WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
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Genus ACILIUS Leach. 
Acilius (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 semisulcaius 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 larvas were obtained from pond 7D when it was drained and were kept 
in an aquarium for 10 days. Nearly every kind of larvse 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 trachese 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 larvse 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 antenna, maxilke, 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 
