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.

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 EmpidÆ, of which numerous species are native to Great Britain. Less common, but very observable when met with, are the closely related robber-flies, AsilidÆ, which are hardly ever to be seen on a hot midsummer day without a captured fly held between their strong front legs. The largest of the robber-flies, A. crabroniformis, is a conspicuously fine insect. It equals the hornet in length, but is more slender in body, tapering throughout the abdomen towards the tail; it resembles the queen hornet in colour. The dragon-flies also eat flies, but they mostly feed upon winged aphides, gnats, and the like small game.

More secret destroyers of the fly brood are a few rather obscure creatures akin in their nature to the ichneumon flies, which are parasitic mainly but not exclusively on lepidopterÆ. Likewise, certain insectivorous beetles share in the good work of fly destruction.

Flies are often observable encumbered with minute vermin; some of these are true lice, and some are allied parasites called false-scorpions or chelifers. These are acquired whilst frequenting dung and refuse heaps, where they abound; thus, probably cheese-mites and the like are conveyed by flies into our larders and warehouses.

It is not only the web-weaving spiders which prey upon the proverbially "silly" flies; there are also roving spiders which do not contrive webs, and some of these are nocturnal feeders; the latter can only be seen in daylight by looking for them underneath stones and in other hiding places.

Other fly destroyers are internal parasites, and these include thread worms (Nematoidea), as well as Protozoa of obscure kinds. These are being scientifically studied by experts, and their life-history is as curious as that of others of the same order, in that they pass from one host to another, which fact for long helped to baffle investigation. Some have now been proved capable and others are suspect of baneful possibilities.

The house-fly fungus, Empusa muscÆ, which is prevalent in autumn, has ever attracted popular wonder and much scientific attention. It has been much written about and plentifully illustrated, but the complete life-cycle of this peculiar parasitic growth is not yet understood; much that has been published as of fact is mere "copy" repeated in one book after another, originally in fact rather a matter of conjecture, based upon the idea that fungic propagation must be on exactly parallel lines with known biologic processes of a botanic order.

The house-fly fungus seems to have a superficial resemblance to some of the common "moulds," but mycologically examined there is good reason for classifying it with a family (EntomophthoreÆ), which may be capable of an alternative form of fructificative development.

The originating germ somehow at some time must be supposed to have effected a lodgment in the body of the fly or possibly that of the maggot. Later on, one cannot say when, fungic spawn (a pulpous mycelium) starts a course of development, invades every part of the body, quickly kills the fly, and fixes it to the spot to which in its last moments it has crawled, often a window-pane. Its corpse is now swollen with the spawn developed into masses termed hyphal; but these should not be called hyphal (thread) but quasi-sclerotia, bodies intermediate in a process of normal development between the mycelium and the fruiting stage; fungic fructification ensues with great rapidity; and the corpse becomes suffused with an appearance of white mouldy excrescences, filiform conidiophores, of which the club-like tips make a copious Ærial discharge of white spores; when these adhere to a glass or window-pane they imprint thereon a remarkable halo. Attempts to artificially infect other flies with these spores are common failures or have led to contrary conclusions.

The period from the first symptoms of distress to the death of the fly, and from that time to the spore discharge is wonderfully short. The infectious germs may have been long dormant in the fly, and very likely may have been acquired in the maggot stage. In the absence of exact knowledge, we can only make conjectures from observations of some kindred fungic parasites, which are not very uncommon amongst the chrysalids of certain moths, beetles, and the pupÆ of some annual wild bees. In these cases it seems very unlikely that the infection was incurred otherwise than in the caterpillar or larval feeding stage. Dampness and insanitary conditions seem to favour the spreading of such disease, especially amongst artificially reared larvÆ when crowded together and closely confined.

Other kinds of flies do not, so often as the house-fly, perish from Empusa muscÆ, but I have seen a common yellow cowdung-fly, as early as June, thus affected.

In spite of all antagonists the brood of the house-fly flourishes and multiplies, but this is because great opportunity and encouragement is provided by the neglect of good sanitation by mankind.