SCJENCE-GOSS/J'. 



327 



:i sirnmls to 2nd seconds, mid ihe time it un- 

 exposed bore to the time it was <|ii iesconl a ratio 

 of 1: I. •!',." 



The female of 1). jnnirl i/Iatus (tig. 12), although 

 generally spending more time at the surface than 

 I), marginal 'is, occasionally remains submerged for 

 longer periods, sometimes refraining from aera- 

 tion for considerably over 40 minutes. She also 

 swims nearer the surface than our commonest 

 species, spending a great deal of her time with a 

 portion of the back slightly protruding from the 

 water. 



In very severe winters the Dytisci probably bury 

 themselves in the mud ; but retirement from an 

 active piratical life is of short duration, and a 

 slight rise in the temperature suffices to again 

 bring the insects abroad in their favourite haunts. 

 In mild winters it seems unlikely the beetles 

 hibernate at all. Both at Ferndown, Dorset, in 

 the south, and at Hoylake in the north-west of 

 England, I have taken our two commoner Dytisci 

 throughout the winter months, even when the 

 weather has been decidedly cold. 



Our beetles of the genus Bytiscus may be con- 

 veniently divided into two groups, in one of which 

 the underside is black and in the other some shade 

 of yellow or yellowish-red. To the first of these 

 groups belongs one species. 



Bytiscus pimctulatm Fabr. (tigs. 8 and 12) is 

 a flattened, long-oval insect, measuring from an 

 inch to an inch and one-eighth in length, by about 

 half an inch in breadth (2<>-29 mm. by 12-14 mm.). 

 The general form is narrower in proportibn to its 

 length than is that of our following and commoner 

 species. The upper surface is black, with just 

 the slightest tinge of brown, wdiich is usually 

 more apparent in the females, the lateral margins 

 of the thorax and elytra, being bordered with 

 yellow. The clypeus is yellow, as in all our British 

 species, and the antennae reddish. The underside 

 is black and polished, the pitchy legs slender and 

 furnished with brown hairs, and the coxal pro- 

 cesses rounded at the apex. The upper surface of 

 the males is smooth and shining, there being but 

 slight punctuation, and that mainly towards the 

 apical area of the elytra. The females arc duller, 

 and have their wing-cases deeply furrowed from 

 near the base to beyond the middle, there being 

 ten grooves on the basal portion of each elytron, 

 the remainder of the upper surf ace being finely and 

 somewhat closely punctured. 



B. pioictitlatiis is a native of Northern and 

 Central Europe. In Britain it is a local insect in 

 some districts, although widely distributed through- 

 out the kingdom. It may be taken at all periods 

 of the year, excepting during severe frost, in 

 similar situations to 1). marginalia, but is also 

 stated by Dr. llofmann to be "chiefly found in 

 running water." 



To our second group belong, the rive remaining 

 British Dytisci. All of these are easily distin- 



guished from the foregoing species in having the 



underside other than black ami the thorax dis- 

 tinctly bordered on re than two of its margins. 



• Dyti&cus marginalia Linn. (tigs. '.'., '.',. and 4). 

 ah hough varying considerably in -i/.", i- on an 

 average larger thatfthe preceding beetle, rati 

 from an inch and one-eighth to an inch and three- 

 eighths in length, and from five-eighths 'o three- 

 quarters of an inch in breadth (25 •'!! mm. by 

 15-18 mm.). The upper surface is olivaceous or 

 greenish-black, with the lateral margins of the 

 elytra, and the whole of the thorax bordered with 

 yellow. The antennae and legs are red. and the 

 underside of a, uni-colorous yellow, with the coxal 

 processes both shorter and more pointed than in 

 the former species. The males are highly polished ; 

 but in this and the three following species is pre- 

 sented to us the peculiar and as yet unexplained 

 circumstance of the females assuming two distinct 

 and well-known forms, whilst very rarely specimens 

 have been captured which may lie said to occupy 

 the position of " missing links " between them ('). 

 In the first and commonest type the insect is dull 

 and deeply furrowed, as in the preceding species ; 

 whereas in the second it is smooth and shining. 

 like the male, save that it has slight punetuation 

 on the thorax and a rather more liberal allowance 

 towards the apical area of the elytra than i> to be 

 met with in the opposite sex. 



B. marginalia is widely distributed over the 

 middle and northern portions of Europe. Asia, and 

 America. It has also been recorded from Japan, 

 and ranges from Geneva to well within the Arctic 

 Circle. It is by far the commonest of the British 

 Bytisei, being generally distributed in stagnant 

 water throughout the kingdom. 



Bytismts circamflexits Fabr. (tigs. '.) and 10) is 

 on an average rather longer and more slender than 

 our last beetle (27-35 mm. by 15-17 mm.). The 

 upper surface is olivaceous or black, with a more 

 or less greenish tinge, both the size and colour of this 

 insect varying considerably. The antennae and 

 legs are red, and the margins of the thorax and 

 elytra are bordered with yellow, as in 1). margin- 

 alis. This species is. however, easily separable 

 from the latter beetle, on account of its yellow 

 underside being boldly decorated by black mark- 

 ings (fig. 10). The coxal processes are. moreover, 

 longer, narrower, and sharper, and the scutellum 

 often exhibits a reddish tint. The sexes differ as 

 in the last beetle, the females hiMnu' similarly 

 dimorphic. 



B. cvrcumflexus is a native of Central and 

 Southern Europe and Northern Africa. In its more 

 northern range the sulcate form of female pre- 

 dominates, the insects exhibiting also a broader 

 form and darker hue than those of Spain, Algeria. 



(1) •' In Dr. Power's collection there is 11 female /'. circum- 

 cinccus that comes between the two forms, the sulci being only 



rudimentary but distinctly tra ible."- -Fowler, " Col. Brit. Is.," 



i. 205. 



