22 BRITISH BEETLES. 



it being absolutely necessary to understand them in 



working from descriptions. 



The head, bearing the eyes, antennae, and organs of 



mastication, etc., is the first to be considered. The 



accompanying cut of the head 

 of a Water- beetle will show 

 the chief points to be no- 

 ticed on the upper side : here, 

 a is the labrum or upper 

 lip; b, the clypeus or shield 

 of the mouth, often bearing 

 tubercles or even horns ; c 

 or d are the mandibles or 

 upper jaws (these are dis- 

 Fig. 3. sected out in the figure, and 



Upper side of head of D. margi- rf i s reversed) : 6, the CVCS ; 

 nalis (highly magnified). L^L 



f, the base of the antennae ; 

 g, the vertex or crown, and h the occiput. 



The mandibles are hard and sharp, often (as in the 

 male of the Stag-beetle) very much developed. The 

 eyes, which are composed of many facets, situated on 

 the side of the head, and usually large, are normally 

 two in number, being however in some cases aberrant ; 

 for instance, in Dorcus (the small Stag-beetle), each 

 eye is almost divided into two, being interrupted by 

 the lateral ridge of the head ; and in the Gyrini (the 

 "Whirl wigs "), it is distinctly divided by a deep broad 

 channel, containing the antenna, so that the insect is 

 four-eyed, having two on the upper and two on the 

 under surface, an admirable structure for species that 

 pass their lives on the top of the water, arid need extra 

 sharpness of vision, partly to save themselves from foes 

 above, and partly to detect their own food below. 



