is inconspicuous or wanting in the other; its value as a diagnosis of genera is thus in a great measure destroyed. Considerable differences likewise prevail in the appearance of the caterpillars, but these are too imperfectly known to be made the groundwork of an arrangement, even if they were likely to be available for such a purpose by indicating natural groups or affording additional means of characterising them. “Some of them,” says Dr. Boisduval, “such as those of _Machaon_, _Alexanor_, _Asterias_, are cylindrical and smooth; others (_Crassus_, _Philenor_), are protected with rather long fleshy prominences; in a very great number (_Pammon_, _Memnon_, _Chalchas_, &c.) the two first segments are attenuated, and capable of being retracted under the third and fourth, which are dilated and often ornamented with ocular spots analogous to those presented by many of the _Sphingides_; others are short and thick, and furnished with numerous rather short fleshy points; finally, there are some (_Podalirius_, _Ajax_, _Antiphates_) which somewhat resemble snails in shape. The caterpillars of _Papilio_ may be distinguished from those of other genera belonging to the same tribe by the following characters:--They differ from those of _Ornithoptera_ in their retractile tentaculum not being enclosed in two exterior cases; from those of _Thais_, in the projections which they sometimes present being never hispid at the extremity; and from those of _Parnassius_, in the body being always free from pubescence; but it is more particularly from the characters drawn from the chrysalis and perfect insect that this last genus is distinguished from _Papilio_[29].” The perfect insects are characterised by very short palpi not passing beyond the eyes, all the joints very indistinct, the third quite invisible; antennæ pretty long, the club slightly curved upwards; abdomen pretty large, the anal valves in the male of moderate size; wings rather strong, the inferior pair having the abdominal margin folded upwards and leaving the abdomen free, their exterior border more or less dentated, and often prolonged posteriorly into a tail.