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The Screwworm, Cochliomyia hominivorax
, is an obligate parasite (meaning it must pass part of its life
cycle in a living host) of warm blooded animals, including humans.
The species name, hominivorax, in fact means "man-eater"
and was given by the French physician, Dr. Charles Coquerel, in
1858 describing the species that he identified as responsible
for the deaths of hundreds of prisoners of the Devil's Island
penal colony in French Guiana. Once an animal becomes infested
with screwworm, death almost inevitably results unless the wound
is cured.
The female fly typically lays an egg mass containing 200-300 eggs
on or near an open wound on a living animal (see the photo). The
eggs hatch in 10-12 hours and the first stage larvae (also known
as maggots or worms, each approximately 0.04" or 1 mm in
length) crawl into the wound and begin tearing at the host's tissue
with a pair of sharp mouth hooks. The larvae feed on the resulting
liquid that oozes into the wound. Once a wound has become infested
with larvae, it also becomes more attractive to female screwworm
flies ready to lay eggs. As a result, a wound may become infested
with hundreds to thousands of larvae. The larvae feed in the wound
for about 5 days and pass through two additional stages before
they crawl out of the wound and drop to the ground. Once on the
ground, the larvae (now approximately 0.2-0.7" or 6-17 mm
in length) burrow into the soil to a depth of about 1-2 inches
and form the pupal stage.
After about 7 days, the adults (flies) exit the pupae and make
their way to the surface of the soil. Both sexes range in color
from dark metallic blue to metallic bluish green to metallic green
and have three dark longitudinal stripes (the middle stripe slightly
shorter than the outer stripes) on their backs between their wings.
Both sexes are larger than house flies with males being about
0.4-0.5" (10-12 mm) and females about 0.3-0.4" (8-10
mm) in length. They then expand and dry their wings and fly away.
After about 3 days the males and females mate and females begin
the cycle of laying eggs on the living hosts. Females may be capable
of laying a mass of eggs once every three days for up to 10 or
11 times during their approximately month long adult life span.
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In this experiment, there are
1,000,000 female and 1,000,000 male flies at the start of
this experiment. The cost of damage to livestock is staggering
if the flies are not controlled. If only 10% of the flies
mate and lay eggs:
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1,000,000 |
Fertile female flies |
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x 0.1 |
10% |
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100,000 |
Number of flies capable of killing cattle |
Continue the calculation in your Lab
Report to determine the cost of damage. |
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Close-up of a screwworm larva. Tusklike mandibles protruding
from its mouth tear the flesh of living warm-blooded animals.
A wound may contain hundreds of larvae. |
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Screworm fly adult |
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