52 Rev. A. EH. Haton’s Monograph 
Last of all, the form and neuration of the posterior 
wings (if there be any) should be taken into considera- 
tion. 
The abdomen may further be examined as to the mode 
in which the last segment is finished off beneath; 
whether by a simple membrane, or by plate-like folds of 
the integument beneath the insertion of the sete (as in 
Hphemera and Baetis). In Lachlania, Oligoneuria, and - 
Cenis, the posterior segments are furnished at the tip. 
with slender lateral processes; on account of this and 
other reasons (e.g., the structure of the sete), I am in- 
clined to consider the first two genera to be allied to 
Cenis more closely than might be inferred from the 
arrangement of the genera adopted by me. Heptagenia 
is the only genus known which possesses a true egg- 
valve. 
In the immature insect, the attitude assumed by the 
subimago during repose is of value in classification. The 
points to be noted are whether it elevates the anterior — 
legs, or not. If it does, are they held together, or 
separated ? Are the wings spread abroad, or held up- — 
right? Are the caudal sete laid together, or separated? — 
The average duration of the subimago stage should also 
be taken into consideration. Does it last only a few 
minutes, or an hour or two, or a whole day, or longer? 
or is it the permanent state of the female ? | 
In the nymph, the habit of the insect is of first import- ji 
| 
| 
ance. Does it burrow, or creep? or does it run and 
swim actively? These points can be inferred from the 
structure of its mandibles, lees (especially the anterior’ 
legs), sete, and antenne. Next in importance is thy 
structure of the branchial organs. Are they lamince 
fringed with short slender processes ? and, if so, are they 
simple (as in Cenis), or compound lamine (as i | 
Hphemera)? Or are the lamine fringeless? If so, are\ 
the lamine simple (as in Baetis), or compound (as im 
Cloeon), or complex (as in Hphemerella)? or are they 
furnished at the base with a fascicle of branchial filaments | 
(as in Heptagenia)? Betisca has the branchie concealed 
under an extension of the dorsum of the thorax. The 
labium, with the two pairs of maxillee and their palpi, have 
next to be considered. Are the palpi of the lower 
maxille two-jointed, or three-jomted ? Last of all, the 
number and position of the branchial organs must be noted, 
