COMPOSITION OF THE INSECT-CIIUST. 



11 



ing the lowest wingless forms such as the Louse, Fk'u, 

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

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

 pervading all these forms, uniting them into a connnou 

 class. 



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

 typically of eight pieces which we Avill now examine more 

 leisurely. Figure 12 represents a side-view of prm 



the thorax of the Telea Polyphemus^ or Silk- pt- 

 worm moth, with the legs and wings removed. 

 ICach ring consists primarily of the tergiim, 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. Tlie 

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

 douin going from before backwards, Fig. n. 



the 'proiscntnm, scutmn, scutellum, 

 and j)ostscutelhim. 



The scutum is iuA'ariably present 

 and forms the larger part of the «" 

 upper portion (tergum) of the tho- <^p^^" 

 rax ; the scutellum is, as its name em 

 indicates, the little shield so promi- 

 nent in the beetle, which is also 

 uniformly present. The other tAvo 

 pieces are usually minute and 

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

 ing rings. As seen in Fig. 11, the pra^scutum of the moth is 

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

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



Fig. 11. Tergal viewof the middle segment of the thorax of Tdea Polyphemus, 

 prm, prrcscutum ; ms-, scutum: scm, scutellum; ptm, postscutellum; pt, patagium, 

 or shoulder tii)i)et, covering the insertion of the wings. — Orif/iixil. 



Fi<;. 12. Side view of the thorax of T. Poh/phemux, the hairs removed. 1, Pro- 

 thorax ; 2, Mesothorax ; ;J, JMetathorax, separated by the wider black lines. Tergum 

 of the prothora.x not represented, ms, mesoscutum ; scm, mesosciitellum; ms" , 

 metascutuin; scm", metascutellum; i)f, a supplementary piece near the inser- 

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

 membrane; em, epimerum of jirothora.x, the long u)iright jiiece above being the 

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

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

 pieces; c', o", c", co.xae; te', ce', le", trochantiues ; tr, tr, tr, trochanters. 

 — Original. 



tr te c" tr c " tr 

 12 3 



