158 DR. R. J. TILLTARD ON THE 
are of no significance except to indicate the essential variability of the 
tracheation in many of its details. 
In this instar, the line of fusion of the wing-sheath with the thorax ceases 
to be straight, and shows a definite convex bulging between the subcosto- 
radial and cubito-anal groups of trachese. The alar trunk is still complete, 
but thrown into a strong loop. The large anterior callus already noted in 
previous instars now appears in close contact wiih the axillary area of the 
thorax, this position corresponding with that in the iniaginal wing, where the 
callus articulates with the axillary process of the thorax. A new structure 
is apparent in the cubito-anal region, viz. a smaller, somewhat reniform, 
posterior thickened patch, shown shaded in text-fig. 8. This corresponds 
with the opaque and somewhat thickened anal area at the base of the imaginal 
wing ; but it never becomes a true callus, i. e. it does not articulate with an 
axillary process from the thorax. 
The last or (?i-l-4)th instar differs from the previous one in the much larger 
size of the wing-sheaths, the strong development of the anterior callus, the 
very clear indication of the whole of the imaginal venation, including all 
the cross-veins and veinlets, and the presence of all six branches of Ej. 
In this instar, the tracheation is not easy to follow, owing to the strong 
-pigment bands of the imaginal venation overlying it. As the general 
tracheation scheme is closely similar to that of the previous instar, and the 
same individual variations are equally apparent, it has not appeared necessary 
to me to figure it. One point of interest needs to be mentioned, viz. that I 
only Succeeded in finding the complete alar trunk in one specimen of 
Oniscigaster distans. In all the others examined, the basal portion cu-a was 
missing, and the single trachea which gives origin to Cu and the anal veins 
appeared to arise from the same trunk as the other trachese above it. This is 
exactly the condition shown in Miss Morgan's figures (1912, pis. 5-7). 
Thus it will be seen that, if she had studied other instars as well as the last, 
she would not have fallen into the error of failing to homologise correctly 
the cubital trachefe and veins, which it is quite evident she has done. 
The above discussion applies to the fore-wing only. As the hind-wing is 
greatly reduced in size in all recent Mayflies, and differs very greatly from 
that of the Permian Mayflies, as well as from the fore-wing of recent forms, 
I have thought it best to summarise the evidence for its venational homologies 
in a separate section. 
The Hind-wing. 
The dissection of the small hind wing-sheath of a Mayfly nymph is not 
easy, and I only succeeded in getting good preparations from the last two 
instars. Text-fig. 9 shows the tracheation in the hind-wing of Ameletus 
ornatus, penultimate instar. The alar trunk is complete, but there are no 
signs of a callus. The line of fusion with the thorax lies well out beyond the 
origins of the wing-trachere, as shown by the dotted line in the figure. 
