CHAPTER II. 

 THE EXTERNAL ANATOMY OF INSECTS 



I. THE STRUCTURE OF THE BODY-WALL 



a. THE THREE LAYERS OF THE BODY-WALL 



Three, more or less distinct, layers can be recognized in the body- 

 wall of an insect: first, the outer, protecting layer, the cuticula; 

 second, an intermediate, cellular layer, the hypodermis; and third, an 

 inner, delicate, membranous layer, the basement membrane. These 



layers can be distinguished 

 only by a study of carefully 

 prepared, microscopic sec- 

 tions of the body-wall. 

 Figure 41 represents the ap- 

 pearance of such a section. 

 As the outer and inner layers 

 are derived from the hypo- 

 dermis, this layer will be 

 described first. 



The hypodermis. — The ac- 

 tive living part of the body- 

 wall consists of a layer of cells, 

 which is termed the hypo- 

 dermis (Fig. 41, h). 



The hypodermis is a portion of one of the germ-layers, the ectoderm. In 

 other words, that portion of the ectoderm which in the course of the development 

 of the insect comes to form a part of the body- wall is termed the hypodermis; 

 while to invaginated portions of the ectoderm other terms are applied, as the 

 epithelial layer of the tracheae, the epithelial layer of the fore-intestine, and the 

 epithelial layer of the hind-intestine. 



The cells of which the hypodermis is composed vary in shape; but 

 they are usually columnar in form, constituting what is known to 

 histologists as a columnar epithelium. Sometimes the cells are so 

 flattened that they form a simple pavement epithelium. I know of 

 no case in which the hypodermis consists of more than a single layer 

 of cells ; although in wing-buds and buds of other appendages, where 

 the cells are fusiform, and are much crowded, it appears to be irregu- 



(29) 



Fig. 41. — A section of the body-wall of 

 an insect: c, cuticula;/?, hypodermis; 

 bm, basement membrane; e, epidermis, 

 d, dermis; tr, trichogen; s, seta. 



