194 PHYSIOLOGY OF INSECTS. 



requires supplies of oxygen, which is a principal ingredient 

 of atmospheric air : an animal, therefore, in breathing the 

 air, divides the oxygen from the other constituent parts, 

 appropriates the former, and rejects the latter. 



General Summary. — In the changes which insects 

 undergo, not only is the external appearance altered, but 

 the organs of support, motion, sensation, digestion, and 

 generation are also altered, and frequently those of respi- 

 ration. The organs of support in insects are mostly ex- 

 ternal ; they are not bones, but perform the functions of 

 bones ; no experiments have yet proved that they possess 

 the least sensitiveness to touch, except in a few parts in 

 which the nerves obviously ramify to the surface. The 

 organs of motion in insects are enclosed within, and at- 

 tached to, the organs of support : they are evident muscles, 

 partly fleshy, partly tendinous, and differ in no material 

 character from the muscles of vertebrated animals. 



The organs of sensation in insects are distinctly and 

 decidedly nerves, and in most respects resemble the nerves 

 of vertebrated animals ; they do not, however, originate in 

 a common or concentrated brain, but in numerous incras- 

 sated bundles of nerves, which are now termed cerebroids. 

 The usual animal senses are possessed by insects ; most of 

 them obviously. Of hearing, we find slender proof, and its 

 seat is altogether unknown ; we also rather presume than 

 know that insects possess smell ; its seat is also unknown. 



The organs of digestion in insects are peculiarly simple ; 

 the alimentary canal is very short, and the intestines gene- 

 rally without convolutions ; the food passes very rapidly, 

 and undergoes but little alteration. 



The organs of circulation are these : a longitudinal series 

 of little hearts, now termed corciila, which receive blood by 

 lateral apertures from the cavities of the body : the blood 

 is transparent and colourless, it contains numerous con- 



