EXTERNAL ANATOMY OF INSECTS. 563 



this part must not be omitted, namely, that though in 

 many cases the mcdipectus and postpectus are perfectly 

 distinct and may be separated, yet in others, as for in- 

 stance the Lamellicorn beetles, the Hymenoptera and 

 Diptera, &c, no suture separates them; so that though 

 the upper parts, the mesothorax and metathorax, are se- 

 parable, the lower ones just named are not so. 



6. Peristethium*. The first piece of the medipectus is 

 what I have called, after Knoch, the peristethium h . This 

 immediately follows the antepectus ; on each side it is 

 limited by the scapulars, and behind by the mid-legs and 

 mesosternum. Its antagonist above is usually the dorso- 

 lum. In the Coleoptera Order it varies occasionally, 

 both in form and magnitude, but not so as to merit par- 

 ticular notice, except that both are regulated by the sca- 

 pulars — if these are small, the peristethium is ample; and, 

 vice versa, if they are large it is small. In all the fol- 

 lowing Orders, except the Hymenoptera, it is equally 

 inconspicuous, but in them it is often more remarkable. 

 I have a Brazilian species of Cimbex (C. mammifera 

 K. MS.) which appears undescribed, in which this part 

 swells into two breast-like protuberances, terminating 

 posteriorly in membrane, as if it had separate motion : in 

 the golden -wasps (Chrysis L.) it is anteriorly concave to 

 receive the coxce of the mid-legs ; and in Stilbwn, of the 



a Plates VIII. IX. n. 



b At first I had named this piece the antecosta, and the mesoste- 

 thium the postcosta ; and there is certainly some analogy between the 

 thorax of insects, consisting of several pieces that follow each other, 

 and the vertebral column ; between their three sternums and the ster- 

 num, and between their other pieces and the ribs of vertebrate 

 animals. Comp. Chabrier, ubi supr. c. iv. 49. note 1. 

 2 o 2 



