250 
Psyche 
[June 
spots. They also recorded that the wasps only seldom followed 
spiders which fell from their retreats. These differences may be due 
to differences in cues learned by individual wasps. 
Discussion 
The hunting behaviors reported above illustrate selective pressures 
on A. cornutus and other orb weavers. A spider at the hub of an 
orb in the daylight is probably relatively safe from attacks by preda- 
tors not strong and heavy enough to escape easily from its web, but 
it is very vulnerable to attacks by large, visually orienting, versatile 
predators such as S. caementarium. On the other hand, many 
insects fly only during the day, and it is certainly advantageous for 
the spider to use its web during the day. Devices such as stabilimenta 
found in webs of Cyclosa, Argiope, Uloborus, and others (Gertsch 
1949) which obscure the spider’s outline, a “stopping mesh” next to 
the orbs of Nephila , Metepeira, Argiope, Araneus, and others (Mc- 
Cook 1889), and signal threads leading to the web from the hiding 
place of Zygiella , Hyptiotes, Araneus, and others (McCook 1889) 
may all function to hinder attacks by relatively large predators while 
allowing the spider to capture prey caught in the web during the 
day. 
A spider off its web is relatively safe from larger predators because 
it can crawl into places too small for them, and also relatively safe 
from smaller substrate-bound predators because it can escape by 
falling and hanging on a thread. A spider in a retreat is probably 
only especially susceptible to wasps which are relatively good fliers, 
which are about its own size, which hunt by crawling into tight 
spots, and whose behavior can cope with the spider’s escape behavior. 
A larger wasp can attack an orb weaver in its retreat only if it can 
drive or lure the spider from the retreat. The wasp can only drive 
the spider from its retreat if it can find it, and if the retreat is not 
in a sheltered spot. It can probably only lure it out with relatively 
complicated behavior like that of Belanogaster junceus (?) which 
hovers next to the hub of the web and taps it with its antennae and 
perhaps its front legs to lure the spider to the hub (MacNulty 1961 ). 
It is possible that wasp predation has been a selective force in- 
fluencing web site selection, as the observations above indicate that 
S. caementarium might be less likely to encounter well-scattered 
webs in cool places. 
Araenus cornutus appears to have two effective close range defenses 
against wasp attacks: a quick unobserved exit from its retreat, and 
