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brown, tipped with black, with five teeth. Labium with six well-marked 
teeth each side, a central irregular, obscurely emarginate lobe, with two 
introduced teeth (see figure). Posterior margin of the head marked with 
black. 
Abdomen with each segment slightly smaller than the previous one, 
plump. Anal segment with four small blood-gills, two slightly dorsal 
to, the other two between the legs, all projecting caudad. Dorsal tufts 
of anal segment each with not more than four setae, usually only three. 
A large number of specimens were obtained from Merrill 
Springs and Merrill Creek (which is fed by springs) from the 
surfaces of rocks, from sand and gravel, wherever the current 
flows swiftest. I have not found the larvae anywhere else on 
the lake. 
The presence of gills on the anal segment and the color of 
the body is sufficient to distinguish the larva of D. mendotae 
from D. waltlii (3), the larva of which was described by Johann- 
sen (1903). There are additional very marked differences in 
the form of the labium, mandibles, antennae, etc.; but those of 
the mouthparts especially are difficult to examine because of the 
unusual opacity of the larva, which makes the use of clearing 
solutions necessary in the study of this larva. 
Since the laryae of Thalassomyia are said to resemble those 
of D. waltlii so closely as to be indistinguishable on superficial 
examination it is well to call attention to the differences shown 
in the form of the labium. In all other respects, such as the 
presence of gills, the type of antennae, mandibles, and also in- 
cluding the form of the labium, the larva of D. mendotae is much 
more closely related to Thalassomyia ohscura Johannsen than to 
Diamesa. So marked is this relation that on first examination 
the writer determined the larvae positively as those of a Thalas- 
somyia. It was only when pupal and adult characters were exam- 
ined that this identification was reconsidered. 
The question naturally arises: Did Professor Johannsen in 
studying his material perhaps make an inadvertent interchange 
of the larvae of Diamesa waltlii and Thalassomyia ohscura t It is 
only by assuming such an interchange that I can find any solu- 
tion to this taxonomic perplexity. Among the hundreds of 
larvae and pupae and pupal skins collected from Merrill Creek 
and Merrill Springs I cannot find a single one which would indi- 
cate the presence of any other genus but Diamesa. I have 
