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[September-December 
together in the same communal webs. If diet is a major factor in 
the evolution of social phenomena in dictynid spiders, we might 
expect the diet of M. gregalis to differ greatly from that of other 
dictynids. Although differences in prey size seem to occur, perhaps 
the most interesting finding in this study is that there is consider- 
able overlap in prey sizes of different dictynids. We need to con- 
sider the possibility that predation on relatively large and dangerous 
prey is only one among other equally or more important factors 
acting as prime movers in the evolution of social phenomena in the 
Dictynidae and perhaps for other groups as well. 
Summary 
Based on arthropod carcasses in webs and observations of actual 
feeding, Diptera seems to be the major prey of Dictyna and Mallos. 
M. gregalis, a species that routinely feeds in groups, may tend to 
prey upon relatively large prey compared to the other species. 
However, relative prey sizes overlap for species of all types of 
social organization. No apparent differences occur in the degree 
to which prey are dangerous. These observations are not to be 
expected from the hypothesis that the prime mover in the evolu- 
tion of social phenomena in spiders is the ability of predators act- 
ing as a group to handle relatively large and dangerous prey. Al- 
though legs of flies are frequently grasped first, M. gregalis may 
initially grasp almost any part of the fly. Size of feeding groups 
varies greatly, ranging from 1 to more than 20. The hypothesis is 
proposed that prey is captured by Dictyna and Mallos primarily 
when flies use webs as resting sites. Feeding and other activity 
occur especially in the early evening and early morning. 
Acknowledgements 
For valuable discussions and comments on the manuscript, I 
would like to thank P. N. Witt, M. C. Vick, S. E. Smith, and J. W. 
Burgess. Special thanks go to W. J. Gertsch for his assistance in 
the identification of spiders. C. E. Griswold, P. S. Jackson, and 
V. D. Roth are gratefully acknowledged for helping me locate 
spiders in the field. The assistance of the Southwestern Research 
Station of the American Museum of Natural History is gratefully 
