156 THE INTEGUMENTAL SKELETON OE THE IMAGO. 



Audouin's own words, when translated, are : ' If we wish to 

 study the anatomy of an insect's thorax, we ought, after 

 dividing it into three segments, to seek for a sternum on the 

 inferior surface of each ; for an episternum, a parapteron, and 

 an epimeron on the flank. We should also search for an 

 entothorax, a peritreme, and a trochantin. I say that we 

 should seek for them, not that we should find all these in each 

 insect ; very generally their union is so complete and intimate 

 that they cannot be demonstrated, but as in other cases these 

 pieces are present, it is more rational to conclude that the 

 same material is utilised in all than to suppose new creations 

 are perpetually occurring ' [39, p. 126]. Audouin's ideal seg- 

 ment of the exo-skeleton has been generally adopted as typical ; 

 and each segment of the thorax is said to consist of a sternum, 

 two lateral plates, the episternum in front and the epimeron 

 behind united by an oblique internal ridge, and of four dorsal 

 plates, one in front of the other, named respectively the 

 prescutum, the scutum, the scutellum, and the post-scutellum. 



The united episternum and epimeron are also termed the 

 pleuron, and the four dorsal plates form the tergum— hence a 

 segment is described as consisting of a dorsal arch, the tergum, 

 a ventral arch, the sternum, and of a pair of pleura between 

 the dorsal and ventral arches. 



The term ' pleuron ' has been unfortunately applied in quite a 

 different sense in the Crustacea— in these it means a lateral 

 prolongation of the dorsal arch, which forms the gill cover in 

 the decapods. By the rule of priority the term should certainly 

 be used as Audouin used it ; I shall therefore call the united 

 episternum and epimeron the pleuron, and suggest that the 

 pleuron of the Crustacea should be called the epipleuron. 



In the prothorax a sternum and a pleuron are still recog- 

 nisable, but it is doubtful, I think, how far these correspond 

 with the same parts in the meso- and metathorax. There is 

 usually a dorsal arch formed of one or more sclerites in all 

 three thoracic somites, but sometimes this is reduced to a 

 mere rim, and the homologies of the several dorsal plates of 

 the meso- and metathorax are all exceedingly doubtful. 



