WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



243 



On the other hand, cannibahsm 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 Tropisternus 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 



