Orientation of the monarch butterfly

Includes bibliographical references (pages 43-45)A study was designed to determine cue(s) used by the monarch butterfly (Danaus plexippus L.) for orientation on its annual migration. Solar visibility, polarized light, and topographic background were tested as possible cues for orientation. Also considered was the biological significance of the direction of orientation, the seasonal onset of orientation, and the comparative ability of both sexes to orient. From June through December of 1973 and 1974, I conducted orientation trials in a circular arena, using 952 adult butterflies. An analysis of the directional headings indicated that monarch butterfly migration is initiated in late July or early August in northern Illinois. Corresponding with the onset of migration, monarchs begin to express a south-southeasterly directional preference. A series of 315 orientation cage trials, involving both wild-caught and cage-reared adults, demonstrated that monarch butterflies possess an ability to detect and use plane polarized light. Under complete cloud cover, 152 orientation trials failed to show any directional preference. A comparison of the orientation abilities of 59 males and 121 females indicated that both sexes had equal ability to orient successfully. Finally, results from 61 orientation cage trials with background visible and 225 trials with the background obscured indicated that immediate background topography does not have any effect on the monarch butterfly's ability to orient.M.S. (Master of Science