1971] 
Robinson £s? Mirick — Neph'la clavipcs 
I3i 
2. Experimental investigations 
To investigate the possibility that certain aspects of the predatory 
pattern are responses to relatively simple stimuli we carried out a 
number of investigations using ‘modified’ insects as dummies. We 
manipulated the size and weight of some insects and also modified 
such parameters as the strength of their adhesion to the web. We 
were able to compare the responses of the spiders, at various stages 
in the predatory process, to such dummies, using adequate controls 
or using their response to unmodified insects as a baseline. 
(a) The bite & back-off attack behavior 
Our observations on the responses of N. clavipes to a range of 
prey items (see page 123, above) showed that the bite & back-off 
attack behavior was only given to large and heavy prey. Since all 
of these that were presented to the spiders were alive there was a 
possibility that the response could be to size, weight or specific 
activity of large heavy prey, or a combination of any of these factors. 
Preliminary tests showed that the response was given to dead (im- 
mobile) prey so that although it remained possible that activity 
could enhance the response it was not the important stimulus. We 
then decided to manipulate the parameters of size and weight. Using 
small acridiids (25-3omm, 400-550mg) as prey, we added lead shot 
to some to double the weight (approximately), increased the length 
of others by inserting a wooden tooth pick in line with the long 
axis, and with the third group we increased both the length and 
the weight. The dead dummies were presented at right angles to 
the radii of the web. This ensured that the weight of prey was dis- 
tributed over as wide an area of web as was covered by the length 
of the insect (or the insect + toothpick). In fact it meant that the 
maximum dimension of the prey was at right angles to the spider 
as it approached across the web. The insects were vibrated elec- 
trically at 250 cps until the spider left the hub on its predatory 
excursion. At that stage the vibrator was switched off so that the 
prey was motionless when the spider came in contact with it. The 
results of this experiment are shown in Table 1. In addition to the 
form of biting attack we noted whether the spider raised legs I & II 
off the web during the attack (see page 127, above), and whether the 
prey was wrapped in situ, or free wrapped, after the attack. There 
was a significant increase in leg raising during the attack in the case 
of the weighted insects. There was a slight numerical, but not a 
statistically significant, increase in the number of attacks on the long 
dummies that involved leg raising. There were three attacks out 
of ten, on heavy insects, that involved the bite and back off behavior. 
