blow fly

blow fly, (family Calliphoridae), any member in a cosmopolitan family of insects comprising about 1,500 species in around 150 genera. Blow flies are ecologically important as decomposers and can rapidly colonize carrion and other decaying organic matter and thus aid in nutrient cycling. Adults and larvae are also food sources for many other animals. Several blow fly species are of significant economic importance as vectors of disease. Some species have parasitic larvae that cause myiasis (infestation of living tissue) and are serious pests of humans, livestock, and other animals. A few are used medicinally for wound debridement.

(Read Britannica’s essay “Do Blow Flies Spread Disease?”)

Physical description

Like other insects, adult blow flies have six legs and a body formed of a head, thorax, and abdomen. With an average size of 8–10 mm (0.3–0.4 inch), blow flies are slightly larger than houseflies but resemble them in habits. They are commonly metallic blue, green, or black in color and are noisy in flight. Blow flies are bristly, and the strongest bristles have a precise location, particularly on the thorax. The arrangement of bristles and the identification method based on them is called chaetotaxy.

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Natural history and uses

Adult blow flies feed on a variety of materials, but the larvae of most species are scavengers that live on carrion or dung. The adults lay their eggs on the carcasses of dead animals, and the larvae (maggots) feed on the decaying flesh. The presence and life stage of certain blow flies can be assessed by forensic entomologists to determine the length of time a body has been decomposing or even time of death.

The larvae of some species (e.g., Calliphora, Cochliomyia) also sometimes infest open wounds of living animals. Although some species are harmful to infected animals and destroy healthy tissue, others may assist in preventing infection by cleaning away dead flesh and foreign matter and by producing allantoin, which has antibacterial properties. Indeed, blow fly maggots can be raised in sterile laboratory conditions and deliberately introduced onto patients’ wounds for a practice called maggot debridement therapy. In the early 2000s, more than 2,000 centers for maggot debridement therapy were active worldwide.

Major genera and species

Screwworm is the name for the larvae of several North and South American blow fly species, so called because of the screwlike appearance of the body, which is ringed with small spines. These larvae attack livestock and other animals, including humans. The true screwworm or New World screwworm (Cochliomyia hominivorax; formerly, Callitroga americana) and the secondary screwworm (C. macellaria) develop in decaying flesh in surface wounds of domestic animals and occasionally of humans, and the larvae may attack living tissue as well. Each female deposits about 200 to 400 eggs near an open wound. The larvae burrow into the tissue, drop to the ground when mature, and pupate before emerging as adults. Severe infestations (myiasis) may lead to the death of the animal affected. The sterilization of male flies has been successfully used in attempts to control screwworms.

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Among the best-known blow flies are sheep blow flies, or greenbottle flies, of the genus Lucilia. Maggots of L. sericata, for example, feed on small dead animals and in abattoirs and garbage cans; they oviposit in soiled wool around the anus of sheep or in the pus exuding from scratches and wounds, where they are important agents of sheep strike disease. These maggots sometimes occur in soil near buildings in cities; their food source is not known.

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Bluebottle (Calliphora) flies are distinguished by their distinctive coloration and loud buzzing flight. These flies commonly infest carrion or excrement, and the larvae of some species infest and may even kill sheep. The black blow fly (Phormia regina) is another widely distributed species with similar habits. Chrysomyia megacephala, which breeds in excrement and decaying material in Pacific and East Asian regions, is an important carrier not only of dysentery but also possibly of jaundice and anthrax. Protocalliphora sucks blood from nestling birds.

The adult cluster fly (Pollenia rudis) of Europe and North America is sluggish and dark in color. The larvae of this species are parasites of earthworms. In autumn, huge buzzing clusters of the adults gather in attics or other sheltered places to hibernate; they return outdoors in the spring.