914 



INSECTA. 



The pro-thorax, as we have stated, is com- 

 posed of four sub-segments, which on its upper 

 surface, or pro-notiun, are generally confluent, 

 more particularly in Coleoptera, and form a 

 smooth, uniform, and often very broad surface. 

 In shape, the pro-notum is usually more or less 

 quadrate and convex, with its sides arched and 

 dilated. In many species, as in some families 

 of Coleoptera, Orthoptera, and Ilomoptera, 

 the pro-notum is larger than the corresponding 

 part of any segment of the body, being consi- 

 derably broader and longer than the head. 

 This is the case more especially in those in- 

 sects which are much employed in burrowing, 

 as in GryUotalpa, Geotrupidte, Coprida, and 

 Silphidte. Sometimes, as in the Cercopida, it 

 is enormously enlarged. In Membracis foliata 

 it is developed in the median line into an ele- 

 vated crest like that of a helmet, which is not 

 only extended forwards so as completely to 

 conceal the head, but also laterally and back- 

 wards over the whole body. In others of the 

 same family, as in Ledra, it resembles an acute 

 triangle, its sides being developed into two ob- 

 tuse processes, while it is elongated backwards 

 like an acute spine, which completely covers 

 the abdomen. In other instances, as in Dynas- 

 tidie, (fig' 333,) it is developed into a strong 

 horn or process, which is as long as the whole 

 body. In each of these instances, the abnormal 

 form and size depend either upon the exces- 

 sive development of the whole of the sub- 

 segments, as in Gryllotalpa, or upon one or 

 more of them, as in Dynattei, since in those 

 species in which the parts of the pro-notum are 

 all nearly equally developed, and are of moderate 

 size, their lines of separation are very distinctly 

 marked, as in the common green grasshopper, 

 Acrida viridmima. The pro-sternum, or under 

 surface of the pro-thorax, isconsiderablyshorter 

 than the pro-notum. In Dyticus circumflexus 

 (Jig. 384 A), the species selected by Audouin 



Fig. 384. 



to each acetabulum is a broad and somewhat 

 triangular-shaped plate, the episternum (2./). 

 This part is united by suture at its anterior 

 margin to the extended part of the sternum, by 

 its superior border to the dilated margin of the 

 pro-notum, and by its posterior to the epimcron. 

 It is a very distinct piece, and does not enter at 

 all into Ihe formation of the acetabulum, as it 

 appears to do on a cursory examination. The 

 third piece, the epimeron (2 A), is that which is 

 always connected with the coxa, or basial joint 

 of the leg. In this species it is a narrow plate, 

 situated posteriorly to the episternum, and 

 forms the posterior margin of the sternal surface 

 of the pro-thorax. At its sternal end it has a 

 short process, that forms the outer margin of 

 the acetabulum, and articulates both with the 

 sternum and episternum. It is probable that a 

 portion of this process is the proper trochuntin 

 of the leg on each side, since the part, which 

 has been described by Audouin as the tro- 

 chantin in the meso- and meta-thoracic seg- 

 ments, has not been delineated in his drawing 

 of the pro-thoracic. In Dyticus marginal* 

 there is a mark upon the process which resem- 

 bles a suture, and which still further induces us 

 to believe that this part is the analogue of the 

 trochantin. Within the cavity of the pro- 

 thorax, extending upwards from their attach- 

 ment to the pro-sternum, are two bony rami, 

 which at their inferior extremity are developed 

 into two rounded plates (2 s), that form a col- 

 lar, or leave a circular hole between them for the 

 passage of the spinal cord. They constitute 

 the unte-furca, the e&to-thorax of Audouin. 

 These are the parts that enter into the formation 

 of the pro-thorax, exclusive of the anterior pair 

 of legs, the only appendages of this segment. 



Fig. 385. 



A, under-mrface of the first tegment of the 

 thorax or pro-stermtm of Dyticus circumjtexus , 

 (Audouin); 2 g, pro-sternum; 2 /, episternum; 

 2 h, epimeron ; 2 s, ante-furca or ento-thorax. 



for the purpose of illustrating the anatomy of 

 the thorax in Coleoptera, it is divided into 

 three distinct pieces. The sternum, or largest 

 piece, (2 g,) is situated in the middle line, and 

 is of a triangular form. It is extended on 

 each side, at the anterior part of the segment, 

 into two processes, which articulate at their ex- 

 tremities by a distinct suture with the produced 

 margins of the pro-notum. Posteriorly to these 

 it is developed in the median line into a spine 

 or crest, on each side of which it is hollowed 

 out to form part of the acetnbula, into which the 

 coxa; of the anterior legs are inserted. External 



Part of the mesa-thorax. (Audouin. ) 

 A, meso-sternum ; 3 a, praescutum ; 3 b, scutum ; 

 3 c, scutellum ; 3 d, post-scutellum ; 3 e, parapte- 

 ron ; 3 a, meso-sternum ; 3f, episternum ; 3 h, epi- 

 meron ; 3 s, medifurca, or ento-thorax. 



The meso-thorax, (Jig. 385,) or third segment 

 of the body, is usually less developed in this 

 order than the pro-thorax, with which it is freely 

 articulated by a strong membrane. It is, as its 

 name implies, the middle portion of the tho- 

 rax, and in most instances its division into four 

 sub-segments is distinctly marked on its dorsal 



