WING-VRNATION OF THE PLECTOPTERA. 153 
(4) The anal area, or clavus, in greatly reduced size, being about one-fifth 
of the total wing-length in Ameletus, as compared with about one-third in 
Protereisma. The number of branches of the anal veins are correspondingly 
reduced, those of lA to two definite branches, while 2 A and 3 A are only 
indicated by short, weakly-formed veins, as shown in text-fig. i. 
(5) The articulation of the wing, situated at the origin of Sc and R, 
becomes greatly strengthened by the formation of a.hugh ovoid callus 
(text-fig. 4). It appears to be the growth of this callus which has forced 
the basal portions of M and Cu below it into their peculiar curved positions. 
In some recent Mayflies of higher type than Ameletus, these curved basal 
parts .of M and Ou become strengthened and partially fused together; 
uniting with the bases of the an;il vein also, they form an upwardly- 
curving arch convex to the posterior border, from which all the branches 
of Cu and A take their rise. 
We thus see that, in the archaic family Siphluridse, as represented by 
Ameletus, the correspondence with the Lower Permian fossil Mayflies is 
exceedingly close, and the homologies of the veins perfectly clear. This, of 
course, refers to the fore- wing only. The hind-wing, which has undergone 
great reduction since Jurassic times, will be dealt with after the tracheation 
of the larval fore-wing has been considered. 
Tracheation of the Larval Wings. 
The study of the tracheation oE the larval wings in New Zealand genera of 
the family Siphluridas was carried out chiefly, as already stated, in the Cass 
Biological Station. Some material was also obtained around Wellington 
during my stay with Mr. Harold Hamilton, of the Dominion Museum, and 
the dissections of these were caried out at his home in Karori. I wish here 
to thank Prof. Chilton (and Mr. Hamilton for the facilities they afforded me 
for carrying out this work. 
The material studied consisted of numerous larvffi of the following 
species: — Oniscigaster distans Eaton, Colohuriscus humeralis (Walk.), 
Ameletus ornatus (Eaton), and Ameletus perscitus Eaton. The larvae of the 
first and last of these were very abundant at Cass, and provided the bulk 
of the dissections. The number of instars is not known for any of these 
larv9e, but the wing-sheaths can be dissected out for at least five instars, 
showing comparative stages of growth from a tiny triangular bud up to a 
consideribly elongated flap. For compai-ative purposes, I have designated the 
youngest of these last five instars as the nth instar, viz. that in which the 
wing-buds are very small flaps, capable of being dissected off with very fine 
scissors if great care is exercised. The following instars will be termed the 
(ri-|-7)th, (ji-l-2)th, {n + 3)th, (;n + 4)t\i respectively, the (n + J^)th being 
the last larval instar. In order to avoid too many figures, I shall give 
