Morphology

Nematoceran larvae range in length from only a few millimeters to many centimeters, depending on the species, and are usually distinguished by having a conspicuous head capsule with opposable mandibles that move in a pincer-like horizontal plane (Fig. 8.1). The general body shape ranges from minute and eel-like in the Ceratopogonidae to large and fleshy in the Tipulidae. Some nematocerans have thoracic prolegs(z.g., Chironomidae and Simuliidae) and others have caudal structures (e.g., Simuliidae) which assist in attachment to substrates. Although the early instars of many aquatic species depend on cuticular respiration, the later instars generally respire via gills or have various adaptations that permit them to obtain atmospheric air. Mosquito larvae, for example, are highly adapted, air-breathing nematocerans that hang from the water's surface film by respiratory siphons or specialized abdominal setae.

Tabanomorpha and Asilomorpha larvae have fanglike mandibles that move in a vertical plane; the head capsule is frequently described as "incomplete posteriorly," meaning that only the anterior parts are sclerotized (Fig. 8.2). The latter character is best seen in specimens that have been cleared in potassium hydroxide or lac-tophenol. Horse fly larvae are good examples of this group. They often have posterior respiratory tubes.

Muscomorpha larvae lack a sclerotized head capsule (Fig. 8.3A) and are commonly known as maggots. At the narrow, anterior end of the 12-segmented larva is

FIGURE 8.1 Representative head capsule of nematoceran larva, with opposable mandibles; chironomid midge (Chironomidae), ventral view. (Redrawn from Merritt and Cummins, 1996.)

the cephalopharyngeal skeleton (Fig. 8.3C), which usually bears one or two mouth hooks used for feeding and in assisting the insect in movement. The caudal end of the maggot is broader and bears the posterior spiracular plates (Fig. 8.3E); like the cephalopharyngeal skeleton, they often are valuable for identification. The segments of the maggot typically bear spines in regular patterns (Fig. 8.3D), and the larvae of some species may possess structures that vary from simple setae to large protuberances. Others, such as cattle grubs and bot flies, are rounded and

FIGURE 8.2 Lateral view of anterior part of Tabanus margina&Urvs (Tabanidae), showing incomplete head capsule and vertical, fanglike mouth hook. (Prom McAJpine etal., 1981b.)

Frontal suture

FIGURE 8.5 Frontal view of head of female fly, showing frontal suture and ocellar triangle at vertex. (From Greenberg, 1971.)

halteres. Included in the Calyptratae are the hippoboscoid flies, which are sometimes secondarily wingless.

The mouthparts of blood-feeding muscomorphan adults are of the piercing/sucking type. In contrast to other Diptera, both male and female Calyptratae suck blood in those species that exhibit this feeding style (e.g., horn flies and stable flies). Other species generally possess mouthparts that permit liquid food materials to be lapped or sponged. The latter type of mouthparts in some species have structures sclerotized enough to scarify tissue during feeding activities (e.g., the face fly).

The functional pair of wings in the Diptera arises from the mesothorax. The metathoracic wings are modified to form a pair of knobbed balancing organs known as halteres (Fig. 8.6). The wing venation is highly variable between groups and provides valuable taxonomic characters

FIGURE 8.6 Calvpterate fly, showing haltere and calypters. (From Greenberg, 1971.)

for distinguishing the families. Many dipteran adults have characteristic wing patterns, including species of biting midges, deer flies, and horse flies,.

The adults of most Diptera possess distinct compound eyes; ocelli are present in a triangle on the vertex of many species (Fig. 8.5). Adults are identified easily to sex, because most species exhibit some degree of sexual dimorphism. Nematoceran males often possess densely plumose antennae, and the females of blood-sucking species bear stylet-like mouthparts. The eyes of brachyceran males typically meet along the dorsal midline of the head (holop-tic), whereas the eyes of females are more widely separated (dioptic). The female abdomen ends in an ovipositor (larvipositor in some species), whereas the male abdomen typically bears distinct genitalia at the terminus. In the males of some Nematocera and Brachycera, the genital segments rotate a half-turn shortly after the adult fly emerges; thus, the genital capsule appears "upside down" in adults of those species. In the Schizophora, this rotation continues through a full circle, so that the genital capsule is in its normal position. A morphological approach to identification that has proven useful in the Diptera, particularly with the Muscomorpha, is the characteristic appearance of male genitalia. In many species the aedeagus, claspers, and associated structures are unique. "Pulling the tail" of male flies is a technique used by dipterists that permits detailed examination of the genital structures. Descriptions of species in some families, such as the Sarcophagidae, are based in large part on male specimens.

McAlpine (1981c) and Teskey (1981b) present comprehensive reviews of the morphology and related terminology of diptera adults and larvae, respectively.

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