THE SENSES OF INSECTS. 21 



do the tympanum and the membranous portions at the 

 base of the hind legs. When the tergum or dorsal portion 

 of the abdomen falls and the pleurum contracts, the spir- 

 acles open; their opening is nearly but not always exactly 

 co-ordinated with the contractions of the pleurum, but 

 as a rule it is. There were sixty-five contractions in a 

 minute in a locust which had been held between the fingers 

 about ten minutes. It was noticed that when the abdomen 

 expanded, the air-sacs in the first abdominal ring contracted. 

 The air passes into the spiracles during the expansion of the 

 abdomen. In most insects, says Plateau, only the expira- 

 tory movement is active; inspiration is passive, and effected 

 by the elasticity of the body-walls. * 



It is evident that the enormous powers of flight possessed 

 by the locust, especially its faculty of sailing for many hours 

 in the air, are due to the presence of these air-sacs, which 

 float it up in the atmospheric sea, increasing the bulk of 

 the body, without rendering it heavier. 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, constantly filling and refilling its internal buoys or 

 balloons with little muscular exertion, and thus be borne 

 along by favorable winds to its destination. It is evident 

 that the process of respiration can be best carried on in 

 clear, sunny weather, and that when the sun sets, or the 

 weather is cloudy and damp, its powers of flight are less- 

 ened, owing to the diminished power of respiration. 



The Senses of Insects. It is difficult to explain many of 

 the actions of insects, from the fact that it is hard for us to 

 appreciate their instincts and general intelligence. That 

 they have sufficient intellectual powers to enable them to 

 maintain their existence may be regarded as an axiom. But 

 insects differ much in intelligence, and also in the degree of 

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



* See Miall and Denny's "The Cockroach," p. 163. 



