1987] 
Uetz & Hart sock — Micrathena gracilis 
111 
insects which encountered the web (and were thus available as poten- 
tial prey items) were small insects containing very little biomass/ insect. 
Larger insects contain far more biomass per individual, but they 
are much rarer. Since the electivity index measures the degree to 
which prey are preferred in comparison to their availability, the 
results of the comparison between the web and the trap suggest that 
the web selectively retains larger prey. This conclusion is supported 
by the observation that escapes occurred in nearly the same propor- 
tion for all size classes except for the smallest and the very largest 
(Fig. 2). Escapes by very small insects (1-3 mm) can be attributed to 
almost instantaneous escape after contact with the web. It is possible 
that these insects were not flying very fast but were strong enough to 
pull free of the web before the chance of attack. On the other hand, 
75% of the insects > 9 mm escaped before being attacked, with 50% of 
these insects escaping within 3 seconds of hitting the web. This is 
probably due to their faster flight velocities and strength (see Nentwig 
1980). 
As a consequence of web selectivity, Micrathena is presented with a 
preselected array of potential prey items to “decide” whether to 
attack. Within this array, the spiders were observed to attack a 
greater proportion of larger insects than smaller ones (Fig. 2). A 
comparison of prey taken by the spiders with those taken by the web 
shows evidence of greater selectivity (Table 2). The Ivlev index values 
are highest for Diptera in the middle range of size classes (4-8 mm). 
The frequency of insects attacked or ignored vs. insects sticking in 
each size class shows that Micrathena is not attacking (or ignoring) 
Table 2. Spider selectivity ( E s ) values for prey types and sizes caught by 
Micrathena gracilis 
Captured by spider 
No. R t 
Caught in web 
No. Pi 
Taxon 
Diptera 
41 
.759 
89 
.669 
+.670 
Hymenoptera 
10 
.185 
20 
.150 
-.567 
Coleoptera 
2 
.037 
5 
.038 
-.013 
Homoptera 
1 
.019 
4 
.030 
-.224 
Size (mm) 
0-2 
20 
.370 
56 
.475 
-.124 
2-4 
16 
.296 
32 
.271 
+.044 
4-6 
13 
.241 
16 
.136 
+.279 
6-8 
2 
.037 
4 
.034 
+.042 
8+ 
3 
.056 
10 
.085 
-.206 
