[a]Fig. 160.]—Neuration of the genus Thanaos.
Egg.—The egg is somewhat like the egg in the genus Achalarus, but the micropyle at the upper end of the egg is relatively larger and not as deeply depressed below the surface. The sides are ornamented, as in Achalarus, by raised vertical ridges, between which are numerous cross-ridges; in a few cases the vertical ridges are beaded, or marked by a series of minute globose prominences, upon the edge.
Caterpillar.—The caterpillars are cylindrical, tapering from the middle forward and backward, marked with lateral and dorsal stripes, with the neck less strangulated than in the preceding genera.
Chrysalis.—Not greatly differing in outline from the chrysalis of the preceding genera, in most species having the outline of the dorsum straight on the abdominal segments, with the thoracic segments forming a slight hump or elevation; convex on the ventral side, the cremaster being usually well developed.
(1) Thanaos brizo, Boisduval and Leconte, Plate XLV, Fig. 7, ♁; Plate VI, Fig. 38, chrysalis (The Sleepy Dusky-wing).
Butterfly.—The band of postmedian spots on the fore wing is composed of annular dark markings, is regular, crosses the wing from the costa to the hind margin, and is reproduced on the under side as a series of pale-yellowish spots more or less distinct. The hind wings have a double series of faint yellow spots; these as well as the marginal spots of the primaries are very distinct on the under side. Expanse, 1.25-1.60 inch.
Early Stages.—The caterpillar feeds on oaks, Galactia, and possibly Baptisia. The life-history has been only partially ascertained, in spite of the fact that the insect has a wide range and is not uncommon.
Brizo occurs from the Atlantic to the Pacific, ranging from the latitude of New England to that of Arizona.
(2) Thanaos icelus, Lintner, Plate XLVIII, Fig. 17, ♂; Plate VI, Fig. 27, chrysalis (The Dreamy Dusky-wing).
Butterfly.—Prevalently smaller in size than the preceding species. The under side of the wings is paler than the upper side, and the outer third of both the primaries and secondaries is marked with a profusion of small indistinct yellow spots, which do not form well-defined bands as in the preceding species. On the upper side of the fore wing the median area is generally marked by a broad band of pale gray, but this is not invariably the case. Expanse, 1.00-1.20 inch.