The anterior pair of wings alone developed; the posterior pair of wings rudimentary, represented by a pair of clubbed filaments, called u halteres" or "balancers" (fig. 188). In a few the wings are altogether wanting. Mouth suctorial. The metamorphosis is complete, the larvae being generally destitute of feet; but in some cases (e.g., the gnats) the pupae are aquatic and are actively locomotive. In most cases, however, the pupae are quiescent.
The proboscis in the Diptera consists of a tubular labium enclosing the other parts of the mouth, and is placed on the under surface of the head. Ocelli are present in addition to the compound eyes. The wings are generally horizontal and transparent, the nervures not very numerous, and for the most part longitudinally disposed. The anterior wings usually have appended to their hinder margin, at their base, a pair of little membranous flaps (the "alulae"), which are to be regarded as separate and detached elements of the front wings. The antennae are generally small and three-jointed (Brachycera), sometimes many-jointed (Tipulidae), or feathery (Culicidae). The larva is soft and fleshy, with a soft indistinct head, usually apodal, never with thoracic legs, and rarely with pro-legs. The larval skin mostly forms a hardened case for the pupa, but the larvae sometimes cast their skin when becoming pupae, or even spin cocoons. In one section of the Diptera, hence termed Pupipara, the larvae continue to reside within the mother until they are just ready to become pupae, and they are born in a form closely resembling the ordinary pupae of the members of the order. In the Hessian Fly (Cecidomyia) the larva produces asexually a number of secondary larvae, which are developed within the body of the primitive larva, and feed upon its tissues, ultimately causing its death.
Fig. 188. - Diptera. Crane-fly ( Tipula oleracea).
The Diptera constitute one of the largest of the orders of the Insecta. Amongst the more important forms included in this division may be enumerated the House-flies, Bluebottles, and Flesh-flies (Muscidae); the Gnats, Midges, and Mosquitos (Culicidae); the Bot-flies ((Estridce); the Gad-flies (Tabanidae); the Forest-flies and Sheep-ticks (Hippoboscidae); and the Crane-flies (Tipulidae).