110 
INTRODUCTION TO 
scutellum sometimes extends over the anterior edge 
of the metathorax ; in some instances it entirely con- 
ceals its surface. An impressed line sometimes sepa- 
rates an anterior portion from the rest, and in such a 
case this is called the Postscutellum. As the internal 
cavity of the mcsothorax is separated from that of 
the metathorax, by a partition denominated the 
Mesophragma , so the latter is separated from the 
abdomen by a similar parietal wall, called the Mela - 
phragma , a small space only being left for the passage 
of the necessary organs. The ventral plate of the 
metathorax is the melasternum , a portion which 
most commonly assumes a shape similar to that of 
its counterpart above, but it varies greatly in some 
of the orders. On each side of the thoracic segment 
now under consideration, are found two horny pieces, 
which Kirby and Spence have named j dura and 
periplura . In the latter, the hal teres or poisers of 
Dipterous insects are situated. 
We have now to consider the organs attached to the 
thorax, by which motion is effected either on the land 
or in the air, viz. the legs and wings. The former are 
never wanting, and their number is invariably the same ; 
the latter are often absent, and when present, their 
number varies from two to four. Such kinds as possess 
four wings, are called Tctrapteroux insects ; those with 
two, Dipterous ; and when altogether wanting, the 
species are said to be Apterous. The point at which 
the wings are inserted, has been already indicated ; 
they articulate to the thorax by means of numerous 
small pieces. Viewed relatively to each other, the 
