THORAX 



lOI 



the most constant element of a thoracic segment, but it lias 

 sometimes the appearance of consisting of two parts, an anterior 

 and a posterior. The plem-on nearly always consists quite 

 evidently of two parts, the episternum, the more anterior and 

 inferior, and the epimeron.-' The relations between these two 

 parts vary miich ; in some cases the episternum is conspicuously 

 the more anterior, while in others the epimeron is placed much 

 above it, and may extend nearly as far forwards as it. It may 

 be said, as a rule, that when the sternum extends farther back- 

 wards than the notum, the epimeron is above the episternum, 



Fio. 55.— Mesothorax of Dytiscus, after Aiidouin. A, notum ; A', pieces of the notum 

 separated : a, praescutum ; b, scutum ; c, scutellum ; d, post - scutellum : B, the 

 sternum and pleura united ; B', their parts separated : a, sternum ; c, episternum ; 

 d, parapteron; e, epimeron. 



as in many Coleoptera ; but if the sternum be anterior to the 

 notum, then the episternum is superior to the epimeron, as in 

 dragon-flies. We would here again reiterate the fact that these 

 " pieces " are really not separate parts, but are more or less in- 

 durated portions of a continuous uitegument, which is frequently 

 entirely occupied by them ; hence a portion of a sclerite that in 

 one species is hard, may in an allied form be wholly or partly 

 membranous, and in such case its delimitation may be very 

 evident on some of its sides, and quite obscure on another. 

 1 See also Fig. 47 (p. 88). 



