Field Guide to Venomous and Medically Important Invertebrates ...

infested wounds, and a distinct, unpleasant odor may be detected. In some cases, the openings in
the skin may be small with extensive pockets of screwworm larvae beneath.
Figure 172. Screwworm fly, Cochliomyia hominovorax. Illustration: D. S. Kettle.
Larvae reach the terminal or 3 rd instar, exit wounds, and drop to the ground where they burrow
in the soil and pupate. After 3-5 days the flies are ready to mate. Females usually mate once.
Female New World screw worm flies oviposit up to 400 eggs in a single egg mass and one fly
may oviposit 6 to 8 batches of eggs in her life with each egg mass having about 100 to 250 eggs.
Eggs of the New World screwworm are creamy white and deposited in shingle-like fashion on
the border of a superficial wound into which the newly hatched larvae migrate and begin
feeding. Small screwworm larvae up to 0.08 inch (2 mm) in length hatch from the eggs in 8 to
12 hours.
Old World screwworm flies differ from the New World screwworm flies in several ways. One
distinguishing character in adults is the number of longitudinal stripes on the thorax. New World
screwworm flies have three prominent, black stripes while Old World screwworm flies have only
two stripes. A simple character useful for distinguishing between the larvae of the two species is
the number of finger-like processes on the anterior spiracles; Old World screwworm larvae have
4-5 processes, while New World screwworm larvae have 7-9 processes.
Wohlfahrtia magnifica