26 WILLIAM T. M. FORBES 



sides, costa, outer (or hind), and inner margin, and the three angles, 

 base, apex, and anal angle, being indicated in the figure. Sometimes 

 the inner margin of the fore wing is extended out near the base, form- 

 ing a basal angle, or the costa of the hind wing may be similarly 

 extended, forming a humeral angle. The wing is stiffened with a regular 

 pattern of hollow rods, the veins, which are important when the wing is 

 expanding, besides serving to stiffen the mature wing. These veins 

 have a definite arrangement, based on that of the pupal tracheae around 

 which they form (see the figures). From the base of the wing there 

 run out costal, subcostal, radial, medial, cubital, and some anal main 

 stems. Of these the costa is simple and forms the front edge of the 

 wing; the sub cost a may be simple, but in a few low forms it forks once 

 at the tip. The radius forks normally into five branches, primitively 

 as shown in figure 40 but in various ways in the higher forms. Often 

 part of these branches are lost, and in a few of the lowest species the 

 first branch forks again (fig. 36). Media is three-branched, but usually 

 the base of it is lost, and the branches are variously attached to the 

 stems of radius and cubitus ; when one is attached to cubitus, the latter 

 is called trifid (fig. 425), when two, quadrifid (fig. 427), the upper 

 median being always free or attached to radius. The middle branch 

 of media, sometimes called the independent, is often weak or lost. 

 Cubitus is two-branched, and is a very constant feature. The anals 

 are somewhat uncertain in origin, but usually appear in low Frenatse 

 as three radiating veins, the first running along the principal concave 

 fold of the wing. The first anal disappears in higher forms, leaving 

 only the fold ; the second is persistent ; and the third tends to grow 

 short in the hind wing, and to join its tip to the second, or even to 

 disappear, in the fore wing. In the Jugatse (figs. 31, 36, 40) the 

 arrangement is more complex and not fully understood. This descrip- 

 tion, intended mainly for the fore wing, applies also to the hind wing 

 of the Jugatae. In the hind wing of the others the radius is only 

 two-branched, and the upper branch fuses more or less, often almost 

 completely, with subcosta, as indicated by the lettering of the earlier 

 figures. Commonly Sc+Rj is merely marked Sc, and R s , merely R. 

 Besides these veins, which are based on tracheae, there are certain cross- 

 veins; the humeral (h.) at the base of the costa, often pushed to the 

 base of the wing and so lost; the sectorial (s.), running across between 

 the third and fourth branches of radius, and so enclosing the accessory 

 cell, often lost by the fusion of the veins at that point; and the dis- 

 corellular, running more or less irregularly from radius across media 

 to cubitus, enclosing the discal cell (" cell ") between it and the base 

 of the wing, and supplying an attachment for the branches of media 

 when the base of media is lost. The arculus (arc.) connects the media 

 and the cubitus at the base of the wing, but shows clearly only in the 



