268 
Psyche 
[Dec. 
where the joints are much longer than broad. In Ptero- 
narcella the joints become longer than broad much sooner. 
In all of these the hairs on the antennae are very small 
and fine, well described as k ‘ minutely pubescent. ’ ’ Often 
they are erect, but in some slope toward tip of joint. 
In Togoperla the hairs, though longer, are closely ap- 
pressed; in Perlinella, though short, are also appressed. 
In the genotype of Isoperla ( bilineata ) the joints near 
base are as long as broad, the hairs are very short, but 
near the tip or beyond the middle is a longer erect hair, 
which I shall call a sense hair. In other species of Iso- 
perla it is the same, but a few (ehria) that have some- 
times been placed in Isoperla do not have these sense- 
hairs, or at least not well-developed. In Alloperla y in 
Perlesta, it is the same as Isoperla. In Neoperla the 
hairs are longer but the sense-hairs are very prominent, 
also Diploperla, Paraperla, and Kathroperla, as well as 
Chloroperla and Hastaperla have them more or less 
distinct. 
In Iso genus and Perlodes there are no sense-hairs or 
else so small as to be unnoticed, in Hydroperla some spe- 
cies show very tiny erect hairs toward the tip of the joint, 
but in others they are very small or absent, in some 
Diploperla they are also short, but usually distinct and 
near tip of joint, in Clioperla ( similis , slossonce ) they 
are readily seen. In some Acroneuria ( xanthenes , Cali- 
fornia) the sense-hairs are fairly distinct, beyond the 
short joints, in others they are obscure or absent. In the 
Perlincc distinct, but not as prominent as in Neoperla. 
Head. 
In shape the head ranges from twice as broad as long 
(. Peltoperla ) to twice as long as broad ( Paraperla ) ; and 
behind the eyes the head may extend more than twice the 
diameter of the eye ( Kathroperla ) or there may be 
scarcely any space at all (some Hydroperla) . 
The head may be considered as of three areas : occipital 
area, the part behind eyes and ocelli ; the interocular area, 
and the face or area in front of anterior ocellus. The 
occipital area is often very short in middle, where it is 
