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 prephragnia 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- 

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

 (Fig. 106). 



ThefurccB. — Each furca is an invagination of the body-wall arising 

 between the stemiim and the stemelliim (Fig. 1 1 1) ; when the sternel- 

 liim is obsolete, as it is in most insects, the furca arises at the caudal 

 margin of the segment (Fig. 112). 



Fig. Ill . — Ventral aspect of the 

 metathorax of Stenopelmatus. 

 The position of the furca 

 within the body is represented 

 by a dotted line. 



h. 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- 

 litmi of which it is a part in being larger 

 and in possessing the secreting and ex- 

 creting functions; others are multicel- 

 lular, 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. — Ventralaspectof the 

 m3so — and metathorax of 

 Grylltis; the positions of the 

 f ircae wit'iin the boiyare 

 indicated by dotted lines. 



