Hormonal control of male horn length dimorphism in Onthophagus taurus (Coleoptera : Scarabaeidae): a second critical period of sensitivity to juvenile hormone

Male dung beetles (Onthophagus taurus) facultatively produce a pair of horn s that extend from the base of the head: males larger than a threshold body size develop long horns, whereas males that do not achieve this size devel op only rudimentary horns or no horns at all. Using topical applications of methoprene, we identified a sensitive period during the feeding stage of t hird (final) instar larvae when application of methoprene shifted the thres hold body size for horn expression. Male larvae that received methoprene at this time delayed hom production until they attained a larger threshold bo dy size than acetone-treated control larvae. This new sensitive period occu rs earlier than a sensitive period previously reported for male horn regula tion, and it coincides with a morph-specific pulse of ecdysteroid secretion described for this species. It appears that male horn expression is influe nced by endocrine events at two different periods of larval development. We incorporate these results into an expanded model for the endocrine regulat ion of male horn expression. (C) 2001 Elsevier Science Ltd. All rights rese rved.