42 THE CLASS OF INSECTS. 



Hydrocampa stratiolata, they form short stiff bristles placed 

 along the side of the body. Agrion and Ephemera, in their 

 larval stages, afford the second kind of branchiae, and Libettula 

 the third kind, or internal gill, situated in the colon. The 

 Mosquito breathes both by branchiae which form large club- 

 shaped organs, and by lateral filaments. 



In those insects that fly, most of the tracheae are often dilated 

 into air-vesicles, so that by filling and emptying them of air the 

 insect can change its specific gravity. That their use is also 

 to lighten the body is shown by their presence in the heavy 

 mandibles and head of the male of Lucanus cervus. In the 

 adult Humble-bee there are two very large vesicles at the base 

 of the abdomen. These vesicles are not found in the larvae, 

 or in the adult forms of creeping insects. 



The act of respiration consists in the alternate dilation and 

 contraction of the abdominal segments, the air entering the 

 body chiefly at the thoracic spiracles. As in the Vertebrates the 

 frequency of the acts of breathing increases after exertion. 

 "When an insect is preparing itself for flight, the act of res- 

 piration resembles that of birds under similar circumstances. 

 At the moment of elevating its elytra and expanding its wings, 

 which are, indeed, acts of respiration, the anterior pairs of 

 spiracles are opened, and the air rushing into them is extended 

 over the whole body, which, by the expansion of the air-bags, is 

 enlarged in bulk, and rendered of less specific gravity ; so that 

 when the spiracles are closed at the instant the insect endeavors 

 to make the first stroke with and raise itself upon its wings, it 

 is enabled to rise in the air, and sustain a long and powerful 

 flight with but little muscular exertion. In the pupa and larva 

 state respiration is performed more equally by all the spiracles, 

 and less especially by the thoracic ones." 



During hibernation the act of breathing, like the circulation 

 of the blood, almost entirely ceases, and the heat of the body 

 is greatly lowered. Indeed Newport has shown that the devel- 

 opment of heat in Insects, just as in Vertebrates, depends on the 

 "quantity and activity of respiration, and the volume and 

 velocity of the circulation." The Humble-bee, according to 

 Newport, possesses the voluntary power of generating heat by 

 breathing faster. He says, confirming Huber's observations, 



