Walter Ritchie 



305 



The Head (Fig. 4). 



If the head of the larva be dissected out from the first thoracic seg- 

 ment and examined under the binocular microscope, the following parts 

 are seen : in the centre of the field is the triangular region of the frons (/), 

 bounded on each side by the frontal suture (fs), and divided into two by 

 the median suture (ms). 



At the base of the triangular frons is a narrow area, the epistome (eps), 

 on each side of which is a very highly chitinised area, thepleurostome(p>-). 

 Anterior to the epistome is the clypeus (c), trapezoidal in shape and 



et.~ 



rt.— 



Fig. 5. Left mandible of larva, S. carcharias (greatly magnified). a = dorso-lateral view 

 of mandible; b— ventral view of mandible; c=chitinous plate; e/= extensor muscle; 

 rt = retractor muscle. 



bearing a few bristles on the sides; attached to the clypeus anteriorly 

 is the bristly labrum (I) which is almost semicircular in shape. 



On each side of the clypeus and labrum can be seen, in part, the 

 mandibles (m). Each mandible (see Fig. 5) is strong and robust, shiny- 

 black in colour and highly chitinised. Their general form is triangular 

 and their inner cutting edges are produced into two blunt teeth. Each 

 mandible is worked by two powerful muscles, an extensor (et) attached 

 to the dorsolateral surface of the mandible and a retractor (rt) to its 

 inner or ventral surface. The retractor muscle is far the stronger of the 

 two. Embedded in each muscle is a thin sheet of chitin (c). 



