NESTING BIOLOGY OF 
THE SOCIAL WASP MICROSTIGMUS COMES 
(HYMENOPTERA: SPHECIDAE, PEMPHREDONINAE) 
By Robert W. Matthews 
Museum of Comparative Zoology 
Sociality in the Hymenoptera has been achieved independently at 
least ten times (Wilson, 1966). Of these, the majority have oc- 
curred in the Apoidea, which in addition exhibit most of the inter- 
mediate stages in the evolution of sociality. While no truly social 
Sphecidae have been previously recorded, presocial sphecids parallel- 
ing both types of social evolution in bees have been reported (Evans, 
1964, 1966), and undoubtedly more examples will be found as fur- 
ther studies are made. This paper describes the nesting biology of 
a unique pemphredonine wasp, Microstigmus comes Krombein (Fig. 
1 ) , from Costa Rica and presents evidence indicating that it is to be 
regarded as the first case of well-developed social behavior in the 
family Sphecidae. 
Except for brief accounts by Myers (1934) and Howes (1925, 
1933), little is known of the biology of Microstigmus. Myers found 
eleven M. theridii nests suspended from the undersides of Coccoloba 
pubescens leaves in the forests of Trinidad; each was constructed of 
reddish fibers taken from the underside of the leaves, loosely bound 
by strands of a silken material, and suspended from the leaf by a 
slender coiled pedicel. Nests contained one to eight cells, and the 
prey was Collembola; some nests were parasitized by pteromalid 
wasps. Although Myers noted the presence of more than one female 
in some nests, this fact has apparently been overlooked. Howes gives 
a very general account of the nests and biology of M. guianensis 
from British Guiana including photographs of nests, which he thought 
to be made of lichen and moss fragments. Significantly, Howes re- 
cords only one adult wasp per nest, though he does not state how 
many nests he observed. However judging from his nest photographs 
his observations may have involved more than one species. 
During the first two weeks of March, 1967, I studied Micro- 
stigmus comes on the Osa Peninsula of Costa Rica. This wasp 
constructs white bag-like nests suspended from the under surface 
of fan-shaped fronds of the palm Crysophila guagara Allen, a com- 
Manuscript received by the editor April 4, 1968 
23 
