The lesser house-fly has proclivities similar to those of the common house-fly, but probably she travels less far afield although a little more inclined to outdoor life.
Very little is known about most of the common outdoor sweat-flies. Some breed in dung, and may be many-brooded and otherwise resemble the house-fly in prolific increase; others are more consistently vegetarian in the larval stage and slower in development; and some are possibly even single-brooded, like certain foreign large sized flies which fortunately appear only for a few weeks of summer weather, for they have a curious semi-blood-sucking habit of feeding after or alongside the skin-piercing flies, and their suctorial mouths are capable of further inflaming wounds and carrying infection from one animal to another.
The robust blue-bottle very closely resembles the house-fly in an inclination to spread the brood. Mature females, however, do sometimes show a slight temporary kind of "homing" instinct; having secured a cosy corner and a well sheltered retreat in a sunny wall, the occupant will battle for its possession, buffeting new comers.
Some of the smaller filth flies and many of the fungus flies have their lives, in the imago stage, influenced and shortened by their extra early sexual maturity; the females are fertilised whilst newly hatched and their wings limp and unfolded. This fact accounts for our seldom seeing some kinds of these flies abroad except females; and these are never seen to indulge in dances, flirtations, and games of chasing and buffeting each other, after the manner of so many kinds of flies. They habitually fly low; nevertheless they travel very great distances, for, though short, their flights are incessant when searching for their special kind of food.
The most disinclined to roam of all common flies is the stable-fly. None other is a more eager seeker of sunshine, but when basking on a sunny wall it seems unwary and sluggish; it is seldom to be seen far from where horses or cattle are stationed or stabled; however, it will make very long journeys hovering about a driven horse or reposing on the car.
A plague of flies of local origin will not take many weeks of summer weather to spread, but it is generally observable that plagues of flies, like many other occurrences, are simultaneous co-incidences distributed over wide areas and at places remote from each other.
CHAPTER VIII
NATURAL ENEMIES AND PARASITES
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.