FLIES 213 



subtle way, very characteristic and was well described in the Theatrum 

 Insectorum : " They never fly forward but sidelong, as it were, 

 hopping and skipping as they go. " For reasons which defy analysis, 

 louse-flies are particularly repellent insects, and most people experience 

 a shudder of disgust at the sight of them, and are filled with a quite un- 

 reasonable feeling of horror if they happen to dart up their sleeves or 

 into their hair while handling the host. A bite from a louse-fly, which 

 is neither dangerous nor painful, is an occupational risk and keepers on 

 grouse moors and members of the Edward Grey Institute of Field 

 Ornithology are among the few people who are bitten fairly often. 

 Louse-flies are too large to infest the host in great numbers, for a big 

 infestation would kill the bird. 



The usual hazards of a parasite's life make special precautions for 

 the offspring necessary. Instead of laying large numbers of eggs 

 hippoboscids go to the other extreme. Only one young is produced at a 

 time but it is hatched and nourished within the body of the parent fly. 

 It is subsequently deposited in the nest as a fully grown larva which 

 immediately pupates. In this stage it passes the winter and hatches out 

 the following spring. The adult fly is also rather long lived and may 

 survive several months. 



It has already been mentioned that some louse-flies have fully 

 developed wings and some have mere vestiges which are useless for 

 flight; in others again the wings are cast off when the fly reaches the 

 host. The sheep ked {Melophagus ovinus) has no wings at all (Plate IXc) . 

 It was pointed out in the chapter on evolution that it is a great advant- 

 age for a parasite which lives on the body of the host not to have wings. 

 In the case of birds such as the swallows and swifts which return year 

 after year to an old nest, the difficulty of finding a host is greatly 

 reduced. It is therefore not surprising to find that the wings of the swift 

 louse-fly {Crataerina pallida) and the swaflow louse-fly {Stenepieryx 

 hirundinis) are greatly reduced and non-functional (see Chapter 6). In 

 the case of the common louse-fly [Ornithomyia avicularia) which is found 

 on a wide variety of birds, ranging from robins to arctic skuas, the 

 difficulty of finding a host stifl sets a premium on wings, and they are 

 fully developed in this species. 



In Britain there are five species of louse-fly known from avian hosts. 

 Apart from the three already mentioned, the finch louse-fly {Orni- 

 thomyia fringillina) is a common species (previously also known under 

 the name of 0. lagopodis) and is recorded from many hosts, and the 



