The present generic group was first separated from _Papilio_ by Dr.
Boisduval. None of its characters, taken singly, are very strongly
marked, but their aggregate importance is sufficiently considerable to
authorise its adoption. As in _Papilio_ the antennæ form an elongate
club, having the extremity slightly curved upwards, the palpi are
longer than in the genus just named, but they never rise above the
forehead. The prothorax is much developed anteriorly, and forms a
pretty distinct neck. The abdomen is long and robust, that of the
male deeply grooved on the under side, and provided at the anal
extremity with two large rounded valves. The wings are large, of a
strong texture, and furnished with salient nervures; the anterior pair
elongate; the posterior with wide shallow indentations, and never
prolonged into a tail.
We are very imperfectly acquainted with the natural history of the
insects of this group in their early stages. Their remote localities,
and the rarity of most of the species, have, for the most part,
prevented them from falling under the notice of competent observers.
The caterpillar and metamorphoses of one of them (_O. Heliacon_),
however, have been described by Dr. Horsfield. Like the caterpillars
of _Papilio_, it is furnished with two retractile tentacula; the
body is large and thick, and bears eight rows of rather long fleshy
prominences, of a conical shape. The chrysalis into which it is
transformed is somewhat curved, having the head obtuse, and a few
projecting points above the middle. It is suspended by the tail, and a
transverse band, which, however, does not encompass the body, but is
inserted on each side. (See Fig.)
[Illustration]
This group is but of limited extent, but it comprehends some of the
largest and most beautiful lepidoptera yet known to us. With one
exception, which extends over a portion of the Indian continent, they
are confined to the Mollucca, Philippine, and others of the numerous
and extensive islands of the Indian Archipelago. The species figured,
which is the most beautiful of the whole, and to which Linnæus applies
the epithet of _august_, has been long known and highly prized by
collectors. It is named
[Illustration: