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.