204 ADBPHAGA. [Di/fisci'na. 



stigmata of the two last segments much enlarged ; the swimming legs 

 are comparatively slender and furnished with two nearly equal claws ; 

 in the male the three hasal joints are much dilated and joined together 

 so as to form a round disc : only two genera are comprised in the tribe, 

 Hyderodes containing three Australian species which have no yellow 

 margin to the elytra, and Dytiscus in which it is always present. 



SITTXSCUS, Linne. 



This genus comprises about twenty species, which are peculiar to the 

 northern parts of the Old and New Worlds, and do not reach the tropics ; 

 the females are usually deeply sulcate from the base to beyond middle, 

 but are often dimorphic, one form resembling the male in sculpture : 

 one of the best distinguishing characters is the shape of the apex of the 

 process of posterior coxae ; besides the yellow marginal stripe of elytra, 

 the clypeus is always yellow, the colour being sharply divided off from 

 the dark colour of the front of head ; in all our species the labrum is 

 distinctly emarginate. 



The larva of Dytiscus marginalis is a very common object ; it has been repeatedly- 

 figured, but the best figure is by Schiodte (ii., PI. iii., Fig. 6)'; it is also depicted by 

 Westwood (Classification, i. 95, Fig. 5, 14) : it is fusiform, narrowed in front and 

 behind and broadest in the middle ; the head is very large and almost orbicular, and 

 attached to the prothorax by a distinct neck ; the mandibles are simple and very 

 large, and furnished on the inner side towards apex with a suctorial orifice ; the pro- 

 thorax is trapezoidal, narrowed in front, and much longer than broad ; the seventh 

 and eighth abdominal segments are much constricted, cylindrical, and very long, and 

 the eighth is terminated by two short cerci which are thickly furnished with swim- 

 ming hairs ; by means of these cerci the insect can suspend itself at the top of the 

 water, and they are moreover organs of respiration j the legs are rather long and 

 slender and are ciliated on the inside, so that they serve as oars ; these larvaj are of a 

 dirty brown colour with scattered darker markings ; they are exceedingly voracious ; 

 during the summer the larva is said to attain its full size in about fifteen days, when 

 it leaves the water and makes a cell in the adjoining earth, where it changes into a 

 pupa of a whitish colour, and appears in the perfect state in a fortnight or three 

 weeks ; it is, however, some little time before the beetle arrives at a state of maturity'. 

 The larva of Cyhister Roeseli much resembles that just described, but it is longer and 

 more slender and the cerci are entirely absent. 



Six species are found in Britain, which may be characterized as 

 follows : — 



I. Underside black ; female not dimorphic . . . . D. PUNCTULATUS, F. 



II. Underside testaceous or reddish, sometimes marked 



with black, sometimes unicolorous. 

 i. Apices of coxal processes acuminate ; female di- 

 morphic. 



1. Coxal processes short ; abdomen unicolorous . D. marginalis, L. 



2. Coxal processes prolonged into a sharp point. 



A. Abdomen with black markings. 



a. Head without yellow border round eyes . D. CIEctjmflexus, F. 



b. Head with broad yellow border round eyes D. 1APP0>'ICus, Gyll. 



B. Abdomen unicolorous ; head with a narrow 



yellow border round eyes D. CIBOUMOINCTUS, Ahr. 



ii. Coxal processes quite blunt ; female not dimorphic D. dimidiattts, Bercf. 



H. punctulatus, F. Pitchy black above and beneath with side? 



