54 THE BOOK OF THE FLY 



northern clime for the frequent feeding of their young. 

 The prevalence of flies near houses partly accounts for 

 the partiality of the swallow for nesting under the 

 eaves of our dwellings, where unfortunately the 

 aggressive, the pampered, and the demoralised sparrow 

 in towns generally prevents successful breeding by 

 appropriating its nest and sometimes by eating its 

 eggs. People who desire to favour the breeding of 

 the swallow should destroy the nests immediately 

 after the migratory departure of the builders. Their 

 retention until the following spring is in no way an 

 enticement for rehabitation ; on the contrary, it favours 

 the objectionable habit of the sparrow to use them for 

 a night resort, whereby they become very foul with 

 bird vermin. The cleanly swallow annually desires a 

 newly built habitation, and a bare peg projecting two 

 inches from the house-wall will much encourage swal- 

 lows to start the foundations of a nest thereon. 



Ground feeding birds of many kinds eagerly search 

 for flies, their pupae, and their larvae ; even some 

 finches will add flies to their diet in the nesting 

 season. Chaffinches are very fond of the house-fly 

 at all times. 



The wasps are assiduous hunters of flies, and, though 

 possessing less agility than their prey, they manage to 

 pounce upon many victims. Very common enemies 

 are the predaceous Empidcv, of which numerous 

 species are native to Great Britain. Less common, 

 but very observable when met with, are the closely 

 related robber-flies, Asilidcc, which are hardly ever 

 to be seen on a hot midsummer day without a captured 

 fly held between their strong front legs. The largest 



