Vol. 1.] Woo(lwo>i,li. — Wing Veins of Insects. 19 



sclerites have a different nature and have arisen to meet 

 different needs. The latter consist of linear invagination, 

 which are characterized by harder rather than softer cuticle 

 and result in the stiffening of the region in which they are 

 developed. 



The first and most important result of the development and 

 perfection of the legs is the production of the sterno-pleural 

 suture. The space occupied by the basal joint of the leg greatly 

 Aveakens the segment at a critical point, but it is entirely com- 

 pensated for by the production of this suture and the stiffening 

 of the edges of the pedal foramen. This region, instead of 

 being weakest, thus becomes in fact the strongest and stiffest 

 part of the segment. 



A second change following the development of the leg is the 

 stiffening of the parts of the segment that bear the strain of 

 the muscles of the legs. There is some variation in the arrange- 

 ment and working of the muscles at the base of the legs of 

 insects, but we shall consider only the more conmion, and 

 undoubtedly primitive, arrangement, which consists of three 

 principal muscles: one extending toward the venter and two 

 toward the sides, one of which is anterior and the other posterior. 

 The ventral muscle was necessarily short at first and attached 

 at an unsatisfactory angle, but gradually became accommodated 

 by an ingrowing process from the back of the middle line of the 

 venter, and the difficulty was thus wholly overcome. This 

 process, which is of various shapes and sizes, forms part of 

 the so-called internal skeleton; though of much physiological 

 importance, it affects only very slightly the external appearance. 

 The side muscles did not require such elaborate changes for 

 their attachment, but did require a stiffening of the body wall 

 of the side, which was more conspicuous externally. This was 

 accomplished in the simplest and most effective manner by the 

 stiffening of the edges and of a line down the middle of the 

 side between the muscles, thus furnishing on each side of each 

 muscle the stiff support needed. The sutures thus formed, 

 showing as ridges internally, conspicuously divide the side into 

 the two regions, the episternum and the epimeron, which are 

 distinct below, but not delimited above, where they extend 

 feebly over the back from side to side. This is the condition 

 of highest development reached until structures evidently 

 associated with flight or with some other special requirement 

 arise. . 



