of levers moved by muscles; but in the vertebrates the lever is within and the motor muscles are without, while in insects the muscles are inside and the lever outside. We have already pointed 
Fig. 17.—Digestive Apparatus of a Sucking Insect (Ascalaphus meridionalis). a a, antennæ; b, salivary glands; c, œsophagus; d, stomach; e, gizzard; f, chylific ventricle; gg, tubes of Malpighi; h, intestine; i, rectum; j, last segment of the abdomen. (After Léon Dufour.) out the adaptation of the legs to the habits and the abode of the animals. The hairs with which the tarsi are furnished have a part in permitting flies to move on polished objects and to assist the rapid course of hydrometers, or skippers, on the surface of water. The wings sometimes come into play in propulsion on water; they are then furnished with long cilia, forming broad oars. The mechanism of flight has been the object of important studies by Pettigrew and Marey. A wasp, the ends of whose wings have been gilded, presents the appearance as shown in Fig. 19. The dispositions of the elytra of the Coleoptera during flight are extremely various. Poujade has given some interesting drawings of them in the annals of the Entomological Society of France. In them the cockchafer, the Onthopagus, is shown simply raising its elytra, the Necrophorus lifting them in a plane perpendicular to that of the body, and another genus holding them closed as in a position of rest. Sometimes, also, in moving through the air, the middle legs are raised above the body.
Fig. 18.—Disposition of the Stigmata of a Silk-worm. S, stigmata of the first ring; S', stigmata of the fourth ring; S" to SVIII, stigmata situated on the fifth, sixth, and eleventh rings.
A considerable number of insects secrete products, some of which are useful, while others are injurious. First in order
