Male Reproductive Behavior in the Anthomyiid Fly Hylemya alcathoe (Diptera)

Abstract

On early evenings in June males of the anthomyiid fly Hylemya alcathoe can often be found in large numbers at the edge of woodland clearings. In these locations the males perch on low-lying vegetation and dart out after passing insects. Mating pairs were occasionally seen on leaves. Each male tends to repel intruders from his leaf perch, but individuals rarely spend more than a few hours in any one spot. Some locations are far more attractive to males than others, apparently because of their proximity to conspicuous tree trunks and to overhanging canopy vegetation. The mating system of males of this fly appears to be analogous to that of those aerial swarming insects whose males gather over or by a prominent landmark and wait for receptive females to approach the marker-beacon. INTRODUCTION Males of the anthomyiid fly Hylemya alcathoe Walker form aggregations at certain times of the day. There are many examples of insects whose males become concentrated spatially in places where they may encounter females (reviewed by Downes, 1969; Sullivan, 1981). Certain mosquitoes, midges and other aerial swarming flies are familiar examples. Males of H. alcathoe do not, however, form aerial swarms but instead gather in favorite locations where they perch on leaves fairly close to one another. This paper describes the behavior of perched individuals, their interactions with one another, the daily cycle of aggregation formation and the properties of locations that attract relatively high densities of males. These data help define the mating system of the fly. Some speculations are presented on the adaptive significance for males of this and other insects that gather at visual landmarks to perch or swarm while waiting for females. MATERIALS AND METHODS Flies were observed beside dirt roads in forested locations from 15-22 June in both 1980 and 1981 at Lake Itasca, Minn., at the Biological Station of the University of Minnesota. Members of my classes in field ethology assisted in both years. The vegetation in the study area consisted of few scattered prominent trees and a low ground cover dominated by wild sarsaparilla (Aralia nudicaulis), a favorite perching substrate for the flies. Three sites where flies were consistently numerous were selected for intensive observation. Transects 10 m long by 1 m wide were divided into 10 1 m2 squares. Data were collected by walking past the transects and recording the number of flies in each square at various times of the day from 0700-2030 hr. A total of 50 censuses were conducted from 16-20 June 1981. The census results were used to determine the daily cycle of male perching activity and to establish whether certain squares within the transects were more attractive to perching flies than others. In addition to the census, between 15-18 June, 66 males were marked with dots of liquid paper or acrylic paint placed on the dorsum of the thorax. A few individuals were marked by daubing them while they perched on a leaf; most were netted, removed by hand, marked, then released. Although the flies were not large (ca. 7 mm long), it was possible to capture and mark individuals without damaging them. In the course of census counts, records were made of the location of any marked individuals to determine their site fidelity and longevity. Interactions between perched males were recorded to determine the tolerance of perched individuals to the arrival of an intruder on their leaf. An observer watched an