354 CHAPTERS ON EVOLUTION. 



simplification of structure, and not towards that progressive advance 

 and evolution which, in the case of the higher crustacean races, have 

 evolved from the relatively simple " nauplius " of the past the crabs, 

 lobsters, shrimps, and prawns of to-day. 



In groups of the animal series, however, both nearly allied to 

 the crustacean class, and far removed from it in structure, equally 

 interesting and often curious examples of degradation may be found. 

 The class of insects, and the nearly related group (Arachnida), includ- 

 ing the mites, spiders, and scorpions as its representatives, number in 

 their ranks instances of degraded and degenerate forms. Amongst the 

 insects which are parasitic in habits a notable absence of wings is 

 discernible, and this 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 organised 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. 253, a) is an active 

 insect, possessing a 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 (c, d\ 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-lod.ger 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 is limited to one sex alone, and that the 

 free -winged males of stylops are as highly organised as most of their 

 neighbour insects. 



The class of the spiders (Arachnida) offers collective examples of 

 degeneration and retrogression, which show how large numbers of 



