"No order surpasses this in the number of species or in the immense swarms of individuals belonging to the same species which are frequently met with. The wings, which are variously veined, though appearing naked to the unaided eye, are often thickly covered with very minute hairs or hooks. As an order the Diptera are decidedly injurious to man, whether we consider the annoyances to ourselves or our animals of the Mosquito, Buffalo-gnat, Gad-fly, Breeze-fly, Zimb or Stomoxys, or the injury to our crops of the Hessian-fly, Wheat-midge, Cabbage-maggot, Onion-maggot, etc. There are, in fact, but two families, Syrphidae and Tachinidae, which can be looked upon as beneficial to the cultivator, though many act the part of scavengers. No insects, not even the Lepidoptera, furnish such a variety of curious larval characters, and none, perhaps, offer a wider or more interesting field of investigation to the biologist. It is difficult to give any very satisfactory arrangement of these Two-winged flies, though they easily fall into two rather artificial sections. These are: 1st, NEMOCERA, or those with long antennae, having more than six joints, and palpi having four or five joints. The pupa is naked, as in the Lepidoptera, with the limbs exposed. This kind of pupa is called _obtected_. 2d, BRACHOCERA, or those with short antennae, not having more than three distinct joints, and palpi with one or two joints. The pupa is mostly _coarctate_, _i. e._, is formed within, and more or less completely connected with, the hardened and shrunken skin of the larva. [Illustration: FIG. 26.--The Sheep Bot (_[OE]strus ovis_). 1, 2, flies; 3, puparium; 4, 5, and 6, larvae or bots.] "The most anomalous of the Diptera are the Forest-flies and Sheep-ticks (_Hippoboscidae_). They have a horny and flattened body, and resemble lice in their parasitic habits, living beneath the hair of bats and birds. Their mode of development has always attracted the attention of entomologists. The larvae are hatched in the abdomen of the female, which is capable of distention. There it remains and, after assuming the pupa state, is deposited in the form of a short, white, egg-like object, without trace of articulation, and nearly as large as the abdomen of the female fly. Closely allied to these are the Bat-ticks (_Nycteribidae_), which possess neither wings nor balancers, and remind one strongly of spiders. "In this order we may also place certain wingless lice (such as _Braula c[oe]ca_, Nitzch), which infests the Honey-bee in Europe, northern Africa, and western Asia, but which has not yet been detected in this country. [Illustration: FIG. 27.--A Flea (_Pulex_). (From Packard.)]