Larviparous Flies

The Diptera are holometabolous. Most dipteran females lay eggs and are thus oviparous. Others are ovoviviparous, hatching their eggs internally and thus producing motile early-instar larvae. Such flies are called larviparous, as represented by flesh flies (Sarcophagidae). In a few dipteran groups, the developing larvae are retained within the female's body until they are ready to pupate. These flies are called pupiparous and include the louse flies (Hippoboscidae) and tsetse flies (Glossinidae). The number of offspring produced per female by larviparous and pupiparous species is low compared to oviparous and ovoviviparous species.

Many dipteran species inhabit aquatic or semiaquatic environments during their immature stages. Typical examples are mosquitoes, black flies, and most horse flies and deer flies. The females of many of these

Frontal suture

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

hematophagous flies are capable of producing an initial batch of eggs before obtaining a blood meal; this is known as autogeny. In contrast, those species that must feed on blood prior to their producing eggs are referred to as ancmtogenous.

While the number of larval instars varies within the Diptera, it remains generally constant for a given species. Mosquitoes and most other nematocerans have four larval instars, whereas most muscoid Diptera pass through three observable larval instars, with a fourth instar, the prepupa, occurring cryptically inside the pupal case. The muscoid instars usually can be distinguished morphologically: the first instar lacks anterior spiracles and generally has only one slit in each caudal spiracular plate; second and third instars bear anterior spiracles and have two and three slits, respectively, in the caudal spiracular plate. These slits are lacking in some groups; instead, the spiracular plate has many small openings (e.g., cattle grubs).

The pupae of Nematocera are obtect, with the appendages and other external body structures of the developing adult being discernible externally. Pupae are typically immobile; significant exceptions are the pupae of mosquitoes and a few other nematoceran families that can move by means of caudal paddles. The Brachycera have coarctate pupae, in which the pupa is encased within the hardened exuviae of the penultimate larval instar. The latter structure, called a puparium, is most frequently brown in color and is often said to resemble a "pill." It retains many morphological features of the larval integument. Adult flies emerge from the pupal case by employing hydrostatic pressure from the hemolymph to generate splits along predetermined lines. The head of most Schizophora has an eversible sac, the ptilinum, which facilitates the fly's escape from the puparium. After emergence, the ptilinum retracts through a fissure proximate to the lunule at the antennal bases.

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