158 THE ENTOMOLOGIST. 



stand as a separate genus, — Enicocerus, Curtis, — and then dividing 

 the rest according to their size, which is very constant, as a rule, 

 in this genus. With the help of the sculpture of the thorax, the 

 membranous border or posterior angles of the thorax, and the 

 striation, the species may then be very easily separated. 



Before, however, proceeding to deal with the species in detail, 

 it may be convenient to point out that the sculpture of the 

 thorax (leaving out the lateral impressions, which are com- 

 paratively unimportant) are chiefly of two kinds : first, a central 

 furrow, with a transverse impression above it and below it, like 

 an H laid on one side, the under line being more or less 

 curved (T); and secondly, a central furrow, with two depressions 

 on each side, giving the appearance of a straight line with a semi- 

 colon, more or less distinct, on each side of it ( [ \ ] ). 



I. Second joint of the antennae almost quadrate, but slightly 

 narrowed at the base ; membranous border hardly, if at all, 

 visible ; sculpture of thorax different in the sexes ; elytra much 

 dilated, very strongly striated. 



O. exsculptus, Germ. — This species is very variable in colour, 

 some specimens being of a dull black, others shining bronze; 

 while occasionally specimens are met with in which either the 

 thorax or elytra, or the base of both, are of a bright metallic 

 blue colour. The male has the thorax very strongly convex, 

 with a central furrow, and two impressions springing from its 

 base, like a government broad arrow, and is comparatively 

 smooth. The thorax of the female has a central furrow, with a 

 semicolon on each side, and is very strongly punctured. In both 

 sexes the thorax is strongly constricted at the base, and so heart- 

 shaped. Specimens, however, occur in which this character is 

 not so marked as in others. Taking these variations into con- 

 sideration it is hardly to be wondered that confusion has arisen 

 regarding this insect. The male is the 0. tristis of Curtis, and 

 the var. a. viridiceneus of Stephens ; the female is the var. /3. 

 Gibsoni of Curtis. It appears to be found chiefly in the North of 

 England in running streams, and is very local. 



II. Second joint of the antennse narrowed from the centre to 

 the apex, ending in a fine point at its junction with the third 

 joint ; thorax with membranous border, or with posterior angles 

 excised and filled with membrane, or both; elytra with sides 

 usually more or less parallel, never very widely dilated, as in the 

 preceding division. 



