TYMPANAL ORGANS 143 



familiar studies in larvae of the Diptera, Lepidoptera, Coleoptera 

 and Tenthredinidae. 



In adult insects simple ehordotonal organs are likewise very 

 widely spread among different orders, where they occur chiefly 

 on the antennae, wings, halteres and legs. They are especially 

 constant in relation with the bases of the wings, and in this 

 position they are known to be present in all the major orders 

 of Pterygota. 



Johnston's Organs. Organs bearing this name are located in 

 the pedicel, or second segment, of the antenna, and are prevalent 

 in all the main orders of insects, and also in Lepisma among the 

 Thysanura. They are not the only ehordotonal organs present in 

 the antennae, and smaller groups of sensillae may occur in other of 

 the segments of those appendages. A Johnston's organ consists 

 of a group of usually very numerous ehordotonal sensillae disposed 

 in the form of a sheath around the antennal nerve, and when it is 

 highly developed, as in the Culicidae and Chironomidae, the pedicel 

 of the antenna is greatly enlarged. The distal extremities of 

 the sensillae are usually attached to the articulatory membrane 

 between the second and third antennal segments, and are indicated 

 by a series of pits or annuli in that membrane. Proximally, the 

 sensillae are directly connected by means of nerve fibres with the 

 main sensory nerve of the antenna (Fig. 60, C). 



Tympanal Organs. As their name implies, these organs consist 

 of a sound amplifier or tympanum developed in close relation 

 with one or more defined groups of ehordotonal sensillae. They 

 are present in grasshoppers and cicadas, which are well known to 

 be capable of producing loud and often shrill sounds, as well as in 

 some other insects whose sound-producing capacity is far less 

 evident. 



Some of the most highly specialised of these organs are found 

 in the saltatorial Orthoptera, but since their structure has so often 

 been described and figured, this aspect of the subject will not be 

 dilated upon here. It will be recalled that in the Acridiidae a tense 

 vibratory membrane or tympanum surrounded by a cuticular ring 

 is clearly visible externally on either side of the first abdominal 

 segment. In the Tettigoniidae and Gryllidae there is often a pair 



