284 



BULLETIN OF THE BUREAU OF FISHERIES. 



They are also cannibals like the Cy bister larvae, and whenever two come together they fight until 

 one or the other gains the victory, and the winner then sucks the body of his vanquished foe. 



Description of the larva. — General form a flattened spindle 70 nun. in length, tapering both an- 

 teriorly and posteriorly, widest through the mesothorax and metathorax, 7 mm. Each of the three thoracic 

 and the first six abdominal segments has a pair of dorsal sclerites which meet at the center and cover 

 nearly the whole dorsal surface. Head (fig. 34) wider than long, considerably flattened, its anterior 

 margin evenly rounded and fringed with short spines. Antennae filiform, the second, fourth, and 

 sixth joints several times the length of the first, third, and fifth, the terminal joint but little more than 

 a spine. Mandibles similar to those of Cybister, with a basal process on the ventral surface, but without 

 a fringe of bristles along the inner margin. Maxillae (fig, 36) uniramose, eight-jointed, the first, fourth, 

 and sixth joints longer than the others; the basal joint stout, somewhat curved, with a rudimentary 

 process on the inner margin at the distal end. Labium (fig. 37) wider than long, deeply emarginate 

 anteriorly; labial palps three- jointed, the basal joint longer than the other two; ligula wanting; inside 

 the base of each palp are two short and stout spines. Legs long and fringed with short hairs. 



Last two joints of the abdomen much narrowed and heavily fringed with stout swimming setae. 

 Terminal joint with two long cerci on the ventral surface near the distal end, which are about half the 

 length of the joint and are also heavily fringed with setae. These are chiefly used to suspend the larva 

 from the surface film of the water when it is breathing. Color a dark olive-brown, deepest on either 

 side of the mid line, along which extends a much lighter stripe. 



Pupation. — After the larva is fully developed it leaves the water and breathes now through the 

 thoracic and abdominal spiracles, which have opened for use. We are told that the Dytiscus larva 

 makes a rough cell in the earth along the pond shore or on the bank of a stream, but we are not informed 

 whether this pupal chamber is simply hollowed out in the soft earth or is made of mud pellets like that 

 of Cybister. Theoretically, the latter seems much the more probable, and the chamber from which 

 the larval skin and newly emerged adult were taken was made of pellets. 



Description of the pupa. — So far as known the pupa of this species has never been seen except by 

 Wickham (1894), and he has not given us any description of it. We can only give the characters of 

 Dytiscus pupae in general. The prothorax projects over the head, hiding a portion of the eyes. The 

 elytra and wings are short and reach only to the third or fourth abdomen segment. The hind legs are 

 also short and do not reach beyond the seventh segment. Each of the first six abdomen segments is 

 raised into a ridge on its dorsal surface near the posterior margin, but these ridges are much less prom- 

 inent than those on the Cybister pupa. In the figure of a European species published by Miall (1895) 

 from Sclii0dte the dorsal surface is nearly flat and there are no perceptible ridges. At the posterior end 

 of the abdomen are two cerci, which are oblong and project for a considerable distance. In the European 

 species they are fringed with setae, but in an American species figured by Kellogg (1908) they are without 

 setae. 



Miall (1895) said that the pupal stage in summer lasts only a fortnight or so, but Kellogg (1908) 

 said that it lasts three weeks. Fortunately we have a statement with reference to the present species. 

 Wickham (1895a, p. 71) said in the Canadian Entomologist: 



The length of time spent in this latter pupa stage must vary greatly in different broods and with the various species, butit 

 was foimd to be 10 or 11 days in the case of Dytisciis verticalis, of which a larva, taken at Bayfield, Wis., pupated July 18, the 

 beetle appearing on the 28th. 



Habits of the adult, — The long hind legs are admirably adapted for swimming, though they are not 

 as powerful as in Cybister ; but they are not suited for walking, and the beetle flounders about clumsily on 

 the land. It flies well and is frequently captured at night around trap lanterns. With reference to its 

 food, two of the quotations already given under the habits of the larva apply also to the adult. In 

 addition, Cooke in his ''Molluscs" (1895, p. 59) cited the case of a Dytiscus in an aquarium that killed 

 and devoured seven Lymnaea stagnalis in one afternoon. These beetles also ate Lymnaea peregra, but 

 seemed to prefer stagnalis. 



This beetle was abundant in two of the ponds and common in eight of the others; yet only two 

 larvae were found in three seasons of work on the water beetles, and in spite of the abundance of the 

 adult beetles not one of them has ever been seen attacking young fish either in the fishponds or in 

 aquaria. 



Description of the adult. — Like Cybister the adult when it first emerges is soft and pale in color, the 

 only pigment appearing in the legs and eyes. Several days elapse before it hardens and assumes the 



