ASSOCIATIONS BETWEEN FLIES AND SPIDERS: 
BIBIOCOMMENSALISM AND DIPSOPARASITISM?* 
By 
Michael H. Robinson and Barbara Robinson 
Smithsonian Tropical Research Institute 
P.O. Box 2072, Balboa, Canal Zone, Panama 
There are numerous records in the arachnological and entomo- 
logical literature of relationships between spiders and flies other 
than the simple case of predator and prey. Bristowe ( 1 94 1 :362— 370) 
reviewed a number of cases of parasitism and commensalism. Flies 
of the superfamily Drosophiloidea are involved in a number of more 
or less complex relationships with spiders. Chloropids parasitize 
spiders’ egg cocoons and may actually perch on adult spiders (Bris- 
towe, 1941:367) while milichiids share food with spiders (Richards, 
1953). McMillan (1975) has recorded an association between mili- 
chiid flies of the genus Desmometopa and two species of large Aus- 
tralian orb-weaving spiders. The flies moved about the host web 
and fed on prey items as they were being consumed by the spiders. 
In addition, the milichiid moved onto the host and apparently 
cleaned the mouthparts and anal region of the spider. McMillan 
does not state whether the flies remained on the spiders when they 
were not actively cleaning them nor does he state how many flies 
were present on the spider at any one time. We here report on 
several different associations between flies and spiders, all of which 
are commensal (in the broadest sense). We found milichiids asso- 
ciated with the golden-web spider Nephila clavipes, unidentified 
flies were found as commensals of Argiope savignyi, and chloropid 
flies were found in a similar relationship with Argiope argentata. 
All these relationships were discovered in Panama. We describe a 
case of milichiid commensalism with a predatory hemipteran and 
suggest that the complex relationship between Nephila and the 
milichiids may have evolved from such a relatively simple stage. 
We think that the term commensalism is not sufficiently specific 
to describe some of the relationships reviewed here and suggest two 
possible additions to the terminology of symbioses. 
* Manuscript received by the editor October 31 , 1977. 
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