Egg -- The white or pale yellow eggs of blow flies may occur in small batches or large masses. Individual eggs are about 1.5 mm long, elongate and slightly tapered toward one end.
Larva -- These white to yellowish maggots have pointed heads. They develop through three instars and become 12 to 18 mm long when fully grown.
Pupa -- Blow fly pupa are encased in light brown to black puparia (the tough, hardened skins of last instar larvae). Each puparium is 9 to 10 mm long and 3 to 4 mm wide.
Feeding Habits -- Most blow fly larvae feed in carrion or other decaying organic matter. They often infest wounds of sheep, goats, cattle, and other animals. Unkempt sheep are particularly subject to attack. Adult blow flies are attracted to nectar, carrion, garbage, and other refuse and soggy, bloody or soiled hair, fur, or wool.
Damage -- Blow flies deposit eggs in castration and dehorning wounds or on dirty, wet wool. The larvae which soon hatch feed on decaying flesh or matted hair. Infested wounds often become inflamed and the hair or wool falls out. Blood poisoning may result.
As a rule, most blow fly larvae do not attack healthy tissue. Mature larvae of green-bottle flies, however, have been known to burrow deep into healthy tissue after spending earlier developmental stages in superficial wounds. Fortunately, this particular species is usually of minor importance in North Carolina.
Life History -- Blow fly biology varies among species and with environmental conditions so the following information is general. The black blow fly differs from the other two species covered here in that it is a cool weather insect which overwinters in the adult stage and is most numerous in early spring or in autumn. Other blow flies are abundant during the warm, humid, summer months and overwinter as larvae or pupae.
In general, each female blow fly deposits thousands of eggs over her 2- to 8-week life span. Egg masses may consist of 40 to over 1,000 eggs, but the larger masses are usually the result of oviposition by several females at the same location. Incubation may last 4 to 4.5 days, but hatching usually occurs in less than 24 hours when conditions are warm and humid.
Depending on temperature and the substrate upon which they are feeding, maggots usually complete development in 4 to 10 days. At the end of this period, larvae typically burrow in the upper centimeters of the soil and pupate for up to a week. Adult flies emerge from puparia and make their way to the soil surface. About 1 week later, females begin to deposit eggs and the life cycle is repeated. Blow flies usually develop from egg to adult in only 10 to 25 days and complete 4 to 8 generations each year.