COMPOSITION OF THE INSECT-CRUST. 11 



ing the lowest wingless forms such as the Louse, Flea, 

 Podura, and Bark-lice, where we see a transition to the Or- 

 ders of Spiders and Myriapods, that we can perceive the plan 

 pervading all these forms, uniting them into a common 

 class. 



A segment of a winged six-footed insect (Hexapod) consists 

 typically of eight pieces which we will now examine more 

 leisurely. Figure 12 represents a side-view of 

 the thorax of the Telea Polyphemus, or Silk- pt 

 worm moth, with the legs and wings removed. 

 Each ring consists primarily of the tergum, the 

 two side-pieces (epimerum and episternum) and 

 the sternum, or breast-plate. But one of these 

 pieces (sternum) remains simple, as in the lower orders. The 

 tergum is divided into four pieces. They were named by Au- 

 douin going from before backwards, 

 the prcescutum, scutum, scutellum, 

 and postscutellum. 



The scutum is invariably present 

 and forms the larger part of the 

 upper portion (tergum) of the tho- 

 rax ; the scutellum is, as its name 

 indicates, the little shield so promi- 

 nent in the beetle, which is also 

 uniformly present. The other two tr te c" tr c'" tr 



pieces are usually minute and 



crowded down out of sight, and placed between the two oppos- 

 ing rings. As seen in Fig. 11, the praescutum of the moth is 

 a small rounded piece, bent vertically down, so as not to be 

 seen from above. In the lowly organized Hepialus, and some 



FIG. 11. Tergal view of the middle segment of the thorax of Telea Polyphemus, 

 prm, praescutum; ms, scutum; scm, scutellum; ptm, postscutellum; pt, patagium, 

 or shoulder tippet, covering the insertion of the wings. Original. 



FIG. 12. Side view of the thorax of T. Pohjphemus, the hairs removed. 1, Pro- 

 thorax ; 2, Mesothorax ; 3, Metathorax, separated by the wider black lines. Tergum 

 of the prothorax not represented, ms, mesoscutum; scm, mesoscutellum; ms" , 

 metascutum; scm'", metascutellum; pt, a supplementary piece near the inser- 

 tion of patagia; w, pieces situated at the insertion of the wings and surrounded by 

 membrane ; em, epimerum of prothorax, the long upright piece above being the 

 episternum; epm", episternum of the mesothorax; em", epimerum of the same; 

 epm", episternum of the metathorax; em", epimerum of the same, divided into two 

 pieces; c, c", c'", coxa?; te, le", le", trochantines ; tr, tr, tr, trochanters. 

 Original. 



