WING-VENATION OF THE PLECTOPTEEA. 151 
definitely triangular, the three angles being the base, the apex, and the tornus. 
In the evolution of: this shape of wing there is a definite reduction of the 
areas served by the cubitus and anal veins, and a definite increase in the 
area served by the media and radial sector. Consequently, if Comstock and 
Needham's theory of interpolated veins is justified^ we should certainly 
expect to find a nunaber of them developed along the termen in Amelehts. 
At first sight, this appears to have happened, since we can find at once no 
less than five long veins, viz. 2R2a, 4R2a, E'2b, R^b, and M2, which are not 
connecied basally with their adjacent main veins. On the evidence of 
recent Mayflies alone, Needham's conclusions were perhaps warranted. But 
when wp come to compare the venation of Ameletus with the fossil 
Protereisma, we find that every one of these veins corresponds exactly with 
a true branch of a main vein present in the fossil ; and, further, the total 
number of veins present in Ameletus, and their alternate convexity and con- 
cavity, agrees exactly with the conditions in Protereisma. I have therefore 
no hesitation in assigning to ihe veins in the fore-wing of Ameletus the same 
names as I have already given to their homologues in Protereisma. 
In the section dealing with larval wing-tracheation, it will be shown that 
disconnection of the base of any branch vein is correlated with loss of its 
precedent trachea in the larval wing, and may well be a direct result of this 
larval condition. 
By comparing the wings of Ameletus and Protereisma., we are now able to 
see that the lengthening of the termen has taken place chiefly in the region 
served by II4-I-5 and M, the branches of these veins standing considerably 
further apart in Ameletus than in the fossil, whereas the .area served by 
II2+3 h^s altered very little. 
Turning our attention to Guj, we note that this remains a strongly convex 
vein, but its original triadic system of branching has been changed to a 
simple pectinate series, though the number of branches, seven in sW, remains 
the s.ame, the branches still being alternately convex and concave. Though 
superficially striking, this change is really a very slight one, consisting only 
of a movement basad, on to the main stem, of the triad called Cujc i" 
text-figs, la, 3. 
The greatest changes that have taken place in the evolution of the existing 
type of Mayfly «ing are those in the basnl region of the wing. For these, 
we must refer to the enlarged basal area of Ameletus shown in text-fig. 4, 
and this must be compared with the basal portion of the wing of Protereisma 
in text-fig. 3. Here we note the following changes : — 
(1) The separate costal vein C of the fossil has become fused with the 
costal margin, forming a thickened basal margin ; only its posterior distal 
branch, Jim, remains free, and forms the strong oblique brace, lim, in Ameletus 
and all recent Mayflies. The intermediate stage in this evolutionary 
process is well seen in the Jurassic Mayflies, the condition in Mesepliemera 
cellulosa (Hagen) being shown in text-fig. 4, upper figure. 
