Blow-flies (commonly known as carrion flies, bluebottles, greenbottles or cluster flies) are insects in the Order Diptera, family Calliphoridae. The family is known to be non-monophyletic, but much remains disputed regarding proper treatment of the constituent units, some of which are occasionally accorded family status (e.g., Bengaliidae, Helicoboscidae, Polleniidae, Rhiniidae).
The name blow-fly comes from an older English term for meat that had eggs laid on it, which was said to be fly blown. The first known association of the term “blow” with flies appears in the plays of William Shakespeare: Love’s Labour’s Lost, The Tempest, and Antony and Cleopatra.
Most species of blowflies studied thus far are anautogenous; a female requires a substantial amount of protein to develop mature eggs within her ovaries (about 800 µg per pair of ovaries in Phormia regina). The current theory is that females visit carrion both for protein and egg laying, but this remains to be proven. Blow-fly eggs, usually yellowish or white in color, are approximately 1.5 mm x 0.4 mm, and, when laid, look like rice balls. While the female blow-fly typically lays 150-200 eggs per batch, she is usually iteroparous, laying around 2,000 eggs during the course of her life. The sex ratio of blowfly eggs is usually 50:50, but one interesting exception is currently documented in the literature. Females from two species of the genus Chrysomya (C. rufifacies and C. albiceps) are either arrhenogenic (laying only male offspring) or thelygenic (laying only female offspring).
Hatching from an egg to the first larval stage takes about 8 hours to one day. Larvae have three stages of development (called instars); each stage is separated by a molting event.The instars are separable by examining the posterior spiracles, or openings to the breathing system. The larvae use proteolytic enzymes in their excreta (as well as mechanical grinding by mouth hooks) to break down proteins on the livestock or corpse they are feeding on. Blowflies are poikilothermic, which is to say that the rate at which they grow and develop is highly dependent on temperature and species. Under room temperature (about 30 degrees Celsius) the black blowfly Phormia regina can go from egg to pupa in 150-266 hours (6 to 11 days). When the third stage is complete the pupa will leave the corpse and burrow into the ground, emerging as an adult 7 to 14 days later.