THE THORACIC EXO-SKELETON. 177 



edge of the scutum. A small apodeme projects between the 

 parascuta and unites them. It gives insertion to a slender 

 muscle with a long tendon, the action of which is to depress the 

 alar edge of the parascuta and assist in the elevation of the wing. 



In removing the dorsum from the thorax, it will have been 

 observed that the prescutum was divided by a horizontal incision 

 just above the anterior spiracle, so that only its posterior part 

 enters into the formation of the dorsum. The prescutum is 

 very convex in front, so that its anterior part forms the anterior 

 wall of the thoracic cavity (Plate VIII., Figs, i and 2). The 

 inner surface of this portion of the thorax is best examined by 

 cutting off the anterior part of a skeleton, or artificial exuvium, 

 just behind the anterior spiracle. The prescutum will be 

 seen in such a preparation to be bounded in front by the 

 prophragma and the two paratremes. 



The Prophragma {pp) is a membranous inflection of the edge of 

 the prescutum. The attached margin of the prophragma corre- 

 sponds with the junction of the anterior edge of the meso- 

 thorax and the posterior margin of the rudimentary prothorax. 

 Its extremities are continuous with the superior and inferior 

 edges of the paratremes, so that the lateral parts of the pro- 

 phragma split into two lamina;, like the e.xtremities of the 

 hypotremes. 



The central portion of the prophragma has a free convex 

 posterior border, and exhibits a median projection, strengthened 

 by a distinct bifurcate sclerite. Its upper and lower surfaces 

 are horizontal. 



The Prodorsal Arch {pd) is the inflected edge of the cervical 

 opening of the thorax. It is divided in the median line above 

 into two lateral halves by a distinct suture. Its outer extremities 

 articulate with the anterior margins of the epitrochlcar sclerites, 

 and with the inner and lower part of the paratremes. 



Each lateral half is divided into two by a distinct oblique 

 suture, which extends from the attached edge of the pro- 

 phragma downwards, outwards and backwards, and termi- 

 nates in a strong apodeme, which projects into the thoracic 

 cavity, the prothoracic apodeme. 



