394 Wisconsin Academy of Sciences, Arts, and Letters. 
2. Hydraclinida. (Table 5, p. 474.) 
The great bulk, I may say 99%, of the Hydrachnid contin¬ 
gent of the lake is composed of members of the Hygrobatidae. 
The commonest of all forms is Limnesia histrionica (Hermann), 
which occurs as practically the sole representative of the sub¬ 
family. Other common forms are Curvipes turgidus Wolcott, 
Atax ypsilophorus (Bonz), etc. The genus Arrhenurus espe¬ 
cially offers a large field, for quite a number of species have 
been taken, several of them apparently new to science. 
Except for an occasional Arrhenurus, Limnesia histrionica 
forms the chief Hydrachnid contingent of fish food. How¬ 
ever, on a number of occasions I have found Eylais and Hygro- 
bates present, especially in creek fishes. It is of interest that 
altho the red Hydrachnida and their scarlet eggs are shunned 
by fish, insects, and other Hydrachnida, they—the red ones— 
appear to enjoy their own eggs as food. They also will readily 
attack other Hydrachnida, but usually not the red species. 
In general, the Hydrachnid food consists of their own kind, 
of Chironomid larvae, and of other animals whose pellicle they 
can pierce. All of the nymphs, most of the larvae, and a few 
of the adult mites are parasites. The symbiotic relation with 
sponges suggested by some writers is probably inapplicable; 
the sponge merely offers transient shelter, for mites will hide 
as readily in empty shells, in larval cases, and in egg clusters 
(Odonata, Trichoptera, Gastropoda, etc.). 
Of all groups present in the lake their distribution seems 
most uniform. Respiration seems a slight factor with them; 
most of them have lost their tracheae entirely and breathe 
through their skin. Their free swimming habit makes them inde¬ 
pendent of a substratum except as a source of food, while their 
small size permits easy access to shelter. 
III. INSECTA. 
A. Order Plecoptera. 
At least three species of stone-fiies occur in Lake Mendota. 
Each of these appears to be fairly abundant, the adults ap¬ 
pearing in April, June, and July, respectively. Yet for 
some reason unknown at present, I have failed to find any evi- 
