24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. -jy 



THE STRUCTURE OF THE BODY WALL AND ITS APPENDAGES 



The foundation of the body wall is the ectodermal layer of cells 

 commonly known in insect anatomy as the hypodermis (fig. lo A, 

 Hy). The cells are lined on their inner surfaces by the basement 

 membrane (BM), which is either a homogeneous product of the 

 cells themselves, or is perhaps itself, in its origin, a cellular tissue 

 derived from the mesoderm. The outer surface of the hypodermal 

 layer is covered by the cuticula (Ct), a secretion from the cells con- 

 taining chitin as its characteristic component. In sections, the cuti- 

 cula usually shows a horizontally lamellate structure, and a differen- 

 tiation into an outer denser part, the epidermis, or cxocnticiila (Epd), 

 and a clearer, softer internal part, the dermis, or endocutictda (Dm), 



MbCl HrCl B 



Fig. 10. — Diagrammatic structure of the body wall, and of a tuber- 

 culate hair, or seta. 



A, piece of body wall bearing two setje. B, section of base of a seta 

 showing two cells associated with it, the seta-forming cell, or tricho- 

 genous cell (HrCl), and the cell of the hair membrane {MbCl). 



The commonest types of external appendages of the body wall 

 have the form of hollow hairs or setce. A seta (fig. lo B, Hr) is an 

 outgrowth of the cuticula formed by a large hypodermal cell, known, 

 on account of its special function, as a trichogcnous cell, or tricho- 

 gene (HrCl). The base of the seta is commonly attached by a mem- 

 branous ring, the hair mejnbrane (HrMb), to the surrounding cuti- 

 cula. Sometimes the hair stands on the surface of the body wall, 

 but more usually it is sunken into a cup or alveolus (Alv). The 

 rim of the alveolus may be flush with the general cuticular surface, 

 or it may be elevated to form a tubercle (Tu) supporting the hair 

 base. 



Beneath the hair and its membrane there is a large cavity in the 

 cuticula, open proximally, but distally prolonged into the hollow of 



