270 
Psyche 
[Dec. 
behind eyes, in Alloperla further back than usual, and 
sometimes behind the eyes. In Isogenus, Hydroperla, 
Clioperla and some related species the hind ocelli look 
more or less laterally from the head of a broad groove, 
there being a ridge on their inner edge as high as the 
ocelli; in Isoperla the hind ocelli look more upward; in 
Acroneuria, Alloperla, Harrisiola the hind ocelli project 
above the surface of the head. The distance of the hind 
ocelli from the eyes is, at most, only a character of species. 
In nearly all Perlids there is a small swollen spot a little 
in front of each hind ocellus, and often further out. This 
is the lateral tubercle or boss. It is more or less char- 
acteristic in shape and position for each species. Some- 
times hardly further than the diameter of the ocellus, 
again it may be five to ten times as far away. In a few 
cases it is circular, but usually longer than the ocellus, 
and one end may be enlarged, or narrowed at outer end. 
In front of the anterior ocellus is the face. 
There is usually a transverse groove or a depressed 
spot in front of the anterior ocellus. In front of this 
there is on each side a smooth area, or raised area, usually 
elongate and sloping outward; frequently they are pyri- 
form. These are the middle part of what Needham calls 
the M-mark; however it is better called the V-mark, since 
the outer sides are faint or absent in most species. Some- 
times the two are joined together above in a broad curve. 
They never reach the front margin of head, and near each 
lateral corner may be a dark rounded spot. The shape 
and divergence of the V-mark are very good characters 
for species and species-groups. But sometimes widely 
separated genera may have the V-mark of very similar 
shape. I have figured a number of these, and I think they 
should be considered in descriptive work. 
In some species the head is without marks, or only dark 
around the ocelli, but in many there is a pattern of dark 
marks, which has long been recognized of specific value. 
There is, of course, variation in the development of a pat- 
tern. When the same pattern occurs in two or more spe- 
cies, there are differences in structure ; often in shape of 
the ocellar triangle or on the lower face. 
