ON THE WING-VENATION • OF THE PLECTOPTKEA. 143 
The Wing-Vetiiition of the Order Plectoptera or Mayflies. By R. J. 
TiLLYARD, M.A., Sc.D. (Cantab.), D.Sc. (Sydney), F.L.S., C.M.Z.S., 
F.E.S., Entomologist and Chief of the Biological Department, 
Cawthron Institute, Nelson, N.Z. 
(With 10 Text-figures.) 
[Read 30th November, 1922.] 
The present paj)er is intended to be read in conjunction with the series of 
papers being prepared by me on the Wing-Venation of the Order Odonata 
or Dragonflies, the first of which has already been published (Tillyard, 1922), 
the remainder having been kept back to allow of the present results being- 
considered first. 
At the present time, considerable donbt appears to exist as to the true 
interpretation of the homologies of the wing-veins in Mayflies. This is not 
to be wondered at when we consider that Comstock and Needham originally 
offered one interpretation (1899), which became widely accepted, but that, 
following on Needham's discovery (1903) of the crossing-over of the supposed 
Rs in Anisopterous Dragonflies, Miss Anna Morgan (1912), working clearly 
under the influence of this discovery, ofirered a new interpretation bringing 
the venation of the Mayflies into line with that of the Dragonflies in this 
very important respect, although the evidence brought forward in her paper 
to support this view is admittedly of the slenderest kind. Space will not 
allow us to give here a full critical discussion of Miss Morgan's paper. It 
will only be necessary to make a single quotation from it, as follows 
(L t-. p. 98) :— 
" An actual connection between R and Rs trachea (in Mayflies) cannot be 
shown by constant structures. Moreover, Mayflies and Dragonflies are 
closely-allied groitps, and their general tracheation' is similar in many points. 
Furthermore, this condition of the radial sector trachea is exactly the same 
as that just described in the Damsel-flies, where there can be no doubt that 
such a crossing has taken place. It is, therefore, highly probable that the 
radial sector is present in Mayflies, and that both the sector trachea and the 
vein Rs have been stranded on Mj, and have left no positive trace of their 
origin." 
On these very slender and debatable grounds. Miss Morgan proceeded to 
alter the homologies of the wing-veins in the Order. While recognising 
and admiring the painstaking work shown in her study of the tracheation 
and venation of so many little-known types, we have to confess that, like 
Prof. MacGillivray, under \\hom the work was done {I.e. p. 89), we 
" disagree with some of the interpretations .... presented." 
Now that I have shown the strong improbability that the supposed Rs 
of Needham in the Anisoptera is really not that vein (Tillyard, 1922), and 
definite proof is forthcoming that, in the Zygoptera, the supposed Rs never 
LINN. JOUKN. — ZOOLOGY, VOL. XXXV. 10 
