1978] 
Jackson & Smith — Mallos and Dictyna 
75 
(Jackson, 1977a). Thus large prey populations would seem neces- 
sary to support some of the larger aggregations of dictynids. Diguet 
(1909a, b, 1915) and Burgess (pers. comm.) noted numerous adult 
Diptera in the vicinity of M. gregalis webs in Mexico during the 
rainy season. Studies are needed to determine whether Diptera 
populations are larger and/or more predictable in habitats con- 
taining communal webs of M. gregalis compared to neighboring 
areas without communal webs. Great numbers of Diptera were 
noted in the metal culvert in Arizona that contained the enormous 
web complex of M. trivittatus. Another consideration in this case is 
that flies entering the culvert may be especially vulnerable to 
capture, since inside the culvert they were almost completely 
surrounded by web. Great masses of nematocerous flies were active 
in the vicinity of web complexes of D. calcarata in Chapala, 
emerging from nearby Lake Chapala in the late afternoon and early 
evening. Diptera were also numerous in the vicinity of web com- 
plexes of D. albopilosa at San Anton Falls. In general, wherever 
there were web complexes, there were also numerous Diptera. 
However, the question of why some species live in large, dense 
populations, while others do not, cannot be answered simply on the 
basis of prey densities in the habitats of different species. Riechert 
and Tracy (1975) have shown relationships between density within 
spider populations and prey availability. However, they found prey 
availability to depend not only on the absolute abundance of prey 
but also on factors that influence how the spider experiences prey 
abundance, especially its thermal relations with its environment. 
Another consideration is that species of Dictyna and Mallos with 
differing aggregating tendencies may occur side-by-side in the same 
habitats (Jackson, 1978a). For example, M. niveus were found in 
individual webs on the same trees with web complexes of M. 
trivittatus in the Chiracahua Mountains. It would seem that dictynid 
species that routinely occur in aggregated states (communal, terri- 
torial and communal, non-territorial) and those that generally live 
more widely dispersed (solitary) are somehow adapted to exploit 
different sets of resources, but we have no clear insights at this time 
concerning what these different resources might be. 
The adaptive advantages and disadvantages for animals related to 
living in groups have been subjects of considerable interest in recent 
years (see Wilson, 1975). For a review of ideas concerning spiders, 
