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BULLETIN OF THE BUREAU OF FISHERIES. 
Habits of the adult . — The habits of the adult are the same as those of the other two species, and the 
food is the same as that of the preceding species. This beetle is not as agile as lateralis but is rather 
clumsy when out of the water. 
Description of the adult . — The adult is from 8.5 to 9 mm. long. This species is distinguished from 
lateralis by the lack of the yellow margins on the thorax and elytra and from glaber by the fact that the 
elytral punctures are of quite different sizes intermingled with one another, and the beetle is consider- 
ably smaller. The anterior portion of the sternal crest and the apical halves of the femora and tibiae 
are brownish-yellow instead of black. The species is only found in a few of the ponds and then not 
abundantly, but it furnishes as good fish food as the others and is harmless. 
Genus HYDROUS Leach. 
Hydrous (Leach, 1817, p. 165). 
The only representative of this genus found in the F airport fishponds is Hydrous 
triangularis (Say), which has already been fully described and figured by the writer 
in this Bulletin, Vol. XXXIX, pages 9 to 38; 23 text figures. 
Genus DONACIA Fabricius. 
Donacia (Fabricius, 1775, p. 195). 
This genus is represented in the Fairport fishponds by the single species Donacia 
sequalis Say. The complete life history of this species with description and figures 
of the larva, pupa, imago, and cocoon is given by A. D. MacGillivray (1903, p. 321). 
CAP ATTACHMENT ON EGG CASE. 
The name and the function of the attachment to the cap of the egg case has 
provoked considerable discussion amongst the various investigators who have 
studied the water beetles, but without any satisfactory results. It has been desig- 
nated by many as a “mast,” but this is open to several objections. Whatever 
name is selected should be one that will apply to the egg cases of all hydrophilids 
indiscriminately. A mast is tall, upright, rigid, and more or less cylindrical, but in 
some hydrophilids (Tropisternus, Enochrus, Berosus) this attachment is flat, 
ribbonlike, and floats pliantly in the water. In others (Laccobius, Helophorus) it 
forms a hollow tube for the exit of the larvae, and in one genus (Hydrobius) it for ms 
a wide collar or cape around the cap end of the case. The term mast is an evident 
misnomer for anything in the shape of a ribbon or collar. 
This cap attachment has also been called a "balancer” or "equipoise,” but 
either of these names implies that it maintains or at least helps to maintain equi- 
librium. Since the great majority of the egg cases are fastened securely to some 
support, it is manifestly impossible for anything of this sort to affect their equi- 
librium. The only cases to which such a term would be applicable are those of 
Hydrous and Hydrophilus, and other genera certainly have an equal claim to 
recognition. 
In striving to interpret the function of this cap attachment it has usually been 
taken for granted that it was in some way necessary to the development of the eggs. 
The investigator has thus been prejudiced in his observations, and all his efforts 
have been directed toward discovering in what way such an attachment could 
benefit the eggs, rather than toward finding its real function. The first and most 
obvious suggestion has been that this cap attachment contributed in some way to 
