EXTERNAL ANATOMY OF INSECTS. 567 



medipectus from the postpectus ,- — the last type is where 

 it is a ridge formed by a process both of the peristethium 

 and mesostethium meeting between the legs ; an example 

 of this you will see in the common dung-chafer (Geo- 

 trupes Latr.). Upon the two first of these cases I shall 

 offer a few remarks ; — the last affording no variation 

 need only be mentioned. 



If you examine the terrestrial Predaceous beetles 

 (Cicindela and Carabus L.) you will find that the periste- 

 thium is usually flat, terminating towards the postpectus 

 in a kind of fork, the sinus of which receives the anterior 

 point of the mesostethium — this is the mesostemum ; but 

 in the aquatic insects of this tribe, at least in Dytiscus 

 marginalis, &c. the structure at first sight seems diffe- 

 rent, for apparently the prosternum is received by the 

 anterior fork of the mesostethium ,• but if you proceed to 

 separate the manitrunk from the alitrunk, you will find 

 that the true mesostemum of the usual form is quite 

 covered by this point, which curves towards the breast, 

 is longitudinally concave to receive the point of the pro- 

 sternum, and permit its motion in the groove. In some 

 Heteromerous beetles, as the Helopidce, &c. this part is 

 anteriorly bilobed, so as to form a cavity which receives 

 the point of the prosternum when the head is bent down: 

 in Helops uitens {Tenebrio Oliv.) this sinus represents a 

 crescent; in Cistela Ceramboides it is shaped like the 

 Greek letter y; in the Lady-bird (Coccinella L.) it as- 

 sumes nearly the shape of a Saint Andrew's cross ; in 

 Spheniscus K. a the mesostemum is wide, concave and 

 wrinkled, with an anterior and posterior sinus ; while in 



a Linn. Trans, xii. t. xxii./. 4. 



