ECOLOGICAL 145 



case of males so altering its host's constitution that its testes degener- 

 ate, and are replaced by a small ovarian structure, which may 

 sometimes even produce ova, though whether capable of fertilisa- 

 tion or not, we cannot yet say. In short, the Sacculina effects 

 "parasitic castration". There are various related forms, one of which 

 (Sesarmaxenos) occurs on a freshwater crab in the Andamans. As 

 Darwin first showed in his classic Monograph, some of the true 

 barnacles, normally hermaphrodite, have small parasitic, "comple- 

 mentary" males: others are unisexual, yet the males are also 

 minute and parasitic on the females. Even among the higher crus- 

 taceans, parasitic forms occasionally occur, notable in certain Isopods 

 which infest other members of the class. The young forms are free- 

 living and male, the adults are parasitic and female; but it seems 

 that while all females pass through a male stage, without genital 

 ducts, those males that become functional never grow up into 

 females. Many of these Epicarids cause parasitic castration of their 

 hosts. Some of them afford good instances of hyper-parasitism, i.e., 

 of parasite upon parasite. Thus a very common Mediterranean 

 species {Danalia curvata) is parasitic on a Sacculina (5. neglecta), 

 which in turn is parasitic on a spider-crab (Inachus). Somewhat 

 similar Isopods infest the gill-chambers, mouth-cavity, and skin of 

 various fishes. In many of the species, if not in all, the young 

 free-swimming forms function as males, but afterwards settle down 

 and become females — an interesting convergence of physiological 

 changes in two very distinct families. 



Parasitism is not to be expected among the winged insects, but 

 many wingless types, such as lice, are true ectoparasites; and 

 some free-living winged insects, like warble-flies ((Estrus, etc.) 

 have parasitic larvae. The blood-sucking lice (Pediculidse) constitute 

 an order, which Latreille called Parasita, ectoparasitic on mammals. 

 They are marked by absence of wings, small head, with simple eyes 

 or none, and large abdomen, and by the adaptation of the claws to 

 clutch the hairs; yet it cannot be said that lice are particularly 

 degenerate, since still not very far removed from the predatory. 

 This is even more marked in the case of the bitmg lice or bird-lice 

 (Mallophaga), which occur on birds and a few mammals, feeding not 

 on blood, but on skin cells and fragments of feathers and hairs. 

 Some birds shelter numerous Mallophaga, thus the hen has nine; 

 but occasionally a particular species of bird-louse is restricted to a 

 particular species of bird. Related species are often found on related 

 hosts. This specificity is often to be noted in parasites, j^et there 

 are others, such as the liver-fluke, that have many different hosts. But 

 as to the Mallophaga, there does not seem to be much of the parasite 

 about them; they are skin-scavengers, and the same may be said of 

 fleas and of the sheep-tick {Melophagus owwws), a wingless Dipteron. 

 It is different, however, with the larvae of bot-flies and warble-flies, 



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