NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 



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distal process (d) of the sense cell. The third cell (MbCl) is evi- 

 dently the hair membrane cell, since its distal part embraces the end 

 of the trichogene and terminates against the hair membrane, when 

 the latter is present. 



In many sense organs, the hair membrane cell and the trichogenous 

 cell do not reach to the basement membrane, or the sense cell bulges 

 into the body cavity and comes to lie proximal to the hair cell. The 

 membrane cell, likewise, often lies distal to the body of the hair cell, 

 and thus the three cells may come to be arranged serially along a 

 radial axis, as shown at B of figure 13. 



6CC1) 



HrCl(=ECl) SCI Nv 



A 



Fig. 13. — Diagrammatic structure of a hair sensillum. 



A, showing the apparent origin of a sensillum from the hair mem- 

 brane cell (MbCl), the trichogenous cell (HrCl), and a hypodermal 

 sense cell (SCI). B, the three cells in axial arrangement, the mem- 

 brane cell having become the cap cell (CCl) of a typical sensillum, and 

 the trichogenous cell the enveloping cell (ECl). 



A simple sensory complex, comprising the cuticular parts of the 

 sense organ, the hypodermal elements, and the nerve, is known as 

 a sensillum. The membrane cell of a sensillum is called the distal 

 enveloping cell, or cap cell (fig. 13 B, CCl), and the trichogenous 

 cell is distinguished as the basal enveloping cell, or simply the en- 

 veloping cell {ECl). In some sensilla the single sense cell is replaced 

 by a group of sense cells (figs. 18, 24, SCls) ; and some sense organs 

 are compound, each consisting of a group of simple sensilla. 



The three cells of a sensillum appear to be partially telescoped one 

 within the other, the neck of the sense cell being contained within the 

 enveloping cell, and the distal part of the latter surrounded by the 



