Muscidae Dung Flies

The large family Muscidae includes at least seven genera in which species cause myiasis (Table I). Muscid larvae (Fig. 16.9) develop in a wide diversity of decaying organic matter, usually of plant or fecal origin. Occasionally they develop in old or buried carrion that is unsuitable for blow-fly exploitation. All stages of myiasis-causing muscids are typically house fly-like in appearance. Exceptions are Neomyia and Hydrotaea species, in which some adults have metallic coloration similar to blow flies, and the nest flies, which are larger and yellow to yellow-brown in color. Gastrointestinal myiasis caused by muscids usually results from oviposition on wet foods. It also may result from retroinfection through the host's anus following fly attraction to foul odors or soiling by feces. Urogenital myiasis may occur in association with purulent discharges, urine-soaked clothing, and secondary microbial infections.

The genus Musca includes about 60 species, which are confined mostly to the Old World. The two most important species that have invaded the New World are the housefly or typhoid fly (Musca dornestica) (Fig. 16.9) and the face fly (M. autumnalis) in temperate regions. In the Old World tropics, the prevalent species is the bazaar fly (M, sorbens). Both the house fly and bazaar fly oviposit in a wide range of wet, decaying organic matter. Green-berg (1971) suggests that the house fly and bazaar fly were originally adapted, as larvae, to develop in wet ungulate feces. They do show preference for accumulations of animal excrement, especially that from the horse, cow, human, pig, and poultry. Their egg complement ranges from 1000 to 3000 per female and is laid over the adult life span in clusters of 120—150 eggs. The quality of larval and adult diets largely determines the number of eggs produced by any one fly. Maggots develop rapidly in 35 days under wet, warm conditions but are intolerant of desiccation. In the tropics, their life cycle can be as brief as 10—12 days, whereas 3 weeks is more typical in other regions. All stages can overwinter, but in colder areas there is a dramatic winter die-off. Larvae of Musca. species that invade wounds feed primarily on necrotic tissues. Musca species with rasping-sucking mouthparts that feed on blood are not known to cause myiasis.

The genus Muscina includes eight species, three of which are implicated in accidental myiasis (Muscina stabu-lans, M. assimilis, and M. pabulorum). The false stable fly (M. stabulans) (Fig. 16.9) is the most important and is involved primarily in gastrointestinal myiasis. Occasionally their maggots occur in fetid sores or wounds. Adults of this species look much like house flies, except that they are usually larger and more robust. They are attracted to, and feed on, plant juices, rotting fruits, and insect-excreted honeydew. Females oviposit by scattering their 140—200 eggs on the surface of overripe, decaying fruit. They also oviposit on accumulations of dead insects or feces, usually from human sources, and on buried carrion. Early-instar larvae are saprophagous but become preda-ceous as they mature. Third-instar larvae prey on smaller maggots. This transition from a saprophagous to a preda-ceous habit has two advantages over species whose larvae remain saprophagous: first, the maturing larvae can store protein resources obtained from their prey to be used by the adult in reproduction; and second, this habit enables this species to exploit a wider range of protein-poor resources as a larval substrate. Larval development varies from 2 to 3 weeks. They usually overwinter as pupae.

Hydrotaea species are metallic-colored muscids usually found in association with feces and older carrion. Like M. stabulans, their dark, cream-colored larvae (up to 15 mm long) become predaceous on other maggots. In Australia, Hydrotaea rostrata has been implicated as a tertiary invader in myiasis of sheep but appears to feed only on necrotic tissues.

Tropical Nest Flies

These muscid flies include the genera Passeromyia, Mydaea, and Philornis. Species in these three genera usually parasitize the young of cavity-nesting birds. Although few details are known about these flies, their biologies appear to be similar and are discussed collectively here. Species in the first two genera are widely distributed in the Ethiopian, Oriental, and Australian regions. The latter genus is found in the New World Tropics. Currently

Philornis is the valid generic name for those flies formerly listed in the genus Neomusca.

Adults of these tropical nest flies feed on plant juices and wet feces. Gravid females oviposit in bird-nest debris near nesting birds. After the eggs hatch, the maggots crawl to the nestlings, scratch the skin with their mouth-hooks, and imbibe blood. As they feed, they continue to scratch and penetrate host tissues. In heavy infestations they can penetrate the host body cavity, with fatal results for the bird. The larvae develop rapidly, in less than 1 week, before leaving the host to pupate in the nest debris. Postfeeding larvae of Philornis species exude a frothy, sticky, salivary spittle which coats the puparium and to which camouflaging debris adheres. In spite of this defensive measure, pupae are subject to attack by parasitic wasps.

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