246 
Psyche 
[June 
Two wasps were observed attacking spiders which had evidently 
been driven from their retreats and were hanging at the ends of 
threads. Each wasp flew close to (and probably hit) the spider, then 
immediately flew to the floor directly beneath the spider and ran 
around quickly. In both cases this first encounter caused the spider 
to descend only part of the way to the floor, and after a short time 
the wasp flew back up and buzzed the spider again. The quickness 
with which these wasps searched the floor below following their 
encounters with spiders suggests the wasps in the case reported above 
may have been disturbed. 
One wasp’s encounter with an A. cornutus exuvium was also 
observed. The skin hung near one entrance of an empty retreat, 
and when she encountered it the wasp inserted her sting into it two 
or three times. 
Two spiders escaped after a wasp encountered them. There was 
a strong (> 15 kmph) wind blowing when one of them left its 
retreat and hung on a thread as a wasp entered. The wasp flew 
down to the spider, but as the spider let out more thread, a gust of 
wind blew it arund the corner of the house. The wasp did not 
follow it, but landed on the wall. Another spider was at the mouth 
of its retreat in the crack between two shingles as a wasp approached, 
and dropped out just as the wasp walked up the crack. The wasp 
walked on under the overlapping shingle, stayed out of sight for 
about 15 seconds, then walked on. After about a minute, the spider 
climbed back up its thread to its retreat. 
In summary, spiders were always attacked while they were on 
surfaces, and those not on surfaces (on a thread) were driven to 
them and then attacked. The wasps used their superior speed and 
an ability to locate the ventral surface of the spider’s cephalothorax 
to sting the spiders into paralysis. The wasps probably did not use 
vision to locate spiders in their retreats, but probably did use it to lo- 
cate spiders which had fallen from their retreats and perhaps to locate 
the general area in which to search for retreats. Tactile or perhaps 
chemical stimuli from the skin of a spider released stinging behavior. 
These observations of the predatory behavior of A. humilis differ 
dramatically from the description of the predatory behavior of A. 
sericeus (= Pompilus sericeus) by Soyer (1950). He saw these 
wasps hunting the orb weavers Araneus diadematus and Zygiella 
x-notcita, but claimed that the wasps, by flying about until they fell 
into an orb and then searching sites to which the web threads led, 
used the spider’s web to find its retreat. The observations above 
