462 



ARTHBOPODA. 



strictions: head, thorax, and abdomen. The number of abdomi- 

 nal somites, varies with the order and even with the family, 



Fig. 484.— Schematic section of a hexapod through the thorax (orig.). cr, coxa: d, 

 digestive tract: /, femur; h, heart; n, notum; pi, pleuron ; &t, sternum; t, tibia; 

 (a, tarsus ; (r, trochanter. 



ranging between eleven (in some larvfe and embryos twelve) in 

 the Orthoptera and five in many Diptera. Each cuticular abdo- 

 minal segment consists of two plates, 

 tergite (dorsal) and sternite (ventral), 

 united on the sides by a softer mem- 

 brane which contains the spiracles. 

 Head and thorax, on the other hand, 

 have a constant number of somites. 

 The thorax is plainly divided into three 

 segments, pro-, meso- and meiatliorax, 

 each composed of three elements, an 

 unpaired dorsal piortion, notum; a pair 

 if lateral plates, pleura; and an nnpaired 

 ventral sternum (fig. 484). For sim- 

 plicity one speaks of j^ronotum, meso- 

 sternum, etc., to indicate the portions of 

 maxXry%t'ip??';;??:'nltiiiTr: the Separate segments. The head is a 

 ", occiput; I', vertex. contintious capsnle in which the follow- 



ing parts are recognized: in front and dorsal dypeus and frons; 

 dorsal and posterior a vertex and an occiput; laterally gc)w, ven- 

 trally a ijula. 'J'he appendages show that the head is composed of 

 at least four somites. 



The view that the he.ad consists of six somites is based on the existence 

 of two more segments without appendages in the embryo, a preantennal 

 and a postantennal (intercalary, premandibular), as well as the knowledge 

 tliat the brain, in which formerly only antennal ganglia were recognized, 

 consists of three pairs of ganglia (proto-, deuto-, and trito-cerebrum). 



Fig. 48.5.— Head of a grasshopper, 

 c, clypeus; /, frons; (/, gena; t, 

 labium; Ip, labial palpi; ir, 



