4o8 THE BIOLOGY OF INSECTS 



the dog ; in the body cavity of this insect as well as in that 

 of the dog-flea {Ctenocephaliis serraticeps) lives a larval 

 parasitic womi (Cestode), and if the dog, irritated by the 

 presence of lice or fleas bites at its own skin and swallows 

 the insects, the cestode larva (cysticercoid or modified 

 *' bladderworm ") develops in the dog's digestive tract into 

 the mature tapeworm Dipylidium cajiinum, from whose 

 eggs are hatched the tiny six-hooked larvae which have 

 opportunity to invade the bodies of the insects and there 

 undergo the early stages of their life-history. 



Students of the Trematoda or flukes, like the Cestoda, 

 a class of flat- worms, find that an insect may be one in a 

 chain of the hosts which successively harbour a fluke at 

 some stage of its life-cycle. Thus a little tailed cercaria- 

 larva which has developed in the body of a water-snail 

 (Limnaea), may penetrate into the water-beetle Ilyhiiis 

 fuliginosus and encyst ; if the latter is eaten by a frog the 

 cercariae thus swallowed will develop further into mature 

 flukes {Distomum cylindraceum). Another trematode cercaria 

 may migrate into the larva of a mayfly, a stonefly or a chiro- 

 nomid midge, and remain encysted until the insect-host has 

 completed its transformation and acquired wings. Then in 

 its aerial flight this insect may fall victim to a bat, within 

 whose digestive tract the cercaria grows into the mature 

 fluke Distomum ascidia. 



The slender threadworms of the Gordius group make 

 use of insects during the parasitic phases of their life- 

 histories. The tiny larvae of Gordius tolosanuSy as described 

 by Von Linstow (1891) hatched from eggs laid in water, are 

 provided with spines on the broadened head region, by 

 means of which they bore into the larva of the alder-fly 

 {Sialis lutaria), of a mayfly or of some other insect that is 

 aquatic in its early stages, and come to rest in the muscles 

 or the fat-body of the host. Here the Gordius larva shelters 

 and feeds through the winter, and by the ensuing spring the 

 insect-larva may have completed its transformation. If the 

 mature fly containing the young Gordius is eaten by a 

 ground-beetle (Pterostichus) or other predaceous insect, 



