Smithsonian miscellaneous collections . dages andnot limb vestiges is shown by their occurrence on the coxae of themiddle and hind legs (B,5>y). The abdominal styli, therefore, areappurtenances of former limbs, but do not themselves represent ab-dominal legs. The same evidently is true of the eversible styli and vesicles occur also among the other groups of ap-terygote insects, and among the pterygotes styli are present on theninth abdominal segment of the adult male in the cockroaches, man-tids, and termites. We may reasonably conclude, therefore, that the no. 9 INSECT METAMO
Smithsonian miscellaneous collections . dages andnot limb vestiges is shown by their occurrence on the coxae of themiddle and hind legs (B,5>y). The abdominal styli, therefore, areappurtenances of former limbs, but do not themselves represent ab-dominal legs. The same evidently is true of the eversible styli and vesicles occur also among the other groups of ap-terygote insects, and among the pterygotes styli are present on theninth abdominal segment of the adult male in the cockroaches, man-tids, and termites. We may reasonably conclude, therefore, that the no. 9 INSECT METAMORPHOSIS—SNODGRASS 25 immediate ancestors of both the wingless and winged insects hadabdominal styli. In Thysanura and Diplura the abdominal styli and vesicles areprovided individually with muscles that arise on the supporting plates(fig. 2 D,E). The styli are flexibly movable on their bases; the vesi-cles are retracted by their muscles, and protracted probably by bloodpressure. The styli are developed during postembryonic growth; ac-. Fig. 2.—Structural details of Machilidae. A, Machilis sp., whole insect, showing thoracic and abdominal styli. B, Neso-machilis maoricus Tillyard, middle leg, showing stylus on coxa. C, same, ventralsurface of first abdominal segment, vesicles retracted. D, same, ventral surfaceof second abdominal segment, with vesicles and styli. E, same, ventral surfaceof sixth abdominal segment, vesicles everted. Cx, coxa; Fm, femur; PI, pleuron; rvs, retractor muscles of vesicle; S,sternum; Sty, stylus; Tr, trochanter; Vs, eversible vesicle. cording to Heymons (1897, 1906), Adams (1933), Sweetman andWhittemore (1937), and Lindsay (1939) they first appear on thefourth or fifth instar of lepismatids, or even on much later instars. Thysanurans moult many times throughout life, the number ofmoults depending on how long the insect lives. Sweetman and Whitte-more (1937) record as many as 42 observed moults for one individualof Thermobia domestica, and they state
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