98 



AN INTRODUCTION TO ENTOMOLOGY 



thorax, and arise differently in different insects, these terms are mis- 

 leading. No phragma is borne by the prothorax; the mesothorax 

 may bear two and the metathorax one, or the mesothorax one and the 



metathorax two. A more definite 

 terminology is that used by Snod- 

 grass ('09) by which the anterior 

 phragma of any segment is termed 

 the pre phragma of that segment, 

 and the posterior phragma of any 

 segment is termed the postphragma 

 of that segment. 



The lateral apodemes. Each lat- 

 eral apodeme is an invagination of 

 the body-wall between the epister- 



Fig. in . Ventral aspect of the 

 metathorax of Stenopelmatus. 

 The position of the furca 

 within the body is represented 

 by a dotted line. 



num and the epimeron. The lateral apodemes are referred to above 

 (Fig. 106). 



The furc&. Each furca is an invagination of the body- wall arising 

 between the sternum and the sternellum (Fig. in); when the sternel- 

 lum is obsolete, as it is in most insects, the furca arises at the caudal 

 margin of the segment (Fig. 112). 



b. THE HYPODERMAL. GLANDS 



A gland is an organ that possesses the function of either trans- 

 forming nutritive substances, which it derives from the blood, into 

 some useful substance, as mucus, wax, or venom, or of assimilating 

 and removing from the body waste 

 material. 



The different glands vary greatly in 

 structure; many are unicellular, the 

 gland consisting of a single cell, which 

 differs from the other cells of the epithe- 

 lium of which it is a part in being larger 

 and in possessing the secreting and ex- 

 creting functions; others are multicel- 

 hdar, consisting of more than one cell, 

 usually of many cells. In these cases 

 the glandular area usually becomes 

 invaginated, and provided with an 

 efferent duct; and often the invagination is much branched. 



The glands found in the body of an insect can be grouped under 

 three heads; the hypodermal glands, the glands of the alimentary 



Fig. 112. Ventral aspect of the 

 meso and metathorax of 

 Gryllus; the positions of the 

 furcae within the body are 

 indicated by dotted lines. 



