On Aquatic Carnivorous Coleoptera or DytiscidcB. 239 



any Carabidaj, in which the area of the external lamina of the hind coxa is anything 

 like as large as it is even in Pelobius and Amphizoa. In Trachypach3's and Systolo- 

 soma of the Carabidne, the hind coxa is large, but it is made so by the increase in 

 size of the internal rather than of the external lamina. In Silphomorpha the in- 

 ternal lamina is very small, and the external one correspondingly large, thus making 

 a slight approximation to the Dytiscidse. [Second, As regards the arched anterior 

 border of the hind coxa in the Dytiscidse ; such a character is I believe absolutely 

 unknown in the Carabid£e,but unfortunately it is not always presentin the Dytiscidse, 

 being absent in Pelobius, Amphizoa, and Colpius, and very nearly absent in Suphis. 

 Third, The accurate coadaptation of the two internal laminae at their junction on 

 the middle line is quite characteristic of the Dytiscidse and is always present ; it is 

 however approximated by some Carabidse, viz., the Pseudomorphides, Trachypachys 

 and Sj'stolosoma ; but the only one of these that really makes any near approach 

 in this respect to the Dytiscidte is Trachypachys. Fourth, As regards the contiguity 

 of the articular cavities, this is very conspicuous and characteristic in the Macro- 

 Dytiscidte, but in many other Dytiscidse it is not present (Pelobius, Hydrovatini, 

 Hyphydrini, Bidessini, Colpius), indeed a slight separation of the cavities is so 

 common in the lower forms of the various groups, as to strongly suggest the idea 

 that all the species of Dytiscidaj have had ancestors with separated posterior coxal 

 cavities like the Carabidae. 



The Haliplides show not the least approach to tlie Dytiscidfe in the structure of 

 the hind coxae ; on the contrary they possess a peculiar development, which is not 

 approximated by any Carabidse or Dytiscidae, of these parts. 



HiND-BODY, or Abdomex. — The dorsal plates of the hind body are eight in number, 

 and they differ but little from one another in length : they are membranous in tex- 

 ture, but usually dark in colour ; the seventh and eighth are a little thicker than the 

 others, and have thus a leathery consistence, and they are also dull and more or less 

 punctate, especially the eighth, while those in front of them are shining : in Dytiscus 

 the two basal segments bear a large quantity of very fine, elongate hair, and other 

 genera show sometimes a similar development though to a less extent. In Hydro- 

 vatus the dorsal plates are very thin and delicate, and pallid in colour, and the 

 apical one differs but little from the others. The basal segment is attached to the 

 hind margin of the metanotum, and each segment is attached by its sides, b}' the 

 intervention of a very delicate membrane, to the harder side pieces of the body. The 

 metathoracic stigma is placed at the hind margin of the metathorax, at the side of 

 the body, and may be either small and inconspicuous (Cybister), or elongate in the 

 transverse direction (Dytiscus) : there are seven pairs of true abdominal stigmata ; 

 the first abdominal plate is without a stigma, but each of the other plates bears at 

 the outside a stigma placed in its delicate membranous border, the stigma of the 

 eighth or terminal segment, is usually placed quite at its front edge, but in Cybister 

 is placed near its hind margin. The stigmata vary greatly in their size and develop- 



