1 }• THOMAS SAY FOUNDATION 



Three principal crossveins, together with portions of the 

 veins they connect enter into its composition. These are 

 the interradial, the radio-median and the medio-cubital 

 crossveins, — all rather constantly present, unless elimi- 

 nated by fusion of the adjacent veins at the point where 

 they would normally be present. The cord divides the 

 wing broadly into inner and outer fields. The middle por- 

 tion of the wing lying between the arculus and the cord 

 is often referred to as the wing disc. The point of origin 

 (separation from vein Rl) of vein Rs in the fore wing 

 and the point of separation of veins Rs and M in the hind 

 wing are often stated in terms of the distance between 

 the arculus and the cord. This distance is measured 

 along the vein in front (Rl in the fore wing, Rs in the 

 hind wing). 



The costal vein is always simple and borders the wing 

 at the front. 



The subcostal vein is typically forked at its tips with 

 forks that are short and divaricate and that usually simu- 

 late crossveins, one joining costa, the other joining 

 radius. The latter is usually longer and stronger so that 

 the subcosta seems to end in the radius, but in Leuctra 

 and Perlomyia it seems to end in the costa. Its tip sur- 

 passes the cord in Acroneuria and a few of the larger 

 Perlas, but in most species it does not reach the level of 

 the cord, and it is much shortened in Alloperla. Always 

 it is a weak vein, and tends to fade out along with the an- 

 nexed costal crossveins. 



The radius is a strong vein, which with its branches 

 supports the wing apex. The radial sector bears a vari- 

 able number of branches, that are generally in a unilateral 

 arrangement (rarely twice dichotomously forked as in 

 the typical insect wing) and all springing from the pos- 

 terior side. The point of origin of the hindmost branch 

 and the point of separation of the sector from Rl at its 

 base in the fore wing, and the extent of basal fusion of 

 the sector with the median vein in the hind wing furnish 

 useful systematic characters. 



The median vein is usually two-branched, there being 

 a single simple fork the base of which enters into the 

 composition of the cord. Rarely, as in Perla venosa (PI. 11, 

 fig. 8) there appears to be the full typical complement of 

 four branches. Often a deflected bit of the base of media 

 enters into the composition of the arculus. 



The cubital vein is typically two-branched, with often 

 some accessory branches on the tip of Cul. These are 



