INSECTS AND OTHER ORGANISMS 399 



but prey on worms, molluscs, and insects that live in the 

 soil or in water. The female flies lay their eggs in clusters 

 on the leaves of plants or in other situations whence the 

 grubs can easily reach the ponds or damp earth in which 

 they live as hunters of their weaker companions in such 

 situations. 



While the Tabanidae and other groups of Diptera feed 

 in their adult state as they have opportunity on the blood 

 of mammals, there are many muscoid flies which, not 

 themselves directly affecting the large beasts, have maggots 

 which live as parasites of these. Reference has been made 

 in a previous chapter (pp. 112-113) to the heavy mortality 

 caused among sheep by the maggots of green-bottle flies 

 (LuciHa) and allied insects, which lay their eggs on the wool, 

 the larvae when hatched eating their way through the skin 

 and into the flesh of the animals. This repulsive type of 

 parasitism appears to have arisen from the carrion-feeding 

 habit characteristic of blue-bottles and allied flies of the 

 same family ; an occasional laying of eggs on living instead 

 of dead beasts having become largely habitual in Lucilia and 

 the other European and Australian insects whose maggots 

 feed in this way. Such parasitism is necessarily harmful 

 and often fatal to the host- animal and must seem to many 

 observers an example of disharmony in nature. 



On the other hand, there are some well-known muscoid 

 flies whose larvae are adapted in a wonderfully specialised 

 manner to a parasitic life in the bodies of mammalian hosts. 

 These, know^n collectively as the " bot-flies," have generally 

 been regarded as forming a special family, the Oestridae. 

 Several recent systematic students of the Diptera believe, 

 however, that among them are representatives of two 

 distinct muscoid groups, which having adopted similar 

 modes of life have come by convergent evolution to similarity 

 of form both in the larval and adult stages of the life-history. 

 In a previous chapter (pp. 109-110) some account was given 

 of migration through the bodies of cattle of the larv^ae of 

 some members of the Oestridae, the warble-flies (Hypoderma), 

 as affording examples of behaviour conditioned to some 



