FLIGHT OF INSECTS. 



329 



and anal veins. So that the wings of insects act as lungs 

 as well as organs of flight. For the latter purpose, the 

 principal veins are situated near the front edge of the wing, 



< 



Fig. 289. Diagram of the knee-joint of a vertebrate (A) and an insect's limb (B). 

 upper, b, lower shank, united at A by a capsular joint, at B by a folding joint ; 

 d, extensor or lifting muscle ; d 1 , flexor or lowering muscle of the lower jomt. 

 The dotted line indicates in A the contour of the leg. After Graber. 



called the costa, and thus the wing is strengthened when the 

 most strain comes during the beating of the air in flight. 



The wing of an insect in making the strokes during flight 

 describes a figure 8 in the air. A fly's wing 

 makes 330 revolutions in a second, executing 

 therefore GGO simple oscillations. 



The sexes are always distinct in insects, the 

 only known exception being certain very low 

 aquatic Arthropods called Tardigrada, in 

 which both sexual glands occur in the same 

 individual. The testes of the common red- 

 legged locust form a single mass of tubular 

 glands, resting in the upper side of the third, 

 fourth, and fifth segments of the hind body. 

 Figs. 291 and 292 represent this structure in 

 other insects. The ovaries consist of two sets 

 of about twenty long tubes, within which the 

 eggs may be found in various stages of de- 

 velopment. The eggs pass into two main 

 tubes which unite to form the single oviduct 

 which lies on the floor of the abdomen. 

 Above the opening of the oviduct is the sebific 

 gland and its duct. This gland secretes a copious supply of 

 a sticky fluid, which is, as in many other insects, poured 



290. Foot- 



