INTRODUCTION. 



19 



vessels are filled with air, but it does not disseminate itself in them 

 directly, and it is only through the walls of these tubes that the 

 contained gas is exchanged for the air held in suspension by the 

 surrounding water. The oxygen contained in the water passes 

 through certain very permeable membranes of the gill and pene- 

 trates the tracheae, which discharge, in exchange, carbonic acid, 

 which is the gaseous product of respiration. 



Fig. 14 represents the gills or breathing apparatus in an aquatic 

 insect. We take as an example the 

 Ephemera* It may be observed that 

 the gills or foliaceous laminae are placed 

 at the circumference of the body, and 

 at its smallest parts. 



We have now seen that the respi- 

 ratory apparatus is considerably de- 

 veloped in insects ; it is, therefore, easy 

 to foresee that those functions are most, 

 actively employed by them. In fact, 

 if one compares the oxygen they im- 

 bibe with the heavy organic matter 

 of which their body is composed, the 

 amount is enormous. 



Before finishing this rapid examina- 

 tion of the body of an insect, we shall 

 have to say a few words on the 

 nervous system. 



This system is chiefly composed of a 

 double series of ganglions, or collections 

 of nerves, which are united together by 

 longitudinal cords. The number of 

 these ganglions corresponds with that 

 of the segments. Sometimes they are 

 at equal distances, and extend in a chain 



from One end of the body to the Other ; Fig. 14. Branchiae or gills of an aquatic 



at others they are many of them close 

 together, so as to form a single mass. 



The cephalic ganglions are two in number ; they have been 



* May-fly family. ED. 



c2 



larva (Ephemera). 

 A, foliaceous laminae or gills. 



