1980] Sivinski & Stowe — Kleptoparasitic cecidomyiid 345 
Table 1. The sex ratio of Didactylomyia longimana compared to that of 6 species of 
non-kleptoparasitic cecidomyiids hanging in the same webs, chi-square = 10.25, 
p<.005. The difference in sex ratios may be the result of D. longimana males waiting 
near sites likely to attract females. Alternatively, female kleptoparasites may search 
for food and spend less time in webs than either males of their own species or females 
of non-kleptoparasitic species. 
Male 
Female 
D. longimana 
19 
10 
N on-kleptoparasites 
14 
40 
females. It is possible that 
both contribute 
to the differences 
between sex ratios. 
The predictable arrival of females might also account for the 
presence of males at feeding sites in the unusual Greek chloropid 
Trachysiphonella pori. The fly is closely associated with an ant that 
is preyed upon by the spider Zodarium fernatum (Zodariidae), 
which lurks about ant nest entrances at night. T. pori is a 
kleptoparasite of the spider, clinging tenaciously to dead ants even 
as they are moved about by the spider (Harkness and Ismay 1977). 
Flies are found only at particular ant nests, and it is possible that 
they breed in or near these nests (a number of acalypterate flies 
breed in ant nests, e.g. Moser and Neff 1971). If so, males may wait 
at nests to inseminate emerging females. 
Summary 
A diverse kleptoparasitic fly fauna consumes the prey of spiders 
in northern Florida. The most abundant of these Diptera is a 
cecidomyiid, Didactylomyia longimana. Our observations consti¬ 
tute only the second record of a nematoceran kleptoparasite of 
spiders and document a rare case of protein consumption by an 
adult gall midge. Four species of milichiid were also captured. One 
species is phoretic. Two others are notable for the extreme 
enlargement of their mouthparts. Traps baited with spider prey 
captured one species of Ceratopogonidae and three species of 
Phoridae that may also be kleptoparasitic. We argue that intra¬ 
specific competition could lead to intimate spatial relations between 
symbionts and that the mating systems of the flies, as reflected by 
sex ratios near the spiders, are influenced by the relative abundance 
of hosts. 
