344 ZOOLOGY. 



dominal ring contracted. This would indicate that the 

 air rushes into the spiracles during the contraction of the 

 abdomen, and that the air-sacs are not refilled until the 

 spiracles are closed ; thus the air in the air-sacs is perhaps 

 constantly changing. 



It is evident that the enprmous powers of 

 flight possessed by the locust, especially its fac- 

 ulty of sailing for many hours in the air, is due 

 to the presence of these air-sacs, which float it 

 up in the atmospheric sea. Other insects with 

 a powerful flight, as the bees and flies, have well- 

 developed air-sacs, but they are less numerous. 

 It will be seen that, once having taken flight, 

 the locust can buoy itself up in the air, con- 

 stantly filling and refilling its internal buoys or 

 balloons without any muscular exertion, and 

 thus be borne along by favorable winds to its 

 destination. It is evident that the process of 



. * 



res pi ra ^ion can be best carried on in clear, sunny 

 trachea of Hy- weather, and that when the sun sets, or the 



drophiluspiceus ,V . , i T -, .", n- i i 



or water-beetle, weather is cloudy and damp, its powers 01 night 



en, epithelium; , -, . , , n . . . , -, 



cu : cuticuia ;/, are lessened, owing to the diminished power of 

 -Afte/Mfnot 8 ' respiration. The finer structure of the trachea 

 is seen in Fig. 288. 



It is difficult to explain many of the actions of insects, 

 from the fact that it is hard for us to appreciate their men- 

 tal powers, instincts, and general intelligence. That they 

 have sufficient intellectual powers to enable them to main- 

 tain their existence may be regarded as an axiom. But in- 

 sects differ much in intelligence and also in the degree of 

 perfection of the organs of sense. The intelligence of in- 

 sects depends, of course, largely on the development of the 

 organs of special sense. 



The sense of sight must be well developed in the locust, 

 there being two large, well-developed compound eyes, and 

 three simple ones (ocelli), situated between the former, sup- 

 plied with nerves of special sense. 



Fig. 289 represents the eye of a moth greatly enlarged to 

 show the finer structure. 



