1947] 
Banks — Characters in the Perlidce 
269 
separated from the interocular area by a curved line or 
groove, the occipital line, the ends of which curve for- 
ward. Just behind the eyes the head usually narrows 
toward the pronotum, but in a few forms ( Perlodes , 
Atoperla ) the side at first bulges outward before it starts 
to narrow. In the middle there is frequently a line or 
groove from the occipital line to the rear of head; this 
median groove is present in Perlodes , Isogenus, Clioperla, 
some Isoperla, also in P. nona and ohscura. It is short 
or hidden in Perla ( To go perla ), Acroneuria, Harrisiola, 
Perlinella, Perlesta, Atoperla, Alloperla, but is present 
in P. luctuosa, P. modesta and P. phalerata. It is present 
in some very small species as P. ehria. In Hydro perla 
it is not present, at least in sight, but sometimes a small 
groove on the hidden part of head or as a very fine line. 
The occipital line is usually quite distinct in the primi- 
tive forms ; in the more specialized it may show behind 
the hind ocelli, or be entirely obliterated. In Perlodes it 
slopes each side and ends in front of eyes ; in Perliphanes 
it slopes less and runs into lower part of eye ; in Perla 
and Acroneuria it slopes each side just outside of boss 
and is stopped by a ridge. In Calliperla it slopes each 
side a little concavely to a point near eye where it con- 
nects with a line sloping below inwardly, the outer part 
of the M-mark. In Harrisiola and Atoperla it makes a 
full semicircle, the ends running into the boss; in P. 
modestus it makes a broader curve, also ending in the 
boss each side, but not a full semicircle. In Iso genus it 
is lower and broad, stopping near boss. In Perlinella the 
sides disappear a little beyond boss. In Perlesta it con- 
nects top of ocelli, beyond vestigial. In Isoperla it some- 
times shows sloping from behind the hind ocelli toward 
the eye, but more often it is not visible. 
The interocular area has the three (or two) ocelli. 
They form a triangle as long or longer than broad in 
Perla, Acroneuria, Harrisiola, and some Perlodes ; in Iso- 
genus, Pictetia, many Clioperla and Isoperla the triangle 
is broader than long or at least as broad. 
The hind ocelli are usually situated at about the middle 
of the eye-space ; in Perlinella and Kathroperla a little 
