brown. Some species are relatively bare and strongly resemble wasps; others are quite hairy, and resemble bees. The resemblance of many syrphids to various Hymenoptera is often very striking, and it may require a second look to determine that they are flies and not Hymenoptera. Syrphids do not bite or sting. Larvae also vary in appearance and habits; many are predaceous on aphids, many are scavengers (living in dung, carrion, decaying vegetation, or in highly polluted aquatic habitats), and some live in ant nests. Aphid-feeding larvae are maggotlike and usually greenish; some larvae living in polluted aquatic habitats have the posterior end extended as a long tail-like structure and are called rat-tailed maggots; larvae living in ant nests are oval and very flat.

THICK-HEADED FLIES Family Conopidae Identification: Venation as in Syrphidae, but lacking a spurious vein. Proboscis long, slender, and often folding. Abdomen usually narrowed at base.

Thick-headed flies are usually medium-sized and brownish; many resemble small thread-waisted wasps. The head is slightly wider than the thorax and the antennae are generally long. Adults are commonly found on flowers. Larvae are parasites of adult bees and wasps.

Muscoid Flies: Division Schizophora

A frontal suture present.

Adult flies in the suborder Cyclorrhapha emerge from the puparium through a circular opening at the anterior end. This opening is made with a saclike structure called the ptilinum, which is everted from the head of the fly. After emergence the ptilinum is withdrawn into the head, and in the Schizophora the break in the head wall through which the ptilinum was everted is marked by a suture called the frontal suture. This suture is in the form of an inverted U or V, with its apex just above the base of the antennae, and the 2 arms extending downward toward the cheeks.

Muscoid flies have aristate antennae, numerous bristles on the head and body, and most of them are rather stout-bodied. Mus-coids are usually small; many are very small. This group makes up about of the order, and its members occur almost everywhere, often in considerable numbers. Because of their small size and the large number of species and families, their identification can be difficult.

The principal characters used in separating families of muscoid flies are those of the bristles and wing venation. The characteristic bristles and areas of the head are shown opposite and those of the thorax are shown on p. 287. The chief venational characters used are the size and shape of various cells, the development of Sc,

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