Use of Host Aggregations as Mating Sites by Chrysagria Alticophaga (Diptera: Sarcophagidae)

Females of the newly described species Chrysagria alticophaga Lopes and Achoy oviposit on larvae of the chrysomelid beetle Altica sp. (Lopes and Achoy 1986). Each parasitized beetle larva produces a single fly larva. Adult beetles often form loose aggregations of up to several hundred individuals on their host plants. This note reports observations of adult C. alticophagaflies at two such adult aggregations in Costa Rica. Male flies apparently use beetle aggregations as sites to encounter females. Observations were made on 24-25 Jan. 1987 on Cufea sp. (Litraceae) plants at the Wilson Botanical Garden near San Vito de Java, Puntarenas Province, and 23-25 Nov. 1988 on a single Ludwigia octovalvis (Onagraceae) plant in San Rafael de Escazu, San Jos6 Province. Flies were identified by comparing their genitalia with the figures in Lopes and Achoy 1986. The beetles (all of the same species) cannot at present be determined to species (S. Shute and R. White, pers. comm.); voucher specimens are deposited in the U.S. National Museum, the British Museum (Natural History), and the Museo National of Costa Rica. During the day the beetles in both aggregations were relatively dispersed on one or a few plants, with the estimated 100-300 beetles in each aggregation spread over 1-2 square meters. In both aggregations C. alticophaga flies were found perched among the beetles on the plants when the aggregations were checked during the day. The San Vito aggregation was also checked on two nights; the beetles were more tightly clumped and there were 4-6 flies perched among them. Observations of individual flies late in the morning at both sites showed that they spent most of the time motionless or grooming, perched near the tips of small branches or at the tops of plants. They