336 MR FRANK BALFOUR-BROWNE ON 



water and burrows into the ground and after several days becomes a pupa. My larvae, 

 reared in tumblers, indicated their maturity by great activity and by many futile 

 attempts to get out. As soon as they were placed in small glass dishes containing 

 moist earth they burrowed into it. 



In the earth the larvae spins a silken cocoon. My larvae burrowed to the bottom of 

 the glass dishes in perhaps an inch of soil, and there formed the cocoons in contact with 

 the glass. In some cases these were sufficiently thin to allow the larva to be seen 

 within. I was surprised at the little difficulty I had in getting the larvae to burrow 

 into the soil. Lyonnet found the larvae of Hydrophilus very particular in the choice of 

 soil for pupating in, and only succeeded in getting pupa after several attempts. So 

 far I have always failed to get the larvae of Hydrophilus to pupate, although possibly 

 the fact that I have reared them beyond the limits of natural distribution may have 

 affected them. The larvae of Hydrobius accepted garden soil without demur. 



Once the cocoon is complete the larva rests and does not at once change to a pupa, 

 as I have found larvae unchanged after eight or nine days in the cell. 



The Pupa. 



The pupa is of the typical Coleopterous form, but bears on the dorsal side of its 

 thoracic and abdominal segments series of stiff hairs. In Hydrophilus only a few hooked 

 spines exist in place of these hairs, and Lyonnet discovered that they were supports 

 upon which the pupa rests within the cell. The Hydrobius pupa, I found, was very 

 active and turned in all positions in the cells, not by any means always lying passively 

 on its back. 



The pupal stage lasts but a short time. I cannot give the exact duration, as I 

 omitted to note the times at which the larvae changed to pupae, but, from the time 

 the larva burrowed into the ground, it was three weeks before I saw any imagines in the 

 cells, and as at least some of the larvae remained unchanged for eight or nine days, the 

 pupal condition lasted about eleven or twelve days. There are, no doubt, variations in 

 the length of this stage, just as there are in that of the maturation of the egg and of 

 the larva, according to temperature conditions. 



The Life- Cycle. 



Miall (I.e., p. 93) states that "very soon after the emergence of the beetle the 

 cocoon is formed and the eggs are laid," and " all the stages may be found together 

 about the end of August." So far as my experience goes, Hydrobius completes its life- 

 cycle during the one season, the whole cycle occupying about twelve weeks. I have 

 never been able to find eggs or young larvae in August or later, yet, if Miall's state- 

 ment is correct, larvae should be found during the winter, as are the larvae of the 

 Dytiscidyj. It is possible that some of the later larvae may pupate in September and 



