322 



BULL^ETIN OF THE BUREAU OF FISHERIES. 



then moves her spinneret rapidly from side to side, guiding the thread with her hind legs. Where the 

 silk comes in contact with the support it is flattened for a short distance and pressed against the surface, 

 to which it adheres firmly. In this way is formed a little bag with a rounded bottom, one side being 

 flattened against the support and the mouth of the bag left open. These bags are from 6 to 10 mm. 

 long and from 4 to 5 mm. in diameter, and it takes a female about 20 minutes to complete one. The 

 work is interrupted only by the necessity of obtaining fresh air, which happens about twice during 

 the entire period. 



When the bag is completed, the eggs are deposited in it. Each egg is wrapped in silk, leaving 

 the larger end loosely inclosed, while the silk about the smaller end is gathered into a narrow strand 

 which is securely fastened to the side of the case that is flattened against the support. The egg does 

 not stand perpendicular to this side of the case but is inclined a little toward the open end. Each is 

 fastened to the others that come in contact with it, as well as to the side of the case. The number of 

 eggs in a case varies greatly; sometimes there are only 6, and again the nimiber may rise to 20 or even 

 24. The eggs in 31 cases were counted in August, 1919, with the following results: Two contained 6 

 eggs, nine contained 7 eggs, seven contained 8 eggs, three contained 9 eggs, two contained 10 eggs, one 

 contained 11 eggs, three contained 12 eggs, two contained 14 eggs, one contained 17 eggs, and one con- 

 tained 24 eggs. The fastening of the eggs in place occupies three to five minutes according to the num- 

 ber of eggs laid. After completing the bag and before depositing the eggs the female usually takes a 

 short rest of about five minutes. 



As soon as the eggs are deposited she spins a cover for the open end of the case, which is fastened to 

 the side of the case next to the support, but elsewhere is left free. It is pushed into the case a little 

 and is made somewhat larger than the opening, so that the margin is held in place by friction. The 

 open end of the case is then prolonged 0.5 mm or more beyond the cover, and the side next to the sup- 

 port is spun into a narrow flattened ribbon, 20 to 60 mm. in length, which gradually tapers toward the 

 tip, and which is not attached but floats freely in the water. As soon as the ribbon is long enough the 

 beetle transfers her hold from the support to the ribbon, and as she spins rises gradually away from the 

 support. The spinning of the cover and the ribbon occupies about 10 minutes, and the female remains 

 beneath the water the entire time. 



During the spinning of the case, the deposition of the eggs, and the spinning of the cover and ribbon, 

 the female is attended by the male, who clings to her back just behind the head, thus leaving the poste- 

 rior end of the body free. He can not, or at least does not, remain beneath the water as long as the 

 female, but leaves her and rises to the surface frequently for fresh air, returning to her immediately. 

 His function seems to be one of protection against disturbance, particularly from other males of the 

 same species, which he drives away whenever they appear. When the case is finally completed, the 

 male and female separate and rest for a long time. The same pair construct at least several cases, but 

 just how many could not be determined. The eggs hatch in about three days. 



Richmond (1920, p. 31) gave as the first characteristic of the Hydrophilidae, to which the present 

 genus belongs, the following: ''Egg cases characterized by their homy mast and comparatively larger 

 size." Although that statement is correct in general, there are just enough exceptions to prove the 

 rule, and the present species is one of those exceptions. Out of several hundred egg cases not one 

 was found with a "mast," but every one had a long narrow ribbon as just described. This is the more 

 noteworthy since both the species that follow finished their cases with a mast. 



The egg case of the present species may be characterized as follows: The case proper one-and-a- 

 half to two times as long as wide, considerably flattened and fastened lengthwise to the support, with 

 a fag in the shape of a narrow ribbon three to six times as long as the case itself and of the same color 

 and texture as the latter. It is flexible, never fastened to the support but sometimes clinging to it, 

 and usually floats freely in the water. 



Habits of the larva. — This larva swims rapidly, not only using the swimming setae on its legs, but 

 also utilizing the tufts of hairs along the sides of the abdomen by means of a vertical undulatory motion. 

 This latter movement enables it to swim backward as well as forward, and so great is its agility that it 

 can dart forward to capture its prey and then shoot backward to avoid danger. When out of the water, 

 it can also move either backward or forward quite rapidly by means of similar undulations or can even 

 lift its body free from the ground like the larva of Laccophilus or Coptotomus. 



f See p. 332. 



