Summary: ECOLOGY AND POPULATION BIOLOGY
Life Cycle and Growth of Senita Moths (Lepidoptera: Pyralidae):
A Lepidopteran with Less Than Four Instars?
J. NATHANIEL HOLLAND
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721
Ann. Entomol. Soc. Am. 96(4): 519Š523 (2003)
ABSTRACT 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 Želd by labeling cohorts of eggs laid in ßowers and following them through pupation.
All life stages of U. virescens were associated with ßowers, fruit, or cactus stems of L. schottii. Among
the Žve cohorts studied, larval growth consistently conformed to DyarÕs rule. Only three larval instars
were identiŽed 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.
KEY WORDS DyarÕs rule, life history, plant-insect interaction, Upiga virescens
LIFE CYCLES OF INSECTS vary greatly among ametabo-