SOUNDS OF INSECTS. 



93 



from those which surround it, and forms with them a bundle of 

 tubes, each terminated by a nervous thread proceeding from a 

 bulbous expansion of the same Optic Nerve. Nearly all Insects 

 are furnished with two of these compound eyes, usually placed 

 on the sides of the head ; but sometimes they are replaced by 

 simple eyes ; and in other instances these two sorts of organs 

 exist together. The structure of the simple eyes, which are known 

 also by the name of stemmata or ocelli, has the greatest analogy 

 with that of each of the elements of the compound eyes. The 

 simple eyes are generally united in a group, to the number of 

 three, towards the top of the head. We know nothing certain 

 of the manner in which these organs act upon the light that 

 falls upon them ; nor of the mechanism of vision amongst 

 Insects. 



669. Several Insects possess, in common with the higher 

 animals, the faculty of producing 

 sounds : but in general their song is 

 not connected with the movements of 

 the air in the respiratory apparatus, 

 as amongst the former ; and depends 

 upon the rubbing of certain parts of 

 the body against e#ch other, or on 

 the movements produced in certain 

 special instruments, by the contrac- 

 tion of muscles. Thus the monoto- 

 nous and deafening noise of the Cicada 

 results from the alternate tension and 

 relaxation of. an elastic membrane, 

 placed like the skin of a drum upon 

 the base of the abdomen ; amongst 

 the Crickets, there are certain parts 

 of the wings, which, when rubbed 

 against each other, vibrate with in- 

 tensity, and which have a very curious structure for this purpose ; 

 but the humming of Flies appears to depend upon the rapid exit 

 of the air through the thoracic spiracles, during the violent 

 movement of flight. Again, there are some Insects that utter a 



TIU. 411. CICADA. 



