NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 25 



the hair. This cavity is known as the hair can-al, or pore canal. The 

 hair canal is usually occupied by the outer ends of several cells that 

 have formed the distal cuticular parts. One of these is the tricho- 

 gene, another is the formative cell of the hair membrane, the mctn- 

 brane cell (MbCl), whWe others are unspecialized hypodermal cells 

 of the canal wall, being" more prominent where the hair is situated 

 on a tubercle (fig. ii, b) . The distal end of the membrane cell 

 normally surrounds the neck of the trichogene at the base of the 

 hair, though after the hair is formed at any molt the trichogenous 

 cell may withdraw from the seta and retract from its base, leaving 

 a vacuole beneath the seta (fig. 12, Vac). Especially is this true 

 in the adult stage. 



The various cells associated with the hair are not always easily 

 distinguishable in sections, and they have not always been included 

 in descriptions and figures of the cuticular organs ; yet, theoreti- 

 cally, we must assume that they are present, in most cases, though 

 perhaps variously modified. It is especially important to take them 

 into account, particularly the hair cell and the membrane cell, in any 

 study of the morphology of insect sense organs. 



SENSORY HAIRS 



A seta sensitized by a nerve connection is the commonest form 

 of insect sense organ. The innervation is always through a special 

 bipolar sense cell, the proximal process of which is continuous with 

 a sensory nerve, while the distal process is associated with the seta. 

 In adult insects the sense cell is situated within the hypodermis 

 (fig. 12, SCI), being limited internally by the basement membrane, 

 though its size or position may cause it to project into the body 

 cavity beyond the general level of the hypodermis (fig. 13 B). In 

 some larval insects, on the other hand, the sense cell may lie beneath 

 the hypodermis (fig. 11, SCI) some distance removed from the base 

 of the cuticular organ, with which it is connected by a long distal 

 process (d) that penetrates the basement membrane (BM). The 

 sense cells of adult insects are unquestionably modified hypodermal 

 cells ; the origin of the larval sense cells that lie beneath the hypo- 

 dermis has not been determined. The two sets of cells will, most 

 likely, prove to be homologous, for Zawarzin (1912 a) notes that the 

 sense cells of the sense organs of Lamellicorn beetle larvae lie in some 

 cases within the subsetal canal, in others in the hypodermis, and in 

 others beneath the hypodermis ; but, for the present, we apparently 

 must distinguish between intrahypodcrmal sense cells, and subhypo- 

 dermal sense cells. The latter constitute the subhypodermal sensory 

 cells of Type I, already noted. 



