Larva and puparium (Fig. 26): Anterior fold with a wide band of spicules. Metathorax with two groups of a few indistinct spicules on the anteroventral margin (Fig. 30). Dorsal vestiture comprising evenly sized setae except for anterodorsal groups of large, broad setae on the mesothorax and metathorax (Fig. 29) and smaller setae on middle transverse bands on abdominal segments 1–2. Abdominal segments 4–6 with second row of crochets inconspicuous and not as developed as on abdominal segments 1–3. Posterior breathing tube red-brown at base, dark brown at apex, below TR tube nodulate, above coriaceous. AP about a third as high to spiky rim as wide at TR (Fig. 69). Pupal spiracles, about as far apart as a pupal spiracle is long and slightly curved in upper third. Openings coating upper half of one side, those at the apex with a short spiky projection (Figs 72, 73).

Taxonomic notes: Very similar in larval characters to C. posticum from which C. isabellina differs in the pupal spiracles being about as far apart as a spiracle is long and the openings at the apex having a short spiky projection.

Material examined: 15 larvae USA Arizona, Tucson mountains east of Gates Pass, 1.iii.1965 J. F. Burger ex decaying stems of C. giganteus (Cactaceae) (SI); 1 ♀, one puparium USA Arizona, Yuma Co., Kofz mountains 20.i.1969 J. F. Burger ex decaying stems of C. giganteus (SI) 1 ♂, one puparium USA Arizona, Pima Co., 16 km west of Tucson 14.iii.1979 C. T. M. ex decaying stems of C. giganteus (NMS); 2 ♂, 2 ♀ four puparia USA Arizona, Pima Co., 22.5 km west of Tucson 21.iii.1979 C. T. M. ex rot-pocket in C. giganteus (NMS).

Previous records: Hubbard & Schwarz (1899) ex C. giganteus; Ryckman & Ames (1953) ex C. giganteus; Maier (1982) ex C. giganteus; Mangan (1984) ex C. giganteus; Myles (1986) ex C. giganteus.