146 LIFK : OUTLINES Ol- GENERAL BIOLOGY 



which illustrate temporary parasitism. A curiously isolated case of 

 alleged parasitism Is a little beetle found exclusively on beavers. In 

 many of the Ichneumon-flies and related types, which lay eggs in 

 caterpillars and the like, the predatory larva: are themselves paraisit- 

 ised. This "hyper-parasitism" is sometimes carried far. Thus acertain 

 caterpillar [Hcnurocampa Icucostigma) which defoliates many trees 

 in the north-eastern United States has 23 primary parasites (17 

 Hymenoptera and 6 Diptera); these have 13 secondary parasites 

 (all Hymenoptera); and these again 2 (perhaps 5) tertiary parasites; 

 indeed, one of these (As'jcodes) may be in some cases not tertiary, 

 but quaternary! 



Among Arachnids various degrees of parasitism are illustrated 

 by mites and ticks, some externally adlierent, others burrowing in 

 the skin, a few, like the bee-mite, penetrating deeply. Sometimes 

 placed in the vicinity of Arachnids are the vermiform Pentastomids, 

 (e.g. Linguatula), found in the nasal cavities and frontal sinuses 

 of carnivores, crocodiles, snakes and some other flesh-eating animals. 

 Apart from a few Gasteropods, such as Entoconcha mirabilis attached 

 to blood-vessels in Synapta, there are no parasitic molluscs. Nor 

 are there any parasitic Vertebrates except the pigmy males of some 

 Angler-fishes; and the predatory Myxinoids, like the Hag-fish (much 

 lower than a true fish), are sometimes reckoned as temporary endo- 

 parasites of certain fishes. 



A survey of parasitic animals shows the widespread distribution 

 of this mode of life. But it is of rare occurrence {a) among types that 

 are sensitive to lack of ample aeration; thus there are no parasitic 

 Echinoderms; (6) among types that breathe dry air; thus, endo- 

 parasitism is relatively uncommon among insects; and (c) among 

 types whose shape of body is markedly unsuitable, such as the long- 

 legged spiders. 



PARASITIC PLANTS— More briefly let us discuss parasitism in 

 the Vegetable Kingdom. Plants may be parasitic in or on animals, 

 a striking case being the rod-like fructification of a fungus (Cordy- 

 ceps) that grows out to a length of several inches from the head 

 end of a parasitised insect -larva, such as a caterpillar or a grub. 

 The common house-fly is often seen dying from the ravages of a 

 fungus, Kmpusa muscce, the spores forming a white powder around 

 the moribund insect; another species is from man's point of view 

 very useful as a check on the multiplication of green-flies. It is but 

 rarely, however, that wild animals in natural conditions fall victim 

 to fungoid or even bacterial attacks. 



An estimate of the number of parasitic animals that infest plants 

 varies according to the strictness of the definition of parasitism. 

 The gall-mites (Phytoptids), which cause swellings on many plants, 

 have only two pairs of legs, very simple mouth-parts, and a worm- 



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