(^ SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. // 



Berlese (1909) gives a drawing of a group of sensilla from the 

 organ of Johnston of a wasp, Vespa crabro, taken from a mature 

 pupa, in which he depicts the usual chordotonal structure (fig. 30), 

 each sensiUum being shown to consist of a cap cell {CCl), an en- 

 veloping cell (ECl), and a sense cell (SCI). The sense cell has a 

 long neck extending apparently through the enveloping cell into the 

 base of the cap cell, where it ends in a scolopala-like rod attached by 

 a long distal fiber from its apex to a cuticular pit between the second 

 and third antennal segments. Child had noted the presence of rods 

 in the organ of Johnston of the mosquito, but his description and 

 drawing (fig. 31 B) do not show clearly their relation to the sense 

 cells. Lehr describes the organ in the antenna of Dyfisciis nwrginalis 

 and his account of the structure of the sensilla agrees essentially 

 with Berlese's figure of that in the wasp. The enveloping cells, ac- 

 cording to Lehr, are not as well defined as in a typical chordotonal 

 organ ; the scolopal^e are simple fusiform rods, each continued at its 

 apex into a terminal filament attached to the cuticula. Neither writer, 

 however, shows the presence of an apical body in the scolopalas of 

 the organ of Johnston or ribs in their walls, though each rod is tra- 

 versed by an axial filament. 



An organ of Johnston of a primitive nature is described by Zawar- 

 zin (1912) and Eggers (1923) in the antenna of a dragonfly larva. 

 The organ here appears to consist merely of a circle of elongate sense 

 cells in the second antennal segment, the cells being attached by 

 their distal ends to the articular membrane between this segment 

 and the third. According to Eggers there are no enveloping cells 

 present in this organ, and scolopalse are not difl:'erentiated in the 

 sense cells, the distal parts of the latter having a fibrous textvu'e. 



In other insects the organ of Johnston varies much in its develop- 

 ment. Its bundles of chordotonal-like sensilla usually form a simple 

 cylinder within the pedicel of the antenna, as shown in longitudinal 

 section at A of figure 31. (The details of structure are probably not 

 well illustrated in this figure.) The organ reaches its highest degree 

 of development in the males of the families Chironomidse and Culi- 

 cidas, in which the second segment of the antenna is greatly enlarged. 

 The well-known illustration from Child (fig. 31 B) gives a general 

 idea of the appearance of the organ of Johnston in a longitudinal 

 section through the base of the antenna of a male of Corcthra (Moch- 

 lonyx) atUcifonnis. The sensilla of the organ are here not attached 

 in the usual way to pits of the articular membrane between the sec- 

 ond and third segments ; the membrane is chitinized to form a circu- 

 lar plate (a) attached to the base of the third segment (^Seg), from 



