FLIES 221 



{Stomoxys calcitrans), which is widely distributed throughout the British 

 Isles, is also occasionally found in birds' nests, and it is quite possible that 

 if no mammals are available it finds sparrows and swallows satisfactory 

 substitutes. The greatest numbers of the house-fly group, which are 

 found either as obhgate or occasional occupants of birds' nests, prey in 

 the larval stage on other dipterous larvae or eat decaying animal and 

 vegetable matter. They are not parasites of the birds themselves. The 

 majority of the blue-botde group are also parasitic in their larval 

 stages on the larva and pupa of other insects. Many breed in decaying 

 animal matter and sometimes they eat flesh and corpses. The larvae 

 of flesh-flies {Sarcophaga) and the green-bottles and blue-botdes [Lucilia 

 and Calliphora) and certain other genera are quite often found con- 

 suming the decomposing flesh in wounds on the bodies of birds. The 

 original wound may have been inflicted by a blood-sucking insect. 

 They are facultative parasites, and their presence is in the nature of a 

 recurring accident. One genus however, the blue or green metallic 

 flies {Protocalliphora), are true ecto-parasites of birds in the larval stage. 



The bird-bottle fly (P. azurea) feeds on nectar as an adult, and it can 

 sometimes be seen around flowers in the sunshine (see tail-piece 

 p. 5). The larvae live in the nest and at certain intervals attach 

 themselves to the nestlings by their anterior end, which is modified to 

 form a sucker with hooks in the centre. Although they somedmes kill 

 the host they are frequently present in large numbers without apparent- 

 ly causing much harm. In one magpie's nest 373 larvae were counted, 

 but the fledgelings seemed healthy. The species is quite common in 

 Britain and has been recorded from a large number of birds, including 

 the nightingale, redstart, skylark, meadow-pipit, and various tits, 

 wagtails, crows, swallows and martins. 



A frequent parasite of British birds with similar habits is the nest- 

 fly {Neottiophilum praeustum). It is a large yellowish brown fly and the 

 larva, which lurks in the lining of the nest, is a voracious blood-sucker. 

 The anterior end is armed with two strong hooks which it thrusts into 

 the flesh of the young bird while feeding. When fully gorged it drops 

 back into the nest. Too many larvae in one nest kiU the fledgelings and 

 the female fly guards against this disaster by dispersing her eggs in 

 several nests. The principal hosts are passerine birds and Basden has 

 reared it commonly from nests of blackbirds, thrushes, finches, warblers, 

 carrion-crow and the linnet ; but it has also been taken occasionally from 

 nests of the nighdngale, tree-creeper, sparrow and hedge-sparrow. 



