Biology of Microdon Fuscipennis (diptera: Syrphidae) with Interpretations of the Reproductive Strategies of Microdon Species Found North of Mexico

—Two hundred and ninety six adults, larvae, and pupae of Microdon fuscipennis were collected and/or reared from nests of the dolichoderine ant, Iridomyrmex pruinosus (Roger). Observations are made on the distribution of M. fuscipennis in the ant nests, sex ratio (1:1), adult emergence, mating, number of eggs laid (x = 63), larval emergence from the egg, and predation (third-instar fly larvae frequently eat ant larvae). Reproductive strategies for the genus Microdon are: 1) specialist strategy—one host species; and 2) generalist strategy—multiple host species. The species of Microdon found north of Mexico and their ant-associations are listed and used to predict the reproductive strategy of each fly species. Microdons are unusual syrphid flies. The larvae and pupae are domeshaped and develop in ant nests. The larvae exhibit slow, sluglike movements, a characteristic which originally caused them to be described as mollusks or coccids (Wheeler, 1908). As adults, microdons do not show typical syrphid behavior. They do not hover or visit flowers as most syrphids but spend their adult lives close to the ant colonies from which they emerged. More than 350 species of Microdon are known from all zoogeographic regions. The diversity, greatest in the tropics (especially the Neotropics, 174 species), tapers off rapidly towards the poles. The northernand southernmost records for microdons in the New World are Microdon albicomatus Novak from the Yukon (62°41'N) and Microdon violaceus (Macquart) from Chile (37°47'S). Microdons are considered primitive because they represent the first offshoot on the branch which includes all other syrphids (Thompson, 1969, 1972). The phylogenetic position and biologic distinctiveness of microdons clearly support the recognition of the group as a separate family (Thompson, 1969, 1972). For pragmatic reasons, however, microdons are left as an aberrant subfamily of the Syrphidae. Early reviews on microdons (Wheeler, 1901, 1908; Donisthorpe, 1927) VOLUME 83, NUMBER 4 717 were primarily descriptive, speculating on behavioral interactions between the larvae and their hosts. Andries (1912) first provided quantitative data on the life cycle of microdons as well as detailed descriptions of larvae, pupae, and adults. Greene (1955) added information on a number of Microdon-ant associations and described larval and pupal forms. More recently, Jordan (1968), and van Pelt and van Pelt (1972) contributed additional biological data (see Table 3). Akre et al. (1973) determined the sex ratio, size measurements, number of eggs laid per female, and the number of larvae and pupae per colony for two color morphs of M. xanthopilis Townsend (reported as cothurnatus), forming a sound basis for future comparative work on other Microdon species. The biology of microdons is not uniform. Akre et al. (1973) described only one generation per year. Microdon fuscipennis (Macquart) has at least two. Akre et al. (1973) also stated that microdons overwinter as third larval instars, yet these data indicate that this is not true for fuscipennis. Other unresolved questions exist. Are the microdon eggs laid in the ant nest or do the larvae move there? Do the microdon larvae eat the ant larvae and pupae? These questions are discussed with respect to M. fuscipennis which develops in the nests of the dolichoderine ant, Iridomyrmex pruinosus (Roger). Two alternative behavioral strategies for Microdon flies are described. Table 2 lists the species of Microdon found in America north of Mexico and their known hosts and predicts their reproductive strategy. MATERIALS AND METHODS Collection.—Microdon fuscipennis was collected primarily during the spring and summer near Athens, Georgia, from nests of Iridomyrmex pruinosus. Ant nests were excavated with a pen knife. Microdon fuscipennis larvae and pupae were placed in plastic pop-top vials for transport. Entire ant colonies were also transported back to the laboratory. Laboratory rearing.—The fly larvae were reared in plastic ant nests, exposed to natural daylight, and stocked with ant colonies dug in the field. The ants were maintained on honey and mealworms. Water was supplied by means of cotton plugs inside the nests. After the microdon larvae were observed eating the young ant larvae, additional ant larvae were added weekly to the colonies. As the microdon larvae grew and pupated, the pupae were removed and placed in vials. A wooden stick was placed in each vial allowing the teneral adults an elevated surface from which they could expand their wings. All live material was kept at 27°C. As the adults emerged, the pupal cases were removed from the rearing vials and placed in capsules. When an adult died, it was pinned along with the pupal case. 718 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON Table 1. Quantitative data on Microdon fuscipennis. Standard Mean Deviation n Number of M. fuscipennis larvae, pupae per 3.45 3.75 84 /. pruinosus colony Number of eggs laid per female 63.5 18.9 15