78 THE BIOLOGY OF INSECTS 



circular canals of a vertebrate's ear, the motions of the insect 

 being responses to nerve-impulses that result from its 

 position with respect to a plane vertical to the surface of the 

 earth. 



Insects belonging to the large order of the Diptera or 

 two-winged flies have the hindwings reduced to small drum- 

 stick like rods known as halters. Many experimenters have 

 shown that if one or both of these organs be cut away from 

 a fly's thorax, the insect is no longer able properly to control 

 its flight. Microscopical investigation of the base of the 

 halters made by B. T. Lowne (1890) and others shows that 

 they contain numbers of the remarkable sensory structures 

 which have been observed and described by V. Graber 

 (1882) and many subsequent observers, in various parts of 

 insects of diverse orders, and called chordotonal organs. 

 We may therefore regard it as highly probable that the 

 function of some chordotonal organs at least is to receive 

 impressions from the movements of the surrounding blood 

 which lead to " equilibrating " sensation when passed 

 on to the central nervous system. Recent investigations 

 into the minute structure of these organs by W. N. Hess 

 (19 1 7) have shown that they consist essentially of elongate 

 spindle-shaped sense-cells whose axes are prolonged as 

 aflFerent fibres while their distal ends are in contact with 

 peg- like scolopales enveloped by accessory cells, the whole 

 organ surrounded by a delicate outer sheath (Fig. 26). 

 The tips of the scolopales are surrounded by cover-cells, 

 and these delicate sensory endings may either float freely in 

 the blood-spaces of the insect's body or be connected by a 

 terminal ligament with some part of the cuticle (Fig. 26, 

 A, Z). In either case the direction and pressure of the 

 surrounding fluid in contact with the chordotonal organ 

 will vary according to the position of the insect's body in 

 relation to the horizontal and vertical planes, so that 

 such organs afford the necessary mechanism for inducing 

 equilibrating sensations. 



But chordotonal organs have generally been regarded as 

 connected with the sense of hearing, and there can be no 



