WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
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is wider; tlie maxillary and labial palps are shorter and stouter; the wings are about the same length; 
the metastemal spine is longer and not as stout; the cerci are longer and more slender; the third legs 
reach the penultimate abdominal segment; the eyes are not as red and are considerably lighter in color. 
Of 10 larvse reared in 1919, 6 remained in the pupa state four days, 2 remained five days, and 2 six 
days. Of 10 larvse reared in 1920, 2 remained in the pupa state three days, 6 remained four days, and 2 
remained five days. Wickham (1894), in endeavoring to rear the larva of a bombardier beetle which 
had infested the pupa of Dineutes americanus, substituted in place of the gyrinid food a pupa of the present 
species and it was accepted by the parasite. Another glaber pupa was infested with the maggots of a 
species of Phora, one of the Diptera. These two instances would indicate that the pupae of the present 
species are susceptible to the attacks of parasites of this sort. 
The adult beetle. — The adults are larger than those of either of the other two species here described, 
attaining a length of 9.50 to 11 mm., and rarely reaching 12 mm. The form is a much broader oval, and 
the punctures of the elytra are larger and more distinct. The head, thorax, and elytra are entirely black 
on the dorsal surface and more or less bronzed, with no yellow stripe or silky hairs along the lateral 
margins. Ventral surface also entirely black, dull on the body, but shiny on the legs and sternal crest, 
the five basal joints of the antennae, the laciniae and palps of the maxillae, and the labium yellow. Basal 
joint of the antenna considerably longer than in the preceding species and more strongly curved. Ter- 
minal tooth of mandibles shorter and secondary tooth longer; labial palps shorter, the setae at the tips 
subterminal. 
This species has never been seen feeding on Anabasna or Clathrocystis but stays beneath the surface 
and eats Chara, Nitella, etc. While this species is not as abundant as the preceding it is still found in 
every pond but two, and it breeds as prolifically. For this reason it becomes an important item in the 
food of the young fish. 
Tropistemus mixtus (Leconte). Figures 138, 141, 144. 
Hydrophilus mixtus (Leconte, 1855, p. 368). 
Egg case and eggs. — The egg case is from one and a quarter to two times as wide as long, strongly 
flattened and fastened to the support transversely, the mouth usually projecting slightly beyond the edge 
of the support. The fag 9 is in the shape of a slender cylinder made of twisted strands which stands 
rigidly upright in the water, is the same diameter throughout, and is usually frayed at the tip (fig. 142, 
p. 327). When the mouth of the case does not reach the edge of the support, the side next to the support 
is prolonged to the edge as a broad ribbon, securely fastened, and the fag is attached to this ribbon. 
The fag is always darker in color and coarser in texture than the case, the cap, and the bread ribbon. 
Case 6 to 8 mm. long, 7 to 10 mm. wide, and 4 to 6 mm. high at the mouth; fag 8 to 12 mm. long. 
The egg is much shorter and broader than that of lateralis; the latter is three times as long as wide, 
but the egg of the present species is less than two and a half times as long as wide. From 8 to 12 eggs 
are laid in a single case, and they hatch in four or five days. 
Habits of the larva. — The habits of these larvae are very similar to those of the other two species but 
differ in one marked particular. The larvae of the other two species begin their cannibalistic ravages 
inside the case as soon as they get out of the egg. Many of the larvae are eaten by their brothers and 
sisters before they emerge from the egg case, and sometimes out of an entire brood only one will be left 
to crawl forth eventually. The present larvae, however, act in a radically different manner. Thirty 
cases were brought into the laboratory and placed in a small aquarium and allowed to hatch. A few larvae 
appeared at about the right time and then disappeared overnight. At first it was supposed that they had 
crawled out of the aquarium in some way, and a cover was placed over it to prevent this. No more 
larvae appeared, however, and after a day or two the cases were taken out and opened separately. Two- 
thirds of them were entirely empty, containing neither eggs nor larvae. The other third were packed 
full of active vigorous larvae, each case containing from 25 to 35 and one 42 larvae. They were packed 
very closely inside these cases but were not tangled, for the moment the case was opened they separated 
and swam away in different directions. As the cases contained only 10 or 12 eggs apiece, the larvae must 
have crawled from two cases into a third one and bunched with the larvae of the latter until it was as full 
as it would hold. Moreover, they seemed to be living in perfect harmony; there was no evidence either 
then or subsequently that they were at all cannibalistic. In no instance was one of them seen to attack 
i. 332. 
