824 ZOOLOGY. 
dominal ring contracted. The respiratory movements, as 
Plateau states, consist of the alternate contraction and re- 
covery of the figure of the abdomen in two dimensions, i.¢., 
vertical and transverse. During expiration the abdomen 
contracts, while during inspiration it returns to its normal 
shape. (Miall and Denny’s “‘ The Cockroach.”’) 
It is evident that the enormous 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 
Fig. 293, destination. It is evident that the process of 
Longitudinal . respiration can be best carried on in clear, sunny 
section of the 
trachea of Hy- weather, and that when the sun sets, or the 
or tines bectle, weather is cloudy and damp, its powers of flight 
ep, epithelium; : ‘ a aac 
eu, cuticula; ¢, are lessened, owing to the diminished power of 
Petter Moot vespiration. The finer structure of the trachea 
is seen in Fig. 283. 
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 (ocel/i), situated between the former, sup- 
plied with nerves of special sense. 
Fig. 284 represents the eye of a moth greatly enlarged to 
show the finer structure. 
