Cochliomyia is a genus of just 4 species of the New World tropics and sub-tropics where it largely replaces the Calliphora and Lucilia of the temperate zone. It includes the New World screwworm fly, Cochliomyia hominivorax, two little known species, C. minima and C. aldrichi, and the secondary screwworm fly C. macellaria.
There was much confusion over the identity of this species and that of Cochliomyia hominivorax until Cushing and Patton (1933) conclusively provided proof of their separate identities. Earlier references to Cochliomyia macellaria, and to cases of myiasis caused by this species, should be treated with caution.
Adults of the genus Cochliomyia are small to medium sized flies, with dull or bright metallic, green to blue-green to bluish-purple coloration. They all have three dark longitudinal stripes on the thorax. Size cannot be used definitively to separate the four species, but aldrichi and minima tend to be smaller (5.5 to 8.5 mm), hominivorax larger (8-10 mm) and macellaria with an intermediate range (6-9 mm). Detailed descriptions of both sexes of all four species are given by Hall (1948). Cochliomyia macellaria can be distinguished from C. hominivorax by the hairs on the fronto-orbital plate, which are black in hominivorax and pale in macellaria (but their insertions appear as black dots), and the central black stripe on the thorax, extending only slightly forward of the mesonotal suture in hominivorax and well forward of the suture in macellaria.
The larval stages of aldrichi and minima are undescribed. Third instar larvae of macellaria can be distinguish from those of hominivorax by examining the dorsal tracheal trunks, which are pigmented only at the junction with the spiracles in macellaria and pigmented forwards to the tenth or ninth segment in hominivorax.
Cochliomyia macellaria is a typical blowfly, the larvae developing in carrion, and occasionally faeces. It shows a tendency to facultative parasitism and can be found as a secondary invader in cases of myiasis, along with the obligate parasite C. hominivorax. Typically the macellaria larvae occur at the edges of the wound feeding on the edge or surface of the wound and not producing the pocket-like lesions characteristic of the primary screwworms (James, 1947).
Adults prefer warm humid weather and populations levels increase during periods of high rainfall. They are attracted to carrion and garbage, and can be an abundant pest in slaughter houses and outdoor markets in the tropics (Greenberg, 1971-73).
A total of 1000 or more eggs can be laid in a lifetime, in loose masses of 40-250. Sometimes several females lay together, forming large masses of thousands of eggs. In favourable conditions the eggs hatch in four hours and the larvae reach maturity in 6-20 days, when they migrate from the body to search for pupation sites in the soil. The total development time takes 9-39 days depending on temperature and humidity. Adults live two to six weeks (James 1948).
Cochliomyia macellaria occurs in the Neararctic from Southern Canada all the way down through the continental USA, and throughout the Neotropics except in the extreme south of Chile and Argentina.
Cochliomyia aldrichi and C. minima have a restricted distribution covering most of the Caribbean Islands and the Florida peninsular.
- Cushing, E.C. & Patton, W.S. (1933). Studies on higher Diptera of medical veterinary importance: Cochliomyia Americana sp. Nov., the screwworm fly of the New World. Annals of Tropical Medicine and Parasitology 27: 539-551.
- Greenberg, B. (1971-73). Flies and Disease. 2 vols, viii + 856 pp; x + 447pp, Princeton University Press, Princeton.
- Hall, D.G. (1948). The blowflies of North America. Pp 477, Say, Baltimore.
- Hall, M.J.R. (1993) Manual for the control of the screwworm fly, Cochliomyia hominivorax, (Coquerel). Volume 2. Food and Agriculture Organization of the United Nations, Rome, Italy, 38 pp.
- James, M.T. (1947). The flies that cause myiasis in man. United States Department of Agriculture Miscellaneous Publication 631: 1-175.