110 
Psyche 
[Vol. 94 
Table 1. Web selectivity ( E w ) values for prey types and sizes caught in webs of 
Micrathena gracilis 
Caught in spider web 
No. Rj 
Caught in sticky trap 
No. P t 
F 
% 
Taxon 
Diptera 
89 
.669 
268 
.355 
+.307 
Hymenoptera 
20 
.150 
338 
.448 
-.498 
Coleoptera 
5 
.038 
48 
.064 
-.255 
Homoptera 
4 
.030 
43 
.057 
-.310 
Thysanoptera 
- 
.000 
21 
.028 
-1.00 
Psocoptera 
- 
.000 
11 
.015 
-1.00 
Hemiptera 
- 
.000 
8 
.011 
-1.00 
Lepidoptera 
- 
.000 
1 
.001 
-1.00 
Other (Unident.) 
- 
.000 
17 
.023 
-1.00 
Size mm 
0-2 
56 
.475 
600 
.795 
-.252 
2-4 
32 
.271 
138 
.183 
+.194 
4-6 
16 
.136 
13 
.017 
+.778 
6-8 
4 
.034 
2 
.003 
+.838 
8+ 
10 
.085 
2 
.003 
+.932 
“Electivity” index of Ivlev (1961): E — r z - p z /r; + pi where r z = 
proportion of item i taken, and pi — proportion of item i available. 
Values for this index range from + 1 .0 (highly preferred) to - 1 .0 (least 
preferred). For this comparison, the catches of the artificial webs 
were assumed to estimate the proportions of prey available in each 
size class or taxon considered. 
Results 
Of the 1 33 insects observed to strike webs, 1 1 8 stuck to the web for 
at least 3 sec. (for an initial web capture efficiency of 88.7%). Of these 
1 18 insects, 66 were actively attacked by the spider, 44 were ignored, 
and 8 escaped before a “decision” was made. Of these 66 insects 
actively attacked, 54 were captured, 5 were rejected, and 7 escaped 
due to mishandling. A total of 38 insects escaped at some point in the 
predatory sequence (Fig. 1), leading to an overall web capture effi- 
ciency of 71.4% (hit-escape/ hit X 100). 
Comparison of traps and webs indicate that the webs of Micra- 
thena gracilis are selective, and show electivity for Diptera sized >2 
mm (Table 1). However, the majority of insects hitting and then 
sticking to the web was quite small (< 3 mm ): 53% (70 of 133 hits) and 
58% (67 of 1 16 sticks) respectively. Therefore, well over half of the 
