WATER BEETLES IN RELATION TO PONDFISH CtTLTURE. 



299 



Genus ACILIUS Leach. 



AoUius (Leach, 1817, p. 69), 



This is a genus of medium-sized species, which are somewhat obovate in form 

 and have the upper surface in the male smooth and regularly punctate; in the 

 female usually sulcate but still distinctly punctate. The front tarsi in the male 

 are dilated and armed with one large and two medium-sized disks and a number 

 of smaller ones. The middle tarsi are equal, and the hind claws are equal. Neither 

 the larvae nor the adults have been known to attack or injure young fish, and the 

 larvae show no cannibalistic tendencies but live peaceably with one another, even 

 in an aquarium. 



Acilius semisulcatus Aube. Figures 21, 75-78. 

 Acilius semisulcatus (Aub6, 1838, p. 132). 



Eggs. — ^Miall quoted Regimbart as saying that Acilius lets its eggs drop at random upon the mud 

 while swimming about (Miall, 1895, p. 40), but it would seem probable, judging from its ovipositor, 

 that some are inserted in the stems of water plants. 



Habits of the larva. — Thirty larvae were obtained from pond 7D when it was drained and were kept 

 in an aquarium for 10 days. Nearly every kind of larvae in the ponds was offered to them as food, but 

 the only thing they ate, as far as observed, was a few small snails. Efforts were also made to induce them 

 to pupate but without success. From their great similarity to the larvse of the two species of Ther- 

 monectes, from their size and from their close correspondence to the known larvse of Acilius as found in 

 Europe, it is reasonable to assign them to the genus Acilius. And since semisulcatus is the only species 

 found in the ponds they have been referred to this species. 



They are larger and even more graceful and agile than the larvse of the two Thermonectes species 

 just described. They swim rapidly and easily, always keeping the abdomen curved upward, often 

 at an angle of 45° . When resting on the bottom or on any support, the abdomen is curved into a crescent 

 shape, the center of the curve in contact with the support. When they wish to rise to the surface, they 

 can usually do so by simply letting go of the support. The air within the tracheae is then buoyant 

 enough to carry them up tail first. Occasionally they are forced to swim with slight movements of 

 the legs or by a sort of jerking motion of the abdomen, and the tail is still kept upward. At other times 

 they swim up head first and when near the surface elevate the tail and thrust it above the water. After 

 doing this the body is allowed to drop downward, and they hang from the surface film for a long time, 

 supported by their cerci. 



Like Thermonectes they can flex the body suddenly at the first abdominal segment and straighten 

 it with equal rapidity, both movements being imperceptible to the naked eye. The result is a snap or 

 jump that often throws the larva several inches. This is undoubtedly the ''peculiar indescribable 

 motion of the whole body away from the point of disturbance " declared by Needham and Williamson 

 (1907) to be characteristic of one of the larvae studied by them. They added also: "Sometimes it 

 makes just one quick dodge, and sometimes it goes through a series of wriggling movements so swiftly 

 executed that the eye can not follow them" (1907, p. 489). They ascribed their larva to a species 

 of Acilius, which it may well have been, since the figure they gave (fig. 8, b, p. 491) of the maxilla com- 

 pares favorably with Figure 76, although the palp had not yet acquired the short spherical penultimate 

 segment. When a larva dies and sinks to the bottom of the aquarium, it is always found with the body 

 sharply flexed at the first abdominal segment and must be straightened before being preserved. 



Description of the larva. — General form an elongate spindle 24 mm. long and 3.15 mm. wide, narrowed 

 considerably both anteriorly and posteriorly, widest through the third and fourth abdominal segments, 

 whose diameter is a little less than one-seventh of the body length. The color pattern affords the readiest 

 means of recognition and is given in detail. General color olive yellow; head with the following black — 

 the tips of the antennae, maxiliae, and palps, the eye areas, the anterior margin of the labro-clypeus, a 

 spot on either side of the mid line just behind the anterior margin, a broad band connecting the posterior 

 ends of the eye areas, and irregular spots on the sides and dorsal surface at the base of the head. On the 

 dorsal surface of the thorax and abdomen the sclerites cover the entire tergum and extend onto the 

 pleurum, so that their lateral margins are invisible in dorsal view. Each sclerite is narrowly bordered 



