23t) On Aquatic Carnivorous Coleoptcra or DytiscidcB. 



point of connexion form a very acute angle, and proceeding forwards like the 

 branches of a letter V diverge but little till they have become quite near to the 

 middle coxa when they are rather abruptly turned outwards. In Hydrovatus these 

 sutures of the two sides diverge from one another only at a very obtuse angle, and 

 therefore are directed but little forwards at first, but when rather near to the out- 

 side of the body, the direction becomes more abruptly forwards, but only to be 

 almost immediately even more abruptly bent back : a similar form of the coxa is 

 pretty general in the Hydroporini. The anterior border of the coxa of the 

 Dytiscidpe thus forms in front an arch which is very various in its form, in Eretes 

 as we have seen it is a broad, very flat arch ; while in Hydrovatus a naiTow abrupt 

 curve placed near the outside of the body is formed. Yaiious intermediates 

 between these extremely different forms occur ; and it is only in the Noterides that 

 we find a really distinct form of the coxa, owing to the fact that this part attains 

 its greatest extension forwards near to the middle of the body, and then becomes 

 shorter towards the side. In the Cjibistrini the coxa assumes in front a shape 

 characteristic of the group, its greatest anterior extension being gained near the 

 outside of the body, while external to this point it so abruptly retreats as to form 

 almost a right angle. In size tlie external lamina varies even more than it does 

 in foi'm; it is, compared with the average of other Coleoptera, always large, and 

 within the bounds of the family is least in Pelobius and Amjihizoa; in certain 

 species of Agabus (Dytiscus uliginosus. No. 694, Agabus maderensis No. 666), it 

 IS not greatly larger than in Amphizoa, and with every gradation of growth in 

 various species and genera, reaches a truly enormous size in Hyphydi'us, Eretes^ 

 Acilius and Coptotomus. 



Though the external lamina is of much interest and importance, yet the internal 

 lamina on account of its being the seat of the articulation of the portion of the leg 

 used for swimming is much more important, and much more complex and varied. 

 The base of each trochanter is placed in a kind of box, and the larger the opening 

 of the box the greater is the range of motion of the limb, and the more powerful 

 its sweep, and in the higher forms of the Dytiscida? the articulation is constructed 

 so as to allow the leg to make a sweep extending round the complete half circum- 

 ference of a circle, and yet the articulation is so well constructed that no water 

 can obtain entrance by it to the interior of the body where the muscles ai'e situated. 

 The socket is formed above by an arched plate, which appears to be merely a 

 prolongation of the abdominal portion of the external lamina of the coxa, and 

 below by the coxal lobe or process, there being left between these two parts a 

 large circularly transverse cleft looking backwards and laterally ; the internal Avail 

 is concealed owing to its being closely connected with its fellow of the opposite 

 side of the body ; in fact in all the higher forms of the family, there seems to be 

 only a single opening for the two legs — in other words the two transverse 

 clefts appear joined into one — but when a dissection is made, it is seen that they 



