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Psyche 
[Vol. 87 
large orb-web spider). On one disc was placed a cricket ( Gryllus 
assimilis) that had been fed on by the spider for 30-60 minutes. 
Another carried an intact cricket, killed by freezing, then thawed, 
and the third was a blank control. Fourteen replicates were made. In 
addition, we periodically collected by aspiration flies hanging on 
webs and spider draglines independent of webs. 
Results 
We propose various flies to be spider symbionts on the basis of 
either direct or circumstantial evidence. The latter category consists 
of flies caught exclusively, or for the most part, on trap discs baited 
with partially consumed spider prey. 
Known Kleptoparasites 
Cecidomyiidae: Didactylomyia longimana (Felt), length 1.3 mm. 
This most abundant of kleptoparasites was active at dusk and night. 
In certain locations virtually every Nephila clavipes with prey would 
have from one to over a dozen midges perched on the semi-digested 
prey (Fig. 1). We also observed D. longimana in association with 
other araneids; Argiope aurantia, Mastophora bisaccata, and 
Eriophora ravilla. While individuals were sometimes found on 
swathed insects stored in the web, they were more commonly seen 
on fluid-covered prey being consumed by the spider. Midges were 
observed with their heads applied to the surface of the insect and 
their abdomens expanded, leaving no doubt that feeding had 
occurred. Rarely, a fly was found on the dorsal surface of the 
abdomen or the carapace of a spider. Examination of 3X photo¬ 
graphs of flies resting on spiders shows no evidence of feeding on the 
spider. Spiders defended their catch from D. longimana by batting 
at approaching flies with their forelegs. One of us (Stowe) has seen 
Scoloderus cordatus (Araneidae) vigorously brushing nematocerous 
flies off its prey. McCook (1889) describes a similar behavior in 
Argiope aurantia. Since Nephila, Argiope, and Scoloderus are in 
separate subfamilies, the behavior may predate the radiation of the 
family. ( Nephila maculata behaves similarly toward kleptoparasitic 
spiders of the genus Argyrodes, Robinson and Robinson, 1973; 
however in Scoloderus this behavior must be adapted for klepto¬ 
parasitic flies alone since Argyrodes are never in their webs.) Prey 
defense is indicative of a competitive relationship and substantiates 
