138 
Psyche 
[June 
Fastest Mile of Wind, U.S. Weather Bureau) for September and 
October would facilitate a southwestward movement of Atlantic 
Coast migrants in the fall, and for March and April would facili- 
tate a north and northeastward movement of migrants passing 
north through Mexico and Texas. Prevailing wind patterns for 
March and April suggest a possible explanation for why the pop- 
ulation of Monarch butterflies in the United States (excluding the 
West Coast population) is proportionally greater east than west of 
the Mississippi River (Urquhart, personal communication). Strong 
March and April winds from the north, west, and northwest in 
northern Texas and Nebraska, could force spring Monarchs, ori- 
enting by a negative sun-orientation, to the east and northeast. 
Therefore, even a broad northerly orientation for spring migrants 
might still result in biasing the summer population toward the east- 
ern half of the United States. Thus, it might not be necessary for 
Monarchs to possess a restricted negative sun orientation in order 
to assure a northeasterly movement in the spring. 
Summary 
Non-migrant and fall migrant male and female Monarch butter- 
flies, Danaus plexippus L., orient toward the azimuth of the sun 
when confined in circular orientation cages with only celestial cues 
present. When both terrestrial and celestial cues are present, non- 
migrants exhibit random directionality similar to the flight of free- 
flying summer non-migrants while fall migrants orient to the sun’s 
azimuth. Both fall migrants and non-migrants exhibit a random 
cage distribution under overcast sky with or without terrestrial 
cues. The sun orientation of fall migrants is believed to be a mi- 
gratory response resulting in a southward movement. Such ori- 
entation differed from the sun orientation of non-migrants which 
appears to be an escape response. Upon release, migrants and 
non-migrants tend to fly with the wind. No conclusive indication 
of sun-compass or polarized light orientation in migrants or non- 
migrants was evident. No sex differences in orientational responses 
were observed. 
Acknowledgments 
I wish to thank Dr. E. S. Hodgson for his advice and support 
during this study. In addition, I am indebted to Dr. F. A. Urquhart 
of the University of Toronto and Dr. L. P. Brower of Amherst 
