The screwworm, more formally known as the New World Screwworm, is the parasitic maggot of the Cochliomyia hominivorax fly, which is a type of blowfly. It infests mammals, including humans (hominivorax is Latin for 'devouring of man') and differs from most maggots in that it eats living flesh rather than the decaying tissues of a dead animal. Freezing temperatures and prolonged cold weather kills the screwworm, so its range is restricted to warm subtropical or tropical regions, specifically the southern U.S, Mexico, Central America and parts of the northern countries of South America. It also makes seasonal intrusions into temperate areas. As a pest to cattle, screwworms have a devastating economic effect. The health threat to humans is also significant.
The larvae hatch in less than 12 hours from eggs laid near open wounds or open body cavities such as the eyes, ears, nose and mouth, anus, vagina and the navels of the newborn. They crawl into the wound or orifice and begin to feed by ripping into the soft tissue with a pair of tiny fang-like hooks and then lapping up the released fluids. They burrow downward, deepening and widening the wound as they grow in size and destroy more tissue. Their name comes from the physical appearance of the larvae in the second and third stages of larval development: the body tapers from a flat and round rear end down to a pointed front area that is equipped with feeding parts. The flat rear has two organs for breathing, called spiracles.
Up to 400 eggs can be laid each time. The infected wound ravaged by the first invasion then becomes even more attractive to other gravid flies, so re-infection occurs repeatedly until the animal is eventually overcome and dies. A single wound can contain as many as 3,000 larvae. The screwworms become packed together in dense clusters with only their rear parts visible. They tend to burrow deeper when disturbed, but screwworms are ectoparasites and remain close to the surface of the host. Under magnification, their bottoms look much like smiley faces packed into a honeycomb.
After a week or so, the larvae mature, drop out of the wound and burrow an inch or two into the ground for pupation, the final stage in which worm turns into fly. After another week, the fly emerges, dries its wings, and seeks nourishment and a mate.
The adult fly is twice as large as a common house fly. It is easy to recognize by its big reddish-orange eyes. If you see one and want to score points with your cohort, first make sure that what you have is not actually just a C. C. macellaria. You can tell the difference because hominovorax has a middle line that stops just short of the prothorax, while macellaria does not. The females live for about a month, during which time they can deposit about ten egg masses. Females mate once, while the males mate a number of times. The single mating by females is a weakness that is exploited as a way to eradicate the screwworm as explained immediately below.
Bye, bye screwworm
The US Department of Agriculture launched an eradication program, beginning in the state of Florida in the mid-1950s. The program applied the sterile insect approach. Large masses of screwworm flies were raised in special facilities and sterilized by exposure to radiation. The sterile flies were then released in huge numbers in areas where the screwworm had established itself. The sterile males mated with wild females, which then produced no eggs. In a short time, this highly effective program had broken the insect's life cycle and completely eliminated the screwworms. The USDA worked its way westward, finally eliminating the screwworm from the U.S. altogether. They extended the program into Mexico and then into Costa Rica, with the cooperation of the governments of those countries.
Species: Cochliomyia hominivorax (screwworm fly)
Family: Calliphoridae (blowflies)
Suborder: Cyclorrapha (flies)
Order: Diptera (mosquitoes and true flies, with one pair of wings)
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vs. the USDA