INSECTS 



not been determined. In any case, the tracheal method 

 of respiration must be a very efficient one; for, consider- 

 ing the activity of insects, especially the rate at which the 

 wing muscles act during flight, the consumption of oxygen 

 must at times be pretty high. 



The activity of insects depends very much, as every one 

 knows, upon the temperature. We have all observed how 

 the house flies disappear upon the first cold snap in the fall 

 and then surprise us by showing up again when the weather 

 turns warm, just after we have taken down the screens. 

 All insects depend largely upon external warmth for the 

 heat necessary to maintain cellular activity. While their 

 movements produce heat, they have no means of con- 

 serving this heat in their bodies, as have "warm-blooded" 

 animals. That insects radiate heat, however, is very 

 evident from the high temperature that bees can maintain 

 in their hives during winter by motion of the wings. All 

 insects exhale much water vapor from their spiracles, an- 

 other evidence of the production of heat in their bodies. 



The solid matter thrown off from the cells in activity is 

 discharged into the blood. These waste materials, which 

 are mostly compounds of nitrogen in the form of salts, 

 must then be removed from the blood, for their accumula- 

 tion in the body would be injurious to the tissues. In 

 vertebrate animals, the nitrogenous wastes are eliminated 

 by the kidneys. Insects have a set of tubes, comparable 

 with the kidneys in function, which open into the intestine 

 at the junction of the latter with the stomach (Fig. 68, 

 Mai), and which are named, after their discoverer, the 

 Malpighian tubules. These tubes extend through the prin- 

 cipal spaces of the body cavity, where they are looped 

 and tangled like threads about the other organs and are 

 continually bathed in the blood. The cells of the tube 

 walls pick out the nitrogenous wastes from the blood and 

 discharge them into the intestine, whence they are passed 

 to the exterior with the undigested food refuse. 



We thus see that the inside of an insect is not an unor- 



f 116I 



