Despite great variation in instar number among Insecta, no Lepidopteran has been observed to have less than four larval instars. I report in this work on the life cycle and growth of the senita moth, Upiga virescens Hulst, which forms an obligate pollinating predispersal seed-eating mutualism with senita cacti (Lophocereus schottii Engelmann) in the Sonoran Desert of North America. From 1996 to 1999, I studied larval growth and life cycle associations of U. virescens with L. schottii in the field by labeling cohorts of eggs laid in flowers and following them through pupation. All life stages of U. virescens were associated with flowers, fruit, or cactus stems of L. schottii. Among the five cohorts studied, larval growth consistently conformed to Dyar’s rule. Only three larval instars were identified among the >500 larvae for which head capsule widths were measured. I discuss and dismiss the feasibility of a fourth undetected instar. I then discuss selection pressures that may have contributed to the evolutionary loss of an instar, including a time and/or size constraint on larval growth, as well as the nutritional quantity and quality of larval food.