320 MR FRANK BALFOUR-BROWNE ON 



fairly long hairs forming a more or less continuous row right across the front. The 

 palpi are three-segmented, the middle segment of each bearing a thin row of long hairs up 

 its inner side, while the apical segment bears one or two very long hairs on its outer 

 side about one-third of its length from the apex. 



When the beetle is out of the water the maxillary palpi are withdrawn and lie 

 folded against the maxillae under the sides of the head, the antennae being turned 

 forward and constantly waved about. In the water, on the other hand, the antennas 

 are withdrawn and the maxillary palpi extended, obviously being the subaquatic organs 

 of touch and possibly also of smell and hearing. 



The Thorax. 



Only two or three points about the thorax call for remark. The mesosternum bears 

 in the median line a small pointed projection such as is found in certain other members 

 of the Hydrophilidse, e.g. Anaccena limbata, F. ; but whereas it is in the genus Anacsena 

 useful for distinguishing A. limbata, F., and ovata, Reiche, from A. globulus, Payk., 

 where it is represented only by a small tubercle, it does not help in Hydrobius to 

 distinguish H. picicrus from H fuscipes. 



The three pairs of legs have the femora covered with a dense mass of short hairs 

 except in the apical region, which is smooth and shining. 



On the mesothorax and attached both to the tergite and to the elytra is a pair of 

 small alulae. It has been suggested that these organs, which occur in the Dytisoidae 

 and Hydrophilidse, may be responsible for the sounds which some of the species produce, 

 but this suggestion is discounted by the fact that, although both sexes of Hydrobius 

 fuscipes possess them, only the male produces a noise. 



With regard to the abdomen, the only part of special interest for this paper is the 

 sexual armature, especially that of the female. 



The Male Sexual Armature. 



The penis (figs. 10 and 11) is of the general form common to all the Hydrophilidae, 

 possessing two side pieces or valves which are said to be at least partly for the purpose 

 of retaining the penis within the vagina at the time of copulation (Henneguey, Les 

 Insecles, 1904, p. 185). I am inclined to think that they are rather for the purpose 

 of spreading the walls of the vagina so as to allow the seminal fluid to escape rapidly 

 from the duct, and no doubt the valves also function as a penis sheath when in a state of 

 rest. I have used the term " penis " in connection with this organ, but it is perhaps open 

 to question whether it is correctly so used. Some authors speak of it as " the sedeagus." 



There seems to be some difference of opinion as to what abdominal segments are 

 represented in the male of Hydropldlus, and since Hydrobius is very similar to Hydro- 

 philus as to its male sexual armature, the remarks apply equally to both genera. 



