The Prey Of A Population Of Machimus Atricapillus Fallen (Diptera)

Abstract

THOUGH Dr. B. M. Hobby (1931, Trans. ent. Soc. S. Eng. 6 : 1-42) was able to collect 91 records of Machimus atricapillus Fallen with prey, no previous attempt has been made to study a population of the species in one locality.Therefore, when in Cornwall in August, 1941, I found the species to be common in a stretch of about 400 yards of a hedge bordering a wood a t Lelant, I collected as many specimens with prey as I could.The fly was rarely seen except on sunny days. It was found on branches and trunks of trees, telegraph poles and sunny leaves of plants from 1 to 10 feet off the ground. The favourite attitude taken up by the fly was to lie on one side with legs outstretched. Whether this was to enable the sun to shine along the whole length of the body or to keep better watch I could not tell. Sunny patches were preferred for resting and invariably the prey was brought to a sunny spot. The Asilid would alight on a telegraph pole on which were massed a hundred or so Musca corvina, 50 Pollenia rudis and perhaps three or four of its own species. Though several hours were devoted to the watching of the species I did not once see an attempt to capture a resting fly. All captures were flying insects, sometimes made by means of a " capture dart )' but occasionally when just about to alight after a patrol. At these times I imagine the Machimus would suddenly catch sight of a fly alighting and drop upon it. Though movement brought death to the victim, speedy and erratic flight brought safety. The commonest insects flying a t the time were Musca corvina, Pollenia rudis and Eumerus tuberculntus Rondani. Eumerus has a speedy erratic flight as it seeks its host plants. Musca, though fast, is steadier in flight, and Pollenia is fairly slow. No Eumerus was seen to be captured, but Pollenia was a frequent victim. Other slow flyers captured included Xyrphus balteatus, Melanostoma scalare and Beris vallata. Capture darts and flying attacks were made a t pieces of passing, falling leaves and at large wasps, but on close approach the Asilid would not press home its attack. As to colour of prey, both dull and brightly-hued insects were taken.Several times I had a pill-box over captor and prey almost as they alighted and the Asilid generally dropped as if dead. Though the victim would also slide inert over the floor of the box, the Asilid alone recovered. This feigning of death was not a regular habit although it occurred several times.