NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS— SNODGRASS 



to be only a simplified chordotonal organ, but inasmuch as it pos- 

 sesses distinctive features and probably has a specific function, there 

 is no particular reason for its demotion. 



The organ of Johnston consists of bundles of elongate sensilla 

 forming a cylindrical sheath about the antennal nerve trunks within 

 the pedicel. The distal ends of the sensilla are attached in groups, 

 corresponding with the bundles, to the articular membrane between 

 the second and third segments of the antenna. Usually the points 

 of attachment of the sensilla groups are marked by pits in the 

 membrane, which form a circle at the end of the pedicel. The base 



« Fig. 30. — Group of sensilla of organ of Jolmston of mature pupa of 

 a wasp, Vcspa crabro (Berlese, 1909). 



of the organ is connected by nerve fibers with the main antennal 

 nerves. 



The organ was described first in the antenna of a mosquito {Culcx) 

 by Johnston (1855), who gave, however, but a brief account of its 

 structure. Later it was more thoroughly investigated by Child (1894) 

 who found it in ten of the principal orders of insects. More re- 

 cently the organ named after Johnston has been studied by Berlese 

 (1909) and by Lehr (1914 a). The orders of insects in which an 

 organ of Johnston is now known to occur are the Odonata, Orthop- 

 tera, Hemiptera, Anopleura, Neuroptera, Coleoptera, Trichoptera, 

 Lepidoptera, Diptera, and Hymenoptera. 



The structural details of the organ of Johnston have been less 

 studied than have those of the true chordotonal organs, but there ap- 

 pears to be no radical difiference between the two sets of organs. 

 5 



