104 Annals Entomological Society of America IVolV, 
intercalaries as in front wing. The hindmost is longest as in 
front-wing, two others are of intermediate length. The Acci, 
and Rs? are in positions identical with the corresponding veins 
of the front wing. Sub-costa is much reduced; in most cases 
entirely wanting. In Palingenia (Fig. 6, Sc) it is a strong but 
very short vein. - 
Summary. 
This is a study of the ontogeny of wings representing fifteen 
genera of May-flies in which the following facts are shown: 
1. The main veins of May-flies may be homologized with 
the veins of insects of other orders. 
2. The main trachee precede and constantly mark the 
course of the main veins. 
3. The costal and subcostal trachee are simple and parallel 
as are the veins which follow them. 
4. The radial trachea (except in one form studied) and the 
vein which follows it are unbranched. 
5. The radial sector is very probably present in May-flies 
but in an unusual position between the veins M; and M.. It is 
detached from radius, as in the dragon-flies, and stranded 
upon vein M). 
6. The medial trachea and the vein M show four branches 
which are characteristic of media in its primitive condition. It 
is similar to the media in the closely allied dragon-flies. 
7. The tracheal system enters the wing by a single stem. 
The course of the cubito-anal trunk shows a possible trace of 
the double stem of the tracheal system of other orders. 
8. In the series of wing-pads studied a remarkable evolu- 
tion of tracheation is shown. This evolution consists of a 
gradual reduction of main trachee and replacement by small 
branches. 
9. This interpretation of the venation involves the import- 
ant changes of nomenclature shown in Fig. 4. 
