1. Center for Biological NMR, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, Laboratorio de RMN, Facultad de Química, Universidad de la República, Avda. General Flores 2124, C. P. 1157, Montevideo, Uruguay, and Veterinary Entomology Research Unit, FAPRL, Agricultural Research Service, United States Department of Agriculture, 2881 F & B Road, College Station, Texas 77845-2122