FLIES AND THEIR HYGIENIC IMPORTANCE
Flies, scientifically speaking, are only those insects of the order of Diptera, distinguished by having only one pair of fully developed wings. They pass through a complete metamorphosis, and the larva is in all cases a "grub" or "maggot" destitute of legs. It is rarely enclosed in a cocoon but lies buried in the ground, floats in the water, or is protected by the last larval skin which, separating from the pupa skin, remains around it as a hard case. Flies and their larvæ live in the most diverse manner. Some flies attack backboned animals and suck their blood, some prey on smaller insects, some suck honey, and some find their food in decaying animal and vegetable matter. A large number of dipterous larvæ eat refuse, many feed inside growing vegetable tissues, and some prey, or are parasitic, on other insects. More than 10,000 species of true flies have already been named in the United States alone. The order contains all the different species and varieties of fleas, mosquitoes, sand flies, gnats, midges and gall flies. Then come the blood-sucking gadflies, and half a dozen families allied to them; the scavenging syrphus flies, the bots that trouble cattle, the house flies and stable flies of deservedly bad repute; and, lastly, the horseflies, bee parasites, and botflies. The popular interest in these insects is confined to the flies of our houses and stables, and to the mosquitoes. In fact it is in the relation that the flies mentioned, and some others, bear to public health and comfort, that this group of insects is important at all to any but the special student.