THE LIFE-HISTORY OF HYDROBIUS FVSCIPES, L. 323 



Secondary Sexual Characters. 



There seems to be no character by which the sexes can be distinguished at first 

 glance, in fact the only differences I have been able to detect between the sexes are that 

 the antennae and maxillary palpi of the male, when compared with those of the female, 

 are distinctly stouter. In Hydrophilus the sexes are easily distinguished by the anterior 

 tarsi, which are simple in the female and have a dilated apical segment in the male. 

 In Hydrocharis the male has the claws of the anterior tarsi " sharply bent like a 

 grappling hook," * but there is no such distinction in Hydrobius fuscipes. 



Habits. 



The food of the imago Hydrobivs consists chiefly of plant tissues, although it is also 

 very ready to eat up dead insect larvae, snails, etc. Its favourite food appears to be the 

 filamentous algae and also algae growing upon stems of other water-plants, etc. It also 

 readily devours dead plant tissues, for instance, decaying blades of grass, leaving the 

 fibrovascular bundles, or at any rate stripping them first of the softer tissues. Apparently 

 only when driven to it for want of other food will it attack living higher plants, and 

 then it seems to prefer the epidermis. It was quite extraordinary the amount of 

 filamentous algae which it caused to disappear when kept in an aquarium. 



During the colder months of the year Hydrobius disappears more or less completely 

 from its usual habitats but, as with regard to other water-beetles, there seems to be no 

 definite knowledge as to where it disappears to. The beetle passes the winter as an imago. 

 I have never found the larvae in the autumn, although Miall (1895, p. 93) says that 

 eggs and larvae are to be found in August. I have, however, occasionally found the eggs 

 of Helochares, a related genus, in August, but I fancy that autumn eggs are exceptional 

 and that the larvae do not survive the winter. I think that the imago burrows in the 

 mud at the bottom of the pond and passes the winter in the torpid state, but this is only 

 theory. The only evidence I have is that in the aquaria in cold weather very few 

 beetles are visible, and as they cannot get out of the water they must be amongst the 

 stones at the bottom. Also some specimens of Bidessus minutissimus, which I have 

 kept through the winter in a tumbler, have spent the whole time amongst the fine 

 gravel, and I have not seen them once for many weeks. 



With regard to breeding habits the usual egg-laying period seems to be from March 

 to July, although Miall (I.e., p. 93) mentions eggs in August. This year I found several 

 egg-cocoons on 2 1st February, but I think that was exceptionally early owing to the long 

 spell of mild weather in this district (near Belfast). Seeing that the larval period lasts 

 from twelve to fourteen weeks, I doubt if eggs laid later than the end of July would 

 have much chance of getting through before the cold weather, and the absence of larvae 

 during the winter seems further evidence of the limits of the egg-laying period. 



* Fowler, W. W., British Goleoptera, i., p. 220. 



