The site of stridulation in Geotrupes (Geotrupidae): An unsolved problem?

Despite their size and familiarity, there still seems to be much uncertainty regarding the actual site of sound-production in those well-studied insects, the dor beetles. Thus, Fowler (1890) writes that the sound is produced by rubbing the back iof the hind femora against the abdomen; Jessop ( 1986) 'by rubbing the hind coxae together'. Both statements appear very questionable. The hind coxae are no l !naturally in contact and can hardly, or only by an effort, be made to touch one another, being separated by a space of a millimetre or two; nor do their inner surfaces appear to be furnished with the necessary ridges or stridulating-file. Britton (1956), on the other hand, denies that the sound is produced by the hind coxae as commonly stated, and in this he is unquestionably right (adding that a G. ,stercorosus deprived of hind coxae has been observed to stridulate, and remarking, with point,that the mechanism is not obvious).

If a live Geotrupes spiniger (or doubtless any other species) is firmly grasped in such a way as practically to immobilize the hind legs, the familiar high-pitched ,chirping will still be heard; at the same time the tip of the abdomen is seen to be moving rapidly up and down. If this part be then immobilized by pressure, stridulation ceases. The natural inference is that the sound is caused by fnction between the internal apical areas of the elytra and the propygidial area of the abdomen. However, if one removes an elytron from a dead specimen andexamines its inner surface and that of the exposed propygidium, no obvious stridulating mechanism is revealed. Presumably it must lie elsewhere.