I AQUATIC BEETLES 53 



sole to the hind foot, and the usual pair of stout claws 

 become rudimentary.^ The fore and middle legs are 

 shorter and stouter, the fringe of hairs less conspicu- 

 ous, and the terminal claws usually well developed. 



In Dytiscus marginalis, as in most other species 

 of the family, we find a curious modification of the 

 tarsus of the fore legs of the male, which forms, a 

 circular sucker. A similar structure is visible, though 

 much less prominent, in the middle leg of the male. 

 The restriction of these suckers to the male Insect 

 at once suggests that they are accessory reproduc- 

 tive organs, to be distinguished from the many cases 

 of suckers or like organs merely used in creeping 

 on inclined or vertical smooth surfaces. These last 

 occur in Insects of many kinds, in both sexes, and on 

 all the six legs. The clasping suckers, on the other 

 hand, are restricted to the male, are borne only on the 

 fore, or fore and middle legs, and are peculiar to the 

 Coleoptera. 



In the fore leg of the male Dytiscus the three first 

 joints of the tarsus are enlarged to form a circular 

 disc ; beyond these are two small and ordinary joints, 

 of which the last bears the usual pair of claws. The 

 same three joints are somewhat enlarged in the 

 middle leg also, and can be used as suckers. They 

 are not however formed into a disc, nor does their 

 peculiar structure catch the eye at the first glance. 

 When the circular disc of the fore leg is carefully 

 examined, we find that its under surface is roughened 

 by a number of minute stalked bodies, which are 



^ In some other Dytiscidae, e.g. in Cybister, the swimming 

 legs are much more powerful than in Dytiscus marginalis. 



