HYDRADEPHAGA. 
79 
bright in the males and dull in the females, a charac- 
ter also found in many Geodephaga. 
Hyphydrus ovatus, common near London, affords a 
good link to Pelobius, on account of its globose form, 
and the basal joints of its tarsi being very long, as 
well as wider than the apical joints. Fresh specimens 
of this insect have been noticed to have a smell some- 
thing iike that of honey. 
The characters of the other tribes need not here be 
discussed ; they are closely related, and have by 
many authors been classed together. In the genera 
Dytiscus, Cybister, Hydaticus, and Acilius, the three 
basal joints of the front tarsi in the males assume the 
form of a round sucker, fringed with hairs, with little 
cups on the under side ; and in the first and last, 
also, the females have the wing-cases deeply furrowed 
longitudinally, the furrows reaching in Dytiscus half- 
way down, and in Acilius to the apex of the elytra. 
These two peculiarities of structure are evidently to 
assist the sexes in pairing. The females of Dytiscus 
are, however, sometimes found without these dorsal 
furrows ; and somewhat analogous instances of what 
is termed “ sexual di-morphism ” occur rather fre- 
quently in the genus Hydroporus , &c. 
The true Dytisci are well known, being “ the 
Water-beetles ” of the aquarium. They are large, 
olive-brown, with a yellowish side-band, and very 
rapacious, attacking all that comes in their way. 
They swim rapidly, diving with great quickness, and 
may be often seen at the surface of the water, with 
their tail upwards, taking in air, which thus reaches 
the spiracles. D. punctulatus (Plate III., Fig. 1) is 
entirely black beneath. 
