Syrphidae Flower Flies Hover Flies Rat Tailed Maggots

This is a large family (180 genera, 6000 species) that includes only a few taxa that cause gastrointestinal myiasis. Adults also are called drone flies because of their beelike appearance and resemblance to honeybee drones. The terms flower flies and hover flies refer to their common habit of visiting flowers for nectar and pollen and their ability to hover motionless in flight, The larvae of Eristalis (Fig. 16.4) and other aquatic genera are called rat-tailed maggots, referring to the long, telescopic, three-segmented respiratory tube at their posterior end by which they breathe at the water surface.

The syrphid species most frequently involved in myiasis is Eristalis tenax. Its larvae develop in sewage, liquefied excrement, decaying animal carcasses, and other decomposing plant and animal material of a liquid consistency. A number of human cases of gastrointestinal myiasis have been reported, with live larvae being passed in stools. Two other Eristalis species have been identified in human myiasis: E. arbustorum in Europe and E. dimidi-ata in the United States (James, 1947). Leclercq (1969) described several human cases involving rat-tailed maggots, including a man in Germany who passed more than 40 Eristalis larvae in one day and another 10—30 larvae

FIGURE 16.5 Cheese skipper, Piophila sp., larva (Piophilidae). (Original by E. P. Catts.)

FIGURE 16.6 Pomace fly or fruit fly, Drosophila sp., larva (Drosophil-idae). (Original by E. P. Catts.)

FIGURE 16.5 Cheese skipper, Piophila sp., larva (Piophilidae). (Original by E. P. Catts.)

on each of the following 6 days. Eristalis larvae also have been the cause of vaginal myiasis in cattle.

Piophilidae (Skipper Flies)

This is a small family of about 70 species in 35 genera worldwide. Females of the common cheese skipper (Piophila casei) oviposit on putrid, dried, cured, or smoked meats and cheeses, typically depositing 400-500 eggs per female. Adult cheese skippers are small (3-5 mm), slender, glossy black flies with yellow on the lower face and part of the legs. Larvae are slender, cylindrical, white, and truncated caudally with three pairs of short caudal protuberances, the ventral pair being the largest (Fig. 16.5). Larvae require about 5 days to develop under warm conditions. In temperate regions the mature larva overwinters. The name skipper originates from the ability of the larva to flex head to tail in a circle and, following total-body muscular contraction and release, the larva propels itself off the substrate for a considerable distance (up to 24 cm). This behavior is used as a means of escape when the larva is disturbed or is dispersing to suitable pupation sites. The pupal stage lasts about 5 days. The life cycle, egg to egg, can be completed in as little as 2 weeks.

P. casei is a widely distributed species which commonly infests cured meats and cheeses and dried fish. It probably is the species most commonly involved in gastrointestinal myiasis of humans. The tendency of people to leave cured meats and cheeses unrefrigerated makes these foods available to gravid females for oviposition. These flies can survive the rigors of alimentary tract passage and can even pupate and emerge as adults prior to leaving the host. The related P. vulgaris and Stearibia species are common inhabitants of dried carrion.

Neottiophilidae (Nest Skipper Flies)

This small Palaearctic family includes two genera: Neotand Actinoptera. The larvae cause cutaneous

FIGURE 16.6 Pomace fly or fruit fly, Drosophila sp., larva (Drosophil-idae). (Original by E. P. Catts.)

myiasis by sucking the blood of nesting birds. Larvae attack with their mandibles and can penetrate deeply enough into their host to cause septicemia and death. Larvae pupate in the host's nest in the fall and emerge as adults in the spring. The adult is a yellow-brown fly, about 7—8 mm in length, with wings "pictured" with a few brown spots. Hosts typically are passeriform birds. Local strains of Neottiophilum praeustum show narrow host specificity to different avian species.

Drosophilidae (Pomace Flies, Vinegar Flies, Fruit Flies, and Wine Flies)

This is a large family (3000 species in 60 genera) of small red-eyed flies (1.6 mm) whose adults favor the odors of overripe or fermenting plant products, usually fruits. Larvae feed on microorganisms found in such substrates. The genus Drosophila is the largest and includes more than half of the species in this family. Larvae have posterior spiracles on paired caudal protuberances (Fig. 16.6) which also are evident in the pu-paria. The best-known species is the highly domesticated "kitchen gnat" Drosophila melanogaster. An additional seven species also are locally common domestic pests (e.g., D. busckii, D. funebris, D. hydei, D. immigrans, D. repleta, D. simulans, and D. virilis). The life cycle for these species is typically 12—14 days, making these small flies useful as biological models in studies of genetics, physiology, cytology, and population dynamics. Because of their attraction to fruits and vegetables, these species can cause accidental gastrointestinal myiasis.

Chloropidae (Grass Flies and Australian Frog Flies)

The genus Batrachomyia includes 10 species whose larvae occur individually in swollen, subcutaneous pockets on the body (not the legs) of Australian frogs. The adult flies are yellow—brown in color and possess hairy eyes. Adults feed on plant juices. Their eggs require high humidity and are laid near, but not on, the host. After moving to a frog host, the larvae attack and appear to feed on blood, reaching a length of 10 mm when fully mature.

FIGURE 16.9 Typical muscid larva (Muscidae), third instar. as, anterior spiracle; psl, posterior spiracular plate, house fly (Musca domestica); ps2, posterior spiracular plate, false stable fly (Muscina stabulans). (Original by E. P. Catts.)

The mature larvae are peculiar in appearance, having paired anterior and posterior "tentacles," each bearing a spiracle (Fig- 16.7). Seasonal prevalence in frog populations can be as high as 25%, with a parasite load of one to four maggots per host. Death results in about 10% of the frogs at the time of larval drop.

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