94 MK. F. BALFOUR-BROWNE ON THE LIFE-HISTORY 



(5) The Life-Cycle. 



Thus from the hatching of the egg to the appearance of the 

 imago occupies from nine to about fifteen weeks. On this basis, 

 the earliest imagines should appear about the middle of June, 

 and this raises the question whether eggs laid in July and 

 August may not be the commencement of a second annual 

 generation. From an examination of the ovaries of a few 

 beetles in August which emerged in the previous June, I am 

 inclined to think that there is normally only one generation a 

 year. Perhaps also the longevity of the beetle, at any rate of 

 the female, which will live for three years and perhaps longer, 

 suggests a one-year cycle. 



On the otlier hand, the possibility of a few precocious indivi- 

 duals breeding soon after emergence must not be lost sight of 

 (and also the possibility of old females laying a second batch of 

 eggs must be kept in mind), though it does not follow that the 

 resulting larvae would complete their metamorphosis. 



I make this suggestion that a second generation may be 

 commenced, even if not completed, because of a number of 

 experiments I made some years ago with a few species of 

 Hydrophilid beetles. By keeping these in tumblers in an 

 incubator at a high summer temperature, 19° to 21° 0. (66° to 

 70° F.), I found that it was possible to get egg-cocoons from 

 some of them (e. g., Hydrohius fascines L. and PMlhydrus inari- 

 twius Thorns.) in JSTovember, after only a month of treatment — 

 ego--cocoons which, under normal conditions, would not have 

 been produced until the following April or May. A hot June 

 and July might therefore cause the ovaries of Pelohius to mature 

 rapidly, and this would account for the young larvae which I 

 found in August at Beckham. 



The life-history of Pelohius is interesting because of the 

 gill-bearing larva, which does not require to come to the surface 

 to renew its air-supply. In this respect it differs entirely from 

 the larvae of the Dytiscids and Hydrophilids, and resembles those 

 of Gyrinids and Haliplids, but like, apparently, all other aquatic 

 beetles, the pupa requires free air, and therefore, like the majority 

 of them, the full-grown larva leaves the water and burrows into 

 the soil, where it forms its pupal cell. 



The change on the part of the larva from an aquatic to a 

 terrestrial habit does not involve a change in the respiratory 

 apparatus, although I had expected one or more pairs of spiracles 

 to open during the last larval stage, as happens in the last stage' 

 of the nymph of the dragonfly. But, whereas the dragonfly 

 nymph comes from the water into the air with the object of 

 d'rving itself, the Pelohius larva leaves the water and quickly 

 burrows into the earth, and therefore, presumably, is under 

 sufiiciently damp conditions to allow the gills to function until 

 the larval skin is thrown oft'. 



