28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 77 



tinuous with the hollow of the hair. Sihler does not distinguish in 

 this organ a specific membrane cell, but, as just noted, the alveolar 

 membrane is obliterated by the solid insertion of the hair. Several 

 hypodermal cells, however, surround the trichogene and apparently 

 contribute to the formation of the cup. At the side of the trichogene 

 toward the base of the cercus is the large, oval, intrahypodermal, bi- 

 polar sense cell (SCI). Its distal process, Sihler says, penetrates the 

 trichogenous cell and ends against the base of the hair. The proximal 

 process (Nv) is continued into a sensory nerve trunk after follow- 

 ing the basement membrane for some distance toward the base of 

 the cercus. The sense cell and its distal process are invested in a 

 nucleated continuation of the neurilemma of the nerve (Nlm). 



The structure of the setal sense organ depicted in the above de- 

 scription is typical of that of all the hair-like sense organs of adult 

 insects, (fig. 13 A, B) except that usually two cells are to be dis- 

 tinguished in addition to the sense cell, one of which is the tricho- 

 genous cell (HrCl or ECl), and the other probably the hair mem- 

 brane cell' (MbCl or CCl). 



THE FUNDAMENTAL STRUCTURE OF AN INSECT SENSE ORGAN 



A general survey of the structure of insect sense organs shows, in 

 a large number of cases, that a single organ, or a single element of 

 a compound organ, is formed of three cells or the multiples of three 

 cells. We are, therefore, warranted in believing that the foundation 

 structure of most types of insect sense organs consists of three hy- 

 podermal cells. 



The structure of a simple three-cell sense organ, in which the cuti- 

 cular part is of the hair type, is shown diagrammatically in figure 

 13 A. The most conspicuous element in the hypodermal part is the 

 sense cell (SCI). Proximally the sense cell is continuous with the 

 nerve (Nv), while distally it sends out a long, slender, terminal proc- 

 ess (d) that goes to the cuticular part of the organ, in the case of 

 a hair organ either attaching to the hair base or to the hair membrane, 

 or penetrating the cavity of the hair. The body of the sense cell and 

 at least the base of its distal process are invested in a thin, nucleated 

 membrane continuous with the neurilemma of the nerve. Lying be- 

 side the sense cell, or distal to it, and usually surrounding its distal 

 process, is a second cell, which, on account of its relation to the base 

 of the seta, is clearly the trichogenous cell (HrCl), though in the 

 adult organ its protoplasm is often retracted from the hair and from 

 beneath the hair base, leaving here a vacuole (Vac) containing the 



