How they become aware of their surroundings 



response of the hairs of an insect is an incidental and unimportant 

 sideUne to their real purpose of being dehcate organs of touch. 



The special organs of hearing in insects are called tympanal organs, 

 because they have a tympanum, or drum, of thin skin, connected with 

 a group of special cells called chordotonal sensillae. The latter involve a 

 nerve-cell and several other cells, drawn out into a fibre-hke structure, 

 which is stretched between a fixed point on the body and a movable 

 one, such as the base of a hair, or the drum of the tympanal organ. 

 These sensillae respond to any change of tension — i.e. slight tightening 

 or slackening — by generating nerve-currents. Chordotonal sensillae are 

 not especially organs of hearing, but are found all over the body of an 

 insect, and are used to register any kind of movement or a force of any 

 nature: a sort of internal stress-gauge. 



Fig. 38. Surface view 

 of an antennal segment, 

 to show various sensory 

 hairs: a, a spine sensi- 

 tive to touch; 6, thin- 

 walled hairs (sensillae) 

 believed to react to 

 smell or taste; c, d, 

 thick-walled hairs, per- 

 haps reacting to changes 

 of temperature. From 

 Wigglesworth, 1953 



There is little rhyme or reason about which insects have tympanal 

 organs, and whereabouts on the body they are placed. The short-homed 

 grasshoppers have them on the first segment of the abdomen, but the 

 long-horned grasshoppers and crickets have them on the fore-legs. 

 Many bugs and moths have them in the thorax, but cicadas and some 

 moths have them in the abdomen; no butterflies have them at all. 



Like the hairs, the tympanal organs respond to sound, but it is not 



63 



