Flight Activity of Insects within a Jamaican Cave: In Search of the Zeitgeber

Abstract

Flies of several families (Scatopsidae, Phoridae, Milichiidae) exhibit clear diurnal flight activity patterns in constant total darkness in Dromilly Cave, Jamaica. Most scatopsids were caught on adhesive traps during daytime (>95% between 0630 and 1830) with maximum catches about midday; phorids showed little evidence of cyclic activity although slightly more flew during the last half of the night and early morning (59% between 0000 and 1030); and most milichiids were caught during the day (>70% between 0630 and 1630) with slightly larger catches before midday. Apparently, those insects in the lightless region of the cave that display activity rhythms synchronize their flight activity to the diurnal cycle even though they are unable to see daily light intensity changes outside. Bats flew within the cave throughout the night with activity maxima at dusk and dawn, but they never flew during the middle of the day. We suggest that the flies used bat activity as a secondary zeitgeber. Additional key words: cave insects, Diptera, Scatopsidae, Phoridae, Milichiidae, bats, rhythm Small flying dipterans, most no longer than 2 mm, are abundant in many Jamaican caves with large bat populations (Peck 1975, 1992; Meyer-Rochow & Stringer 1997). Little is known about these insects except some aspects of their behavioral ecology (Stringer & Meyer-Rochow 1994) and their distribution in Dromilly Cave (Stringer & Meyer-Rochow 1996). Specimens representing the families Scatopsidae, Phoridae, and Milichiidae (in this text collectively referred to as cave flies) were taken to the Institute of Jamaica for comparisons with fly specimens held in the national insect collection, but identifications failed as the collections present were based on flies caught in the open. This suggested to us that the cave flies were absent, or at least very rare, outside caves. How little is known about Jamaican cave flies is highlighted by the fact that Matile (1994), in a recent review on cavernicolous dipterans of the world, does not list a single from Jamaica. One is inclined to think that the statement by Farr (1984) that nearly every recorded from Jamaica belongs to a separate phyletic line or group ... and only four groups are represented by more than one species (referring to Culicidae) could apply to cave flies as well. Most of the cave flies are saprophages, feeding on bat guano, carrion, and plant debris; a small number, most a Author for correspondence. Present address: Institute of Arctic Medicine, University of Oulu, SF-90220 Oulu, Aapistie 1., Finland. E-mail: vmr@sun3.oulu.fi notably streblids and culicids lead parasitic lives. Worldwide, 30 genera of scatopsids are recognized, but larvae are known for only 8 of them (Werner 1995). We found that the numbers of scatopsids caught in adhesive traps left in a permanently dark chamber of Dromilly Cave varied markedly on different dates and seemed to depend on the time of day. The results suggested to us that the scatopsid flies-and possibly others-might have a daily flight activity peak in the middle of the day, in spite of their permanently dark surroundings. Since extremely evolved cavernicolous animals ... lack circadian periodicities of locomotion (Lamprecht & Weber 1985) and are essentially non-rhythmic (Ford & Cullingford 1976), we investigated whether the Jamaican cave flies showed activity rhythms and, if so, what factor(s) might govern them. One possibility was that the insects' activity was related to the activity (or presence) of the bats within the cave, so the latter were also monitored.