WING-VENATION OF THE PLBCTOPTERA. 
157 
not only with Cui, but in other parts of the larval wing. It seems to 
indicate a certain amount of instability in the tracheation, and may possibly 
be the stage precedent to a more complete breaking down of the tracheal 
supply, as exhibited in many of the forms studied by Miss Morgan. 
Text-fig. 8 shows the tracheation of the fore-wing in the (n + 3)th or 
penultimate instar. In this stage, R, Rs, and M generally appear to arise 
as a single thickened trachea from the alar trunk, at a point somewhat 
removed from the origin of Sc, and beneath the callus. By careful 
manipulation, this apparently single trachea can be separated into its 
component parts, when it will be seen that it is in reality three separate 
tracheae arising from almost the same point. In some specimens, however, 
Ri may still be found arising as a branch of Sc, while Rs and M arise as a 
single trachea well below it. I think the varying conditions seen in the later 
Text-fig. 8. 
Ameletus ornaius (Eaton). Tracheation of fore--wing in (w+5)th or penultimate 
larval instar. For lettering, see p. 162. (x48.) 
instars are almost wholly due to the presence of the callus, one or more of the 
original tracheae taking a devious course to avoid passing under it, or possibly 
becoming displaced at ecdysis by the pressure of the harder mass of chitin. 
In this instar, the number of branches of Rj is still five, and the most 
anterior of these never has any tracheal supply except a mere rudiment. 
Various irregularities may be noted. In the specimen figured, the trachea 
which usually supplies the most posterior branch of R2 arises alongside the 
trachea belonging to the branch above, and runs with it to about half its 
length, when it turns downwards and supplies the course of the most 
posterior branch for its distal half only. The trachea supplying Mj 
sometimes arises from Mg.,.^. Cu2 possesses a complete trachea in this 
specimen ; but in the majority of wings examined, it either has no trachea or 
a mere rudiment. These are all individual variations in the tracheation, and 
