Observations on the Cryptic Behaviour of Machimus atricapillus (Fln.) (Diptera: Asilidae)

OBSERVATIONS

MACHIMUS

ON

THE

CRYPTIC

ATRICAPILLUS

BEHAVIOUR

OF

( F L N . ) (DIPTERA:

ASILIDAE) A . G . IRWIN

ON 22nd September, 1975, while collecting in Butt Plantation near Mildenhall, West Suffolk (Grid Ref. 52/7374), I observed a female Machimus atricapillus (Fln.) hunting. T h e fly was using an oak trunk as a perch and occasionally darted into the grass to try (unsuccessfully) to catch prey. After each dart, the fly returned to the same height on the oak trunk and adjusted the orientation of its longitudinal body axis, so that it was at right angles to the sun's rays. It also bent its legs to make its body lean over so that the edge of the right or left wing (whichever was lower) almost touched the bark. The accompanying figures will help explain these two manoeuvres. Because of this behaviour the fly cast no shadow and thus was difficult to see.

C R Y P T I C BEHAVIOUR OF M A C H I M Ăœ S A T R I C A P I L 1 . U S

119

This sort of behaviour is well-known in the Stone-Curlew, Burhinus oedicnemus (L.). Flies in the families Muscidae, Therevidae and Asilidae which live on open ground can be seen (with difficulty!) spreading out their legs, thus bringing their bodies closer to the ground and causing less shadow. All of these, however, are orientating on flat ground, whereas the Machimus was resting on a vertical surface. The open ground flies (and the Stone-Curlew) orientate by turning until equal amounts of light enter both eyes, i.e. the body axis points towards the sun, and then they simply squat. But Machimus must orientate until a maximum difference between the light amounts for each eye occurs, and then complete an asymetrical manoeuvre. It would be interesting to know whether Machimus can orientate successfully on open ground, and if so, whether it uses the same system.

A. G. Irwin, The Museum, High Street, Ipswich.