WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
243 
On the other hand, cannibalism in so many of the species is a very serious 
obstacle to the attainment of their full life history. Needham and Williamson 
(1907) related a characteristic experience with one of these species. They tried 
various devices to prevent them from eating one another; they even went so far 
as to construct a series of small wire chambers in which individual specimens could 
be kept separately. The partitions between the chambers projected some distance 
above the surface of the water, but in the night the isolated larvae climbed out of 
the water and over the tops of the partitions and ate one another just the same, 
leaving but a solitary victor in the morning. The present author has experimented 
with all but one of the species whose life history is here given and several besides. 
Hydroporus, Enochrus, and Tro'pisternus mixtus are the only ones outside of the 
Haliplidae whose larvae could be induced to live together in peace. 
TIME OF PUPATION. 
This is a critical period in the life of a larva; it must then emerge from the 
water and either build its pupal chamber or find a place where it can hollow out one 
in the mud. The danger varies with the length of time the larva remains uncovered, 
and in this particular those that bury in the mud have a decided advantage. The 
larva of Berosus, Tropisternus, Laccophilus, or Coptotomus does not travel far 
from the water’s edge before it begins to burrow into the mud, where it is soon 
covered and protected. Once beneath the surface of the ground it can take as 
much time as necessary in fashioning its pupal chamber. 
On the other hand, a larva like that of Thermonectes must gather mud into 
pellets and build itself a chamber, and not until the latter is fully completed is the 
larva safe from its enemies. It is even worse in the case of the Dineutes larva, 
whose pupal chamber is constructed with more care, and hence requires a much 
longer time for its completion. Such enemies as parasitic Hymenoptera and 
Diptera, Carabid beetles, and the like, have more time to locate these larvae, and 
it is not unusual to find the beetle pupae infested with the larvae of these parasites. 
Then, too, there are enemies like the frogs and turtles, which sit on the bank and 
watch for their prey. The mud-burrowing larvae quickly bury themselves and escape 
notice, but the movements of Theromonectes, Dytiscus, and Dineutes larvae attract 
attention and reveal the presence of the larvae to their enemies. 
Again rapidity of movement, especially locomotion, becomes a vital factor 
at this critical period. Thermonectes is very agile in all its movements and quickly 
throws together the fragile walls that constitute its pupal chamber. This partially 
compensates for the attention it attracts and helps it to escape danger. Dineutes 
however, is slow and sluggish and does nothing with rapidity, and hence it more 
often becomes infested with parasites. The Berosus larva is another slow mover, 
and many more of them than of the agile species fall victims to the parasites. 
DRAGONFLY NYMPHS. 
The larger nymphs, like those of Aeschna, Anax, and Libellula, eat beetle 
larvae, even including those of Hydrous and Dytiscus. The mature nymphs in 
June and July can easily overpower a dytiscid or a hydrophilid larva an inch or 
