On Aquatic Carnivorous Coleoptera or Dytiscidce. 961 



shallow. In the structure of the hind legs Eretes shows some interesting pecu- 

 liarities, they are much more slender than in any of the other Hydaticides ; 

 the femora especially are slender, the tarsi are elongate and their basal joint 

 not very greatly shorter than the tibia : on the upper face of the femur close 

 to its posterior edge is a band of pubescence, this band traverses the whole length 

 of the limb, the pubescence at its commencement at the trochanter is quite short 

 but gets longer as it nears the knee, and terminates there in a group of very 

 elongate fine hairs ; in Hydaticus there is usually an obscure band of very fine 

 short pubescence, homologous to this band of Eretes, but placed at a distance from 

 the hind margin ; in other Hydaticini, this band is represented only by a series 

 of isolated punctures (Dytiscus seminiger, &c.), and in others cannot be detected 

 in any form : the tibia also shows some interesting modifications ; it is longer and 

 more slender than in the Hydaticini and Thermonectini ; the series of punctures 

 on its upper face bearing elongate furcate setae is very highly developed, and is 

 placed near the extremity (about five-sixths of the length from the knee), where it 

 forms an obliquely transverse band, starting from the inner margin, and extending 

 half way across the tibia ; in the other Hydaticides a very different an-angeraent 

 prevails, the furcate setse are either arranged as a series near to and parallel with 

 the outer margin of the tibia, or as a transverse series on the middle of the face, 

 but are never approximate to the inner margin or to the apex as in Eretes ; it would 

 appear, if this be so, that in the rudimentary stages of development of these furcate 

 setse their position is as a series parallel with the outer margin, and Eretes is of 

 all known Dytiscidse the one which has most widely departed from the primitive 

 arrangement of these sette. 



The tibial spurs in the Thermonectini are bifid at the apex, this character is how- 

 ever frequently so minute that it requires a careful examination to detect it ; these 

 bifid tibial spurs, although wanting in Eretes and Hydaticini, reappear in 

 Laccophilus and Cybistrini, and there this peculiarity of structure attains a more 

 considerable development than in the Thermonectini. 



The straight episternal suture of the Hydaticini is an exceptional feature — a 

 departure from the usual method of stucture of the part — for this suture is curvi- 

 linear, not only in Thermonectini and Eretes, but also in the Colymbetides, 

 Dytiscini and Cybistrini ; in these groups however, the curvilinear suture is not so 

 much curved as it is in the Thermonectini, so that as regards this structure we are 

 brought face to face with the interesting fact, that the Hydaticini and Thermonectini 

 are departures in opposite directions from the average or normal Dytiscid type. 

 Such a consideration renders it difficult to imagine that the Hydaticides, in opposi- 

 tion, say, to the Colymbetides, are descended from any common ancestor, but 

 rather disposes us to believe, that both types of the episternal suture existed in the 

 early ancestry of the Hydaticides, and that the distinctions between the two types 

 would not then be so extreme as they are at present. This view is borne out by 



