ANT-DECAPITATING FLIES

Ant-decapitating flies: nature's executioners
by Brian V. Brown
From Terra magazine, March/ April 1995, vol. 32 (1-2): 4.

Phorid flies live at the edge of our perception. Being tiny (1 to 5 millimeters long), they are seldom seen. Some are herbivores and scavengers, but others live out spectacularly gruesome lives as parasites and predators.

One of the most bizarre groups is the genus Apocephalus, whose species are commonly referred to as the ant-decapitating flies. Darting from above, an Apocephalus female lands on its ant victim, pierces it with a sharp, swordlike ovipositor, and lays a single egg inside the ant's body. Depending on the species of fly, the egg is laid in the head, thorax, or abdomen of the host.

Once the egg hatches, the fly's larva journeys through the internal tissues to the host's head (if it is not already there) and begins feeding on the muscles and tissues that fill the head capsule. Eventually, the ant's depleted head falls off, sometimes while the body is still living and walking about. In Costa Rica, we have found leaf-cutting ants still wandering along trails with their nestmates, in spite of the fact that there is nothing inside their heads except for a mature, fat phorid maggot.

Ant-decapitating flies are anything but a rare and obscure phenomenon: in one forest in Costa Rica I have collected eighty species, almost all of which are new to science. Many of these flies follow the roving packs of army ants in the rain forest; when the army ants raid the nests of other ants, driving them up out of the ground, the phorid parasites are waiting, ready to attack their hapless hosts when they are most vulnerable.

In North America, we have twenty-five species of these flies named and described, but many more await discovery. The known ant decapitators are found from Alaska to southern Texas, but the greatest concentration of species is found in the southwest, where their ant hosts are also most numerous.

Each species of ant-decapitating fly is apparently restricted to a single species (or, in some instances, a few closely related species) of ant host. My studies of the evolutionary relationships of the flies are allowing me to hypothesize the ways in which hosts and parasites have co-evolved over time. Closely related flies that attack relatively unrelated ant hosts probably came to specialize in their respective host species through random colonization, a type of evolutionary "accident." But closely related ant-decapitating flies that are parasites of closely related ant species suggest intriguing scenarios: it is possible that diverging lineages in the ant hosts were soon followed by evolutionary divergences in the fly species. Thus, when an ancestral ant species gave rise to two derivative new species, the ancestral parasite fly also may have soon developed two separate lineages.

There are many surprising discoveries to be made in the study of ant-decapitating flies. Last year in Costa Rica, I found the first species of Apocephalus know to parasitize stingless bees rather than ants. Hundreds of the tiny flies were harassing a mating aggregation of male bees, and from random collections I found that about half the bees were parasitized; each bee's body contained from three to twelve developing larvae, whose feeding was quickly killing the host.

The Natural History Museum of Los Angeles County collection of ant-decapitating flies, by far the largest in the world, is now supporting the work of ecologists attempting to use phorids to control pest species of ants. The numbers of one such pest, a fire ant introduced into the southeastern U.S., might be drastically reduced if parasitic flies for the ant's original home in Brazil were brought to North America. Dr. Donald Feener, Jr., and I have shown that phorids not only parasitize fire ants, but pester them until the ants are virtually unable to forage for food. Because of our study the U.S.D.A. - which has long believed the flies to be ineffective against ants - is now considering using phorids to control fire ants. The ant-decapitating flies are thus moving from a position of relative obscurity to become big players in the field of biological pest control.

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A female Apocephalus (ant-decapitating fly) has landed on a prospective host, a male stingless bee. Drawing by Jesse Cantley; from a photograph by Brian V. Brown.

The pupa of an ant-decapitating fly projects from the hollowed head of its victim. Photograph by Brian V. Brown.

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