254 On Aquatic Carnivorous Colcoptera or Dytiscidce. 



so articulated that it undergoes a process of semirotation when moved through 

 the water, as the result of which when acting as a propeller it presents a broad 

 face to the water, but when it is jsassive or is moved through the water for the 

 purpose of attaining a position from which to commence another stroke it presents 

 an edge ; in other words, what rowers call feathering the oar is performed by the 

 tarsus of the Dytiscidse in a most perfect manner. Both the true upper, and the 

 true lower, surfaces of the tarsus are mere edges, and the former bears elongate 

 swimming hairs, which are depressed Avhen the limb is at rest or moving 

 forwards, but are spread out when the foot is acting as a propeller ; the latter or 

 lower edge being closely set with spines. In Pelobius, however, the sole of the 

 tarsus still exists as it does in the Carabidse ; that is to say the lower face of the 

 foot bears two series of spines and between these series a considerable space 

 intervenes. It is an interesting fact that in many of the higher Dytiscidte there 

 are remnants of this structure still to be seen in the highly changed tarsus ; the 

 sole it would seem, disappears, at any rate in some cases, not by any diminution or 

 degeneration but by a process of growth of the tarsus and concomitant modification 

 of form by means of which the sole becomes j^art of the inner face ; in other words 

 the inner face of the tarsus in the higher Dytiscidse is homologous with both the 

 inner and lower faces of the tarsus of Carabidfe. In the Carabidse the tarsus has 

 two sides curving into one another so that an arch forms the upper portion of the 

 tarsus, and a sole, this latter being bounded on each side by a series of spines. In 

 Pelobius the two sides of the Carabid tarsus and the sole still exist, as in the 

 Carabidse, but form a triangle, of which the outer face is the longest side, and the 

 true inner face of the tarsus is quite narrow, not so broad in fact as the sole, 

 which is still marked off by a series of spines extending the whole length of the 

 foot; in Dytiscus Sturmi (Agabus No. 737) where the tarsus is thoroughly 

 Dytiscideous in form, there may be seen on the inner face of the basal joint a 

 series of three or four spine bearing punctures, placed at a distance from the edge 

 and evidently representing a remnant of the series of spines which serves as the 

 inner boundary of the sole in Pelobius and the Carabidte ; in Scutopterus there is 

 a similar series of punctures on the first and second joints ; and in the interesting 

 Australian genus Hyderodes, this series of spine-bearing punctures actually still 

 exists along the whole length of the four basal joints of the tarsus, on its inner 

 face. Besides these sjaines which exist occasionally, there are on the inner side 

 of the tarsus, other spines more constantly present ; these consist of three or 

 four spines placed on the hind margin of each joint, near the lower edge, and of 

 some others similarly placed close to the upper margin ; in Cybister the lower 

 of these series of spines is reduced to a pair placed on each of the second, third and 

 fourth joints ; in Laccophilus this series is entirely absent, and the upper series 

 consists of a pair of suberect spines, on the first, second and third joints, one of which 

 being much longer tlian the other, projects beyond the lower margin so that its 



