WING-VEKATION OF THE PLECTOPTERA. 
155 
Text-fig. 5 shows a typical £ore-wing of Ameletiis ornatus in the ?ith 
larval instar. There is no costal trachea visible. The alar trunk connecting 
the. subcosto-radial with the cubito-anal group o£ trachese is complete^ 
though of small calibre, and the two groups are widely separated. Instead 
of coming off separately from the alar trunk, the tracheae Sc, R, and M arise 
as separate branches from a single trachea of lai"ge calibre. Rs branches, as 
a simple triad, into Rj, R3, and R.1+5. Trachea JVI arches gently in front 
of the callus already mentioned, and thus comes very close up to Rs just 
where it comes off from R]. Most of the larvae examined in this instar 
show exactly the condition given in text-fig. 5 ; but I found one larva in 
which trachea M had already captured trachea Rs at its base. This 
condition becomes the usual one in the following instars. M is only two- 
branched, as in the Order Perlaria. The trachese of the cubito-anal group 
are very fine in calibre; but I could distinctly make out Oui and lA in all 
the larvae examined, and could see Cu2 in most of thom. In following 
instars, no trachea is usually visible in Cuj, though I found it quite well 
developed in a fine larval wing of the penultimate or (H + 3)th instar shown 
in text-fig. 8. 
Text-piq. 6. 
Ameletus ornatus (Eaton), ■ Traclieation of fore-wing in (n-\-l)tb. larval instar. 
For lettering, see p. 162. ( x 87.) 
Passing on to the next, or (n+l)th instar (text-fig. 6), we note that the 
callus has now come to overlie the base of M, and in the great majority of 
wings examined, trachea M has captured trachea Rs at its point of nearest 
approach basally. I have notes of a few exceptions, chiefly in the genus 
Oniscigastei; in which, both in this and the following instars^ any of the 
follovi'ing conditions may happen to occur : — 
(1) Tracheas Sc, R, Rs, and M all come off in a bunch as separate tracheae ■ 
from a single point on the alar trunk. 
(2) R arises from Sc, but Rs and M arise close together from a point on 
the alar trunk a little below them. 
