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). 
a@, upper, 0, lower shank, united at A by a capsular joint, at B by a folding joint ; 
d@, extensor or lifting muscle ; d}, flexor or lowering muscle of the lower joint. 
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 660 simple oscillations. | 6 
The sexes are always distinct in insects, the a 
only known exception being certain very low box 
aquatic Arthropods called TYardigrada, in “Q ¢ 
which both sexual glands occur in the same va 
individual. The testes of the common red- Vl ; 
Jegged locust form a single mass of tubular p i a 
glands, resting in the upper side of the third, Poh 
fourth, and fifth segments of the hind body. 
Figs. 291 and 292 represent this structure in » a 
other insects. The ovaries consist of two sets “O 1@ 
of about twenty long tubes, within which the | 
eggs may be found in various stages of de- 
velopment. The eggs pass into two main fig, 290,—Foot- 
tubes which unite to form the single oviduct "20, of Meco: 
which lies on the floor of the abdomen. Nataral size—at 
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 
