INSECT PARASITISM 437 



passively by the second, or definitive, devouring the first, or inter- 

 mediate host. Both stages of the parasite exhibit extreme modification 

 of structure, the second being characterized by an enormous develop- 

 ment of the hermaphroditic gonads and of the alimentary surface, which 

 is merely the integument, and is therefore in immediate contact with the 

 food supply. To this type of parasitism we may also refer the Dicyem- 

 ids, and many of the flukes and round-worms. In many of these cases 

 the association with the host may be effected without any effort on the 

 part of the parasite, and the small size, the enormous number and the 

 method of distribution of its eggs are properly interpreted as so many 

 direct and necessary adaptations to chance. 



The Rhizocephalous crustacean Sacculina, which may serve as a 

 paradigm of the second type, or that of temporary parasitism with free 

 early ontogenetic stages, also produces an enormous number of minute 

 eggs. These, however, develop into free-swimming Nauplii, which in 

 turn become Cypris larvae and as such seek out their Decapod or Isopod 

 hosts. Owing to the activity and comparatively high organization of 

 these larvae, the element of chance in bringing about the host associa- 

 tion, though still considerable, is not as great as it is in the tape-worm. 

 When it has joined its host, the Cypris larva, through one of the most 

 remarkable methods of development known to exist among animals, 

 proceeds to undergo structural modifications so extreme that, without a 

 knowledge of the earlier stages, the crustacean affinities of the organism 

 would never be suspected. " In the adult state the body consists of two 

 portions : a soft bag-like structure, external to the host, carrying the 

 reproductive, nervous and muscular organs and attached to some part 

 of the host's abdomen by means of a chitinous ring; and a system of 

 branching roots inside the host's body, which spring from the ring of 

 attachment and supply the external body with nutriment." 6 In Sac- 

 culina, as in the tape-worm, the gonads are hermaphroditic and repro- 

 duction takes place by a continual round of self-fertilization. To this 

 type of temporary parasitism with free larval stage we may also refer 

 the myzostomes and other parasitic annelids and the parasitic mollusks. 

 In all these cases metamorphosis supervenes while the animal is still 

 very small and hence precedes growth and the incidence of the modifi- 

 cations produced by the parasitic habit. 



As an example of the third type or that of temporary parasitism 

 with free adult stage, we may select the Ichneumonid Hymenopteron. 

 The eggs are few in number and rather large and are deposited by the 

 mother directly in or on the host, which is the larva of some other 

 insect. The sluggish, bag-shaped parasitic larva, on hatching from the 

 egg, feeds for some time on the blood-tissues and fat-body of the host, 

 but is careful not to prevent the latter from moving about, procuring its 

 food and growing to maturity. When it has reached this stage, how- 

 6 Geoffrey Smith in the Cambridge Natural History, Vol. IV., p. 95. 



