This genus was first proposed by Mr. Swainson for the reception of a remarkable insect from Siam and Java, to which various situations had been assigned by different naturalists. In external aspect it has all the appearance of an _Erycina_, but, on examining the disposition of the nervures of the wings, the form of the palpi and antennæ, together with the structure of the feet, which are all complete in both sexes, its close affinity to the true _Papiliones_ becomes apparent. The head and body are very thick; abdomen short; eyes large and salient; palpi very short, the articulations very indistinct; antennæ rather long, thickening at the extremity into a club which is slightly curved upwards. The anterior wings are nearly hyaline, and have the discoidal cell closed; the posterior folded longitudinally, and each drawn out into a very long tail curved at the extremity. The expansion of the wings, in the only known species, is about an inch and a half; the inner half of the superior pair black, traversed in the middle by a pretty broad green band; the exterior portion, consisting of a large triangular space, transparent, with the nervures and external border black. The inferior wings are black, edged externally with white, and having a central blue band in continuation of the anterior one. On the under side, the base of all the wings is whitish, and the abdominal margin of the under pair is marked with three curved white streaks: abdomen whitish beneath, and having a double row of black dots on each side. In the female the bands on the wings are white. A few years ago this curious butterfly was to be found in very few cabinets, but it is now received not unfrequently. THAIS MEDESICASTE. PLATE V. Fig. 2. _Godart’s Pap. de France_, ii. Pl. 3, fig. 3, 4.--Pap. Medesicaste, _Hubn. Pap._ 124, fig. 632.--Pap. Rumina, _Hubn._ Pl. 394, 395.--Thais Honnoratii, _Boisd. Icon._ Pl. 3, fig. 3-5, var. The genus Thais is confined to the southern countries of Europe, the north of Africa, and Asia Minor. All the species are of moderate size, and may at once be known by the peculiar design of the colouring of the wings, which are always yellow spotted with red and black, and bordered externally with a dark festooned line. The palpi are composed of three nearly equal articulations, and rise conspicuously above the head; the antennæ rather short and terminating in a club slightly curved upwards. The body is slender, and the abdominal margin of the hinder pair of wings is curved downwards as if to leave room for the movements of the abdomen. The caterpillars are short and cylindrical, covered with fleshy spines and short hairs, the first segment provided with a fleshy bifurcated tentaculum. They live solitarily or in small groups on the _Aristolochia_, and obviously bear a considerable analogy, as well as the perfect insect, to _Parnassius_. The chrysalis is cylindrico-conic, somewhat angular in front.