WATER BEETI^S IN RELATION- TO PONDFISH CULTURE. 



329 



is wider; the 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 larvae reared in 1919, 6 remained in the pupa state four days, 2 remained five days, and 2 six 

 days. Of 10 larvae 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 palpe shorter, the setae at the tips 

 subterminal. 



This species has never been seen feeding on Anabaena 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. 



Tropisternus mixtus (Leconte). Figures 138, 141, 144. 



HydropMlus 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^ 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 react 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 broad 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 e^s 

 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 oi 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 larv^ 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. Thiity 

 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 wae one of them seen to attack 



9 See p. 332. 



