This genus is composed of a selection of species from Colias, to which
it bears a very close relation. It is not long since it was proposed
by Dr Boisduval, who thinks that the following characters entitle it
to this distinction. Palpi approximating and very much compressed,
clothed with short hairs and dense scales, the terminal joint conical
and much shorter than the preceding one; antennæ thickening gradually
from the base to the apex, which is distinctly truncated; body robust,
the abdomen much shorter than the inferior wings; the latter forming
a groove which completely embraces the under side of the abdomen. The
caterpillars are naked and somewhat attenuated at both extremities; the
chrysalis arched, or boat shaped, with the extremities drawn out to a
narrow point; always attached by the tail and a transverse band.
According to the manner in which they have been respectively
constituted, _Callidryas_ therefore is best distinguished from Colias
by the antennæ, which in the latter terminate in an obconical club;
and from _Rhodocera_ (including _Gonopteryx_ of Dr. Leach), which has
these organs likewise truncated, by the shape of the wings, which
never present acute angular projections. The prevailing colour of the
species is yellow, from deep orange to the palest sulphur yellow. The
females are usually of a paler hue than the males, and the sexual
differences are strongly marked in other particulars. On the under
side of the wings there are almost invariably one or two small spots,
near the middle, of a silvery or rusty-brown hue; these are either
wanting or very minute in the males, while they are distinctly marked
in the females. The former sex, in the majority of species, presents a
very peculiar character in having the anterior edge of the under wings
provided with a kind of pulverulent glandular sac, which varies much in
size and colour in different species.
A great similarity prevails among the different species of
_Callidryas_, and they are consequently difficult to determine. They
inhabit the intertropical regions of the two continents.