258 ANIMAL BIOLOGY. [Part II. 



and this by a thorax divisible into a fore, mid, and hind seg- 

 ment (pro-, meso-, and metathorax). Posterior to the thoracic 

 region is the abdomen, in which ten segments may be made out, 

 while two small triangular podical plates, one on each side of the 

 vent, are held by some anatomists to represent an eleventh. 



The surface of the body and limbs is invested by a chitinous 

 exoskeleton, laminar in structure and secreted by a chitinogenous 

 layer of cells (hypodermis). Although the chitinous investment 

 is continuous, it is not everywhere of the same thickness or 

 consistency. Where mobility is required, it forms a yielding 

 cuticle, elsewhere it is thickened into harder and less yielding 

 plates (sclerites) or tubes. The whole cuticle, with its thickened 

 elements, is shed at the periods of ecdysis, a median dorsal 

 longitudinal fissure being formed in the thorax, through which 

 the insect escapes from its old skin. 



A front view of the head is shown in Fig. 80, A. In the 

 upper region is seen the epicranial suture (ep.), the lateral 

 branches of which end in small rounded white spaces (w. s.). Near 

 these spring the long antennce (an.) from an antennary fossa (an. /.). 

 External to these are the large compound (polymeniscous) eyes. 

 The mid-region of the face (clypeus, d.) narrows below somewhat 

 suddenly at the ginglymus,wheYQ the large mandibles (I. mn., r. mn.) 

 are articulated. The flap-like piece hanging from its lower 

 edge is the labrum (lb.), behind which lies the mouth. Below the 

 mandibles, one of which is represented as widely open, are the 

 maxillce (mx.), to which are attached long maxillary palps (mx.p.\ 

 The shorter palps (labial palps, I. p.) belong to the labiuin. 

 Looking down upon the head from above, there are seen on 

 either side of the mid-line the epicranial plates, separated by the 

 epicranial suture. On either side of these are, anteriorly the 

 eyes and posteriorly the gence, which, passing down behind the 

 eyes, form the sides of the exoskeleton of the head, reaching as 

 far as the mandibles, and ending somewhat pointedly above the 

 ginglymus. A posterior view of the head shows a large shield- 

 shaped occipital foramen, the lateral margin of which is 

 strengthened by a thickened rim continuous with the tentorium 

 or internal skeleton of the head. The top and sides of this rim 



