292 



SCIENCE- G OS SIP. 



In all our British species the clypeus and a 

 narrow marginal stripe the elytra are yellow, 

 the labrum being broad and distinctly notched. 

 The legs of Dytiscvs are slender, and appear some- 

 what disproportionate to its size. The posterior 

 pair, which are supplied with powerful muscles, 

 are longer and stronger than the others, and have 

 the tibiae and tarsi flattened and furnished along 

 their edges with long swimming hairs. By a 

 peculiar keel-like prolongation of the thorax these 

 organs are set far back below the centre of the 

 beetle, where their strongly developed flat coxae 

 are firmly fixed to the metasternum. The arrange- 

 ment and articulation of the limbs render it easy 

 for the legs to be brought at right angles to the 

 body, thus allowing of a long sweep being made, 

 the flattened portions of the tibiae and feet with 

 their stiff setae acting like the blades of a pair of 

 oars and serving to propel the insect forwards. 

 These paddles, as a rule, strike the water simul- 

 taneously, although they are also capable of being 

 used singly and in conjunction with the inter- 

 mediate leg on the same side for turning rapidly 

 in a small space ; on the same principle as that by 

 which an expert sculler manoeuvres his rudderless 

 skiff. The anterior and middle legs are shorter ; 

 the latter, in addition to affording some assistance 

 in propulsion, being used for steering purposes. 

 Both pairs are employed in the seizure and rending 

 of its victims, and to anchor the insect when 

 desirous of remaining at rest beneath the surface 

 of the water. For these latter uses the last tarsal 

 joints are, as in the larva, furnished with two sharp 

 hooks, weapons which in the oar-like posterior feet 

 are much less strongly developed. 



The sexes differ considerably in appearance, so 

 much so that they were formerly described as 

 distinct species. The males are slightly larger than 

 the females, have the swimming legs more power- 

 fully developed, and the elytra smooth and highly 

 polished (fig. 2). The females are dimorphous — 

 i.e. exist under two different forms, of which 

 the commoner has the elytra deeply grooved from 

 near the base to about midway between the disc 

 and apex (fig. -1). Females are, however, occa- 

 sionally found with elytra resembling those of the 

 males (fig. 3). but these are of rare occurrence in 

 Britain. Simmermacher has stated that the sculp- 

 tured females belong more particularly to northern 

 latitudes ; and the observation of Redtenbacher, 

 that near Vienna the smooth form of female is as 

 common as the rough, would seem to support this 

 assertion. It may also be interesting to note that 

 in the closely allied Australasian genus, Hydc- 

 rodes, the females, although dimorphic, are almost 

 invariably smooth, like the males. 



No very satisfactory explanation as to the use 

 of the furrowed elytra has yet been advanced. 

 Earlier entomologists (Kirby and Spence) usually 

 associated this roughness of the female with the 

 perpetuation of the species, regarding it as a 



special provision of Providence and accessory to 

 the dilated pads on the anterior tarsi of the males ; 

 but Plateau was the first of several naturalists to 

 prove by experiment the fallacy of these ideas, by 

 demonstrating that a roughened surface was op- 

 posed to the action of the suckers. Careful obser- 

 vation will disclose the fact that the suckers 

 on both the anterior and middle tarsi of the males 

 are always applied to the thorax and smooth edges 

 of the elytra, and never to the furrowed portion of 

 the wing-cases. These sucker-like discs on the fore 

 and intermediate feet of the males constitute 

 another important distinction between the sexes, 

 they being entirely absent in the females. In the 

 anterior pair the three basal joints are very re- 

 markably dilated so as to form large saucer-like 

 discs, the function of which is for mating purposes, 

 as suggested above. They appear to be a special- 

 ised modification of the pulvillus met with in 

 many phytophagous beetles, flies, and other insects, 

 from which, however, they must be carefully dis- 

 tinguished ; the latter being for creeping purposes, 

 and therefore on all six feet, and present in both 

 sexes. In the coleoptera they are particularly 

 noticeable in the family Chrysomelidae, and 

 amongst the Rhynchophora or Weevils, although by 

 no means confined to those groups of beetles. 

 Under a microscope the peculiar arrangement of 

 this curious enlargement of the anterior legs of the 

 Dytiscus becomes increasingly interesting, and the 

 pad of the foot is seen to possess a series of sucker- 

 like prominences, two of which are considerably 

 larger than the others (fig. 6). The under side of the 

 middle tarsi is also densely clothed with small 

 discs raised on foot-stalks which exactly resemble 

 the smaller of those on the anterior feet. Much care- 

 ful attention to the subject has convinced Dr. B. T. 

 Lowne that the structure of these discs is identical 

 with that of an insect's hairs ; that they are, in 

 fact, only different in form. These hairs are, of 

 course, distinct from those with which we are 

 familiar in the higher animals, in being mere pro- 

 cesses of the integument itself. There appear to 

 be nearly 200 disc-bearing hairs on the anterior 

 tarsus of our three commonest species of Dytisous, 

 and Dr. Lowne's observations tend to prove conclu- 

 sively that Blackwall was right in his contention, 

 early in the last century, that these hairs are 

 hollow and exude a viscid fluid, which enables the 

 insect to adhere to polished surfaces. Derham r 

 White, Banks, Home, and many other naturalists- 

 have associated the action of the pulvilli, or 

 '• cushions," with atmospheric pressure ; and, con- 

 sidering the general interest of the subject and 

 the wide prevalence of the latter popular belief,, 

 it may not be out of place to cite an experiment 

 carried out by Dr. Lowne before the Royal Micro- 

 scopical Society so far back as 1871. Following 

 upon an explanation of the nature and structure 

 of the discs, he proceeded : " I have here a male 

 Dytiscus rendered insensible by the action of 



