latter want is seen even in those cases in which one sex alone of a particular insect species assumes the habit in question. An excellent illustration of such a fact, and also of the extreme modification of form which may accompany the degeneracy of highly organized animals, is found in the history of the insects collectively known as Strepsiptera, and of which the genus Stylops is the best-known example. The male stylops (Fig. 13, a) is an active insect, possessing a
Fig. 13.—Stylops. (Fig. c shows the Stylops, in outline, within the body of the Bee; and Fig. b shows the Stylops removed from the body of its host.)
single pair of wings. These wings are the hinder pair; the front pair being represented by a pair of twisted organs (w), which illustrate wing-degeneration, possibly through disuse. Both males and females as they leave the egg are small, active, six-legged beings (d, e), which crawl about on the bodies of bees. Carried into the hive, the young stylops behave like the proverbial viper, injuring the community which gives them shelter by boring their way into the bodies of larval or infant bees. Here the young stylops, casting their skin, become in the larval interior sluggish, footless grubs. Each possesses a mouth, small jaws, and a digestive system of simple construction. Meanwhile, bee-development progresses; and, as the larval bee passes through its chrysalis state with its stylops-lodger contained in its interior, the latter thrusts the front extremity of its body from between two of the hinder body-segments of the bee. Then the male stylops, undergoing development in this position, becomes the winged insect (a) and passes into the world. The female stylops (c), on the other hand, remain in their places on the bees. They undergo but a slight change of form, persisting as mere sac-like bodies (c), without legs or digestive system (b), and develop in their interior the eggs from which succeeding generations of stylops will be produced. Such a case of absolute degeneracy is all the more remarkable in view of the facts that it
