. In the may-bug, for 

 example, we find nine thousand such in a single compound 

 eye, and there are insects which have more than twenty-five 

 thousand. All these small cornese are hexagonal, and unite 

 together so as to form a kind of common cornea, whose sur- 

 face presents a vast number of facettes. Moreover, each of 

 these small constituent parts of these multiple or compound 

 organs is quite distinct from those around it, and forms with 

 them a bundle of tubes, each terminating in a nervous 

 filament, proceeding from the terminal enlargement of the 

 same optic nerve. Almost all insects have a pair of these 

 compound eyes ; but they are sometimes replaced by simple 

 eyes, and at other times both sorts are present. The simple 

 eyes, also called stemmata, have the greatest analogy with 

 the structure of each of the elements of the compound eyes. 

 In general, these simple eyes form a group of three on the 

 summit of the head. Nothing precise is known respecting 

 the manner in which these eyes act on the light, nor of the 

 mechanism of vision in insects. 



519. Several insects have the power of producing 

 sounds, which they do by friction of certain parts of their 

 body on each other, or they may depend on the movements 

 impressed on the special instruments for the contraction of 

 the muscles. The monotonous and deafening sounds of the 

 balm cricket or chirping grasshopper, result from alternate 

 tension and relaxation of an elastic membrane, disposed like 

 the skin of a drum, over the base of the abdomen ; in crickets, 

 the sounds are caused by certain parts of the wings, which, 

 rubbing against each other, vibrate intensely, and the struc- 

 ture producing this is very singular ; but the buzzing of flies 

 appears to depend on the rapid escape of the air through the tho- 

 racic stigmata during the violent movements of flight. Finally, 

 there are other insects which produce sounds by a mode as yet 

 unknown, such as the nocturnal butterfly or death's-head sphinx. 



520. The nervous system of insects has been already 

 in a great measure described ( 509). It is composed prin- 

 cipally of a double series of ganglions, reunited by longitu- 

 dinal cords (Fig. 354); the number of the ganglions correspond 

 to the number of rings; sometimes equidistant, and extending 

 from one end of the body to the other, at other times they ap- 



