SOCIAL AND COMMUNAL LIFE 4 '9 



Various parasites have been referred to in Part II under 

 their proper branch and class. The worms include an 

 unusually large number of them, such as the tape- 

 worms, trichinae and other intestinal forms, all of which 

 live as internal parasites in the alimentary canal or in 

 other organs of higher animals, especially the vertebrates. 

 Many crustaceans are parasitic, usually living, like the 

 fish-lice, as fixed external parasites on fishes, other crus- 

 taceans, etc., but with a free and active larval stage. 

 Among the insects, on the contrary, many of the parasitic 

 forms (as the ichneumon flies) are free and active in the 

 adult stage, but live as internal grubs or maggots in the 

 larval stage. The ichneumon flies (of the order Hymen- 

 optera) are four-winged, slender-bodied insects which 

 lay their eggs either on or in (by means of a sharp pierc- 

 ing ovipositor) some caterpillar or beetle grub, into the 

 body of which the young grub-like ichneumon larvae 

 burrow on hatching. The parasites feed on the body- 

 tissues of the host, not attacking, however, such organs 

 as the heart or nervous system, which would produce the 

 immediate death of the host. The caterpillar lives with 

 the ichneumon grubs within it usually until nearly time 

 for its pupation. Often, indeed, it pupates with the para- 

 site still in its body. But it never comes to maturity. 

 The larval ichneumons pupate either within the body of 

 its host, or in a tiny silken cocoons outside of its body 

 (fig. 1 60). From the cocoons the winged adult ichneu- 

 mons issue; and after mating the females find another 

 caterpillar on whose body to lay their eggs. 



Degeneration can be produced by other causes than 

 parasitism. It is evident that if for any other reason an 

 animal should adopt an inactive fixed life it would degen- 

 erate. The barnacles (see fig. 37) are excellent examples 

 of degeneration through quiescence. They are crustaceans 

 related most nearly to the crabs and shrimps. The 



