494 



Comparatwe Animal Physiology 



The tympanic membrane, when located on the body, generally lies in 

 a cleft which forms an external meatus (Figs. 169 and 170), and it may 

 be further protected by the folded wings or the femur of the third leg, or 

 both. On the tibia there are usually two tympanic membranes on opposite 

 sides of the limb. In many genera these membranes are protected by heavy 

 folds which almost completely enclose them (Figs. 169, 170). In Gryllus 

 the membrane is completely exposed, but is associated with a very long 

 and prominent hair sensillum, which, on slight contact, evokes an immediate 

 avoiding response of the animal. 



The chordotonal organs are the sensory elements of the tympanic organ. 

 On the basis of morphological resemblance the term "chordotonal organ" is 

 also applied to similar structures which are not associated with a tympanic 



Fig. 170. Sections through thoracic tympanal organs of Catocala. A, Horizontal 

 section. B, Detail of organ on left side (after Eggers")- «, thorax; h, abdomen; c, tympanal 

 pit; d, tympanum; e, tracheal air sacs; f, chordotonal organs; g, nerve. From Wiggles- 

 worth." 



organ and, at least in some cases, do not have an auditory function. The 

 noncommittal term "scolopidia" is also applied to all "chordotonal" organs, 

 regardless of whether or not they are auditory. The non-auditory scolopidia 

 are widespread in the insect body and are found in the legs, antennae, palps, 

 wing bases, and the general body cavity. At least one end is fastened to 

 some pliable region of the cuticle, usually the intersegmental membrane. 

 Any movement, active or passive, of the animal which results in a relative 

 change in the position of two segments is likely to cause stimulation of such 

 scolopidia; they are also sensitive to external pressure, pressure or tension 

 of nearby muscles, and general changes of the internal pressure in the blood 

 or tracheal system. Those attached to tympanic membranes are distinctive 

 only in that they are attached to membranes particularly sensitive to vibra- 

 tion by sound waves in air. Vibration of the substrate which may cause 

 a relative motion of the segments of an insect's body or appendages may 

 cause stimulation of the intersegmental scolopidia. It is also possible that 

 some of the intersegmental scolopidia are auditory in function. 



The structure of a typical scolopidium is shown in Fig. 171. Each unit 

 consists of a distal or cap cell which surrounds a scolopale, or sense rod, to 

 which is attached the terminal filament of the sense cell. This terminal 

 filament is surrounded by a sheath cell. The sense cell is a primary neurone 

 with its cell body in the base of the scolopidium. These structures are 

 fundamentally the same in all scolopidia, regardless of whether or not they 



