154 
DR. R. J. TILLTARD ON THE 
here only a series o£ drawings which T made from the wing-slieaths o£ 
Ameletus ornatics, a larva which has fairly transparent wings, is easy to 
dissect, and in which the tracheae can be followed without any trouble. 
Characters in which the other larvae differed from this will be noted 
below. 
The first point to be noticed is that the larval wing, in this Order, 
is soldered to the thorax over a considerable portion o£ its basal area. 
The free distal portion of the wing is that which lies to the right of the 
dotted line in text-figs. 5-9. Great care is necessary, in dissection, to 
remove sufficient of the soldered basal part to ensure that the bases of the 
wing-trachese, and their connections, are not severed or damaged in the 
operation. 
Just distad of the middle of the line marking the junction of the flap 
of the wing with the thorax, there is always present a somewhat triangular 
patch of hard chitin, shown shaded in all the figures. It at first separates 
the subcosto-radial group of trachese, including M, from the cubito-anal 
group, as shown in text-fig. 5 for the nih instar. But, in the next instar, it 
Text-pig. 5. 
Ameletus ornalus (Eaton). Traoheation of fove-wiiig in nth larval instar. 
For lettering, see p. 162. ( x 97.) 
moves forward a little, so as to overlie part of the base of M ; and finally, in 
the last two instars, it comes to overlie the wholly or partially fused bases of 
E and M. From its final position in the last instar, it seems to me that 
this hard piece of chitin represents the large callus of the imaginal wing ; 
its position in relation to the thorax is probably constant, its apparent 
movement forward being probably due to changes in the actual positions of 
the wing-trachese. The chief point of interest appears to me to be that it 
certainly does cause trachea M to run closer up to R than it naturally 
would if this callus were absent. Consequently, it may well be one of the 
factors which helped to bring about the fusion of M with Rs basally, as we . 
find it in the majority of larval wings in the last four instars (text-figs. 6-8). 
