462 



ARTHROPODA. 



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.). ex, coxa; d, 

 digestive tract: /, femur; 7i, heart; n, notum; pi, pleuron ; st, sternum; (, tibia; 

 (a, tarsus ; tr, trochanter. 



ranging between eleven (in some larvae 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 met at h or ax , 

 each composed of three elements, an 

 unpaired dorsal portion, notum; a pair 

 of lateral plates, pleura, and an unpaired 

 ventral sternum (fig. 484). For sim- 

 Fio.485.-Head of a grasshopper, plicity one speaks of pronotum, meso- 



' i f abia s i ; pafprrv'; sternum, etc., to indicate the portions of 



labrum; wid, mandible ; 'mpl fV, ~ e*vnarnfp apo-mpnt TTif> Vi^nrl ia a 

 maxillary palpi; mx, maxilla; tjl ts * 



o, occiput; v, vertex. continuous capsule in which the follow- 



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

 dorsal and posterior a vertex and an occiput; laterally gence, ven- 

 trally a gula. The appendages show that the head is composed of 

 at least four somites. 



The view that the head 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 

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

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



