Flies, which are such insidious and pertinacious persecutors of man and beast, are themselves the prey of innumerable enemies; many species are much sought for by birds, they are devoured by lizards and toads, and they are equally preyed upon by predaceous insects. Those flies which have bodies with banded colours, and which otherwise somewhat resemble bees and wasps, probably escape being the victims of some birds; but the tribe of flies does not, like the beetles, the lepidopteræ, and some other insects, furnish instances of other common protective devices, such as bearing and voiding offensive secretions, or attempted concealment in repose by mimicry of environment. All insectivorous birds are fond of a diet of flies, and we may largely attribute the spring-time immigration of the beautiful swallow tribe to the fact that in the northern parts of the temperate zone swarms of soft bodied dipterous insects abound, and there replace the hard cased and more chitonous insects of hotter countries. The true swallow, the house martin, and the sand martin, all require a special food for their nestlings; and they also then require the longer summer days and the prolonged twilight of our 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 swallows to start the foundations of a nest thereon. Ground feeding birds of many kinds eagerly search for flies, their pupæ, and their larvæ; even some finches will add flies to their diet in the nesting season. Chaffinches are very fond of the house-fly at all times.