1968] 
Matthews — Microstigmus 
43 
is the apparent clumped nest distribution, inasmuch as it would 
increase the likelihood that nest-mates are relatives. Also, evolution 
of a pendent type of nest, unique among the Sphecidae, seems certain 
to have been a significant step, especially as it allows an opportunity 
for parent-offspring interactions. Perhaps correlated with this type 
of nest is the presence in Microstigmus of a distinctive setal brush 
in place of a pygidial plate on the apical tergite of the female. Ex- 
cept for Spilomena , the closest relative of Microstigmus (K. V. 
Krombein, pers. comm.), all genera of the Pemphredoninae I have 
examined show varying degrees of development of the pygidium; 
those for which biological information exists nest in the soil or in 
various preformed cavities and presumably use the pygidium to pack 
soil or tamp cell partitions. 
SUMMARY 
Nests of Microstigmus comes Krombein from Costa Rica contained 
as many as 18 adults of both sexes, and up to 18 cells with brood of 
all ages present; half of the 22 active nests collected at night (when 
all wasps were presumed to be inside) contained more than one adult 
female. Observations on adult behavior in provisioning, defense and 
nest maintenance give evidence of parental care and cooperation; 
ovary dissections indicate some form of reproductive dominance 
(division of labor) among females from the same nest, although no 
external morphological differences are apparent. These facts suggest 
that this is the first fully social wasp of the family Sphecidae. 
M. comes is, furthermore unique among Sphecidae in its pendent 
bag-like nests, constructed entirely of vegetative material scraped 
from beneath fronds of the palm Crysophila guagara Allen and re- 
inforced by a transparent secretion, and in its use of Collembola, as 
prey; cells are mass-provisioned. A dense cluster of hollow setae 
(setal brush) on the apical abdominal tergite and an associated 
sternal gland are described and hypothesized to be involved in nest 
construction. 
An external parasite of M. comes larvae, the braconid Metero- 
spilus microstigmi Richards was reared from some nests, and oviposi- 
tion by this wasp was observed. 
ACKNOWLEDGEMENTS 
This research was done as part of a course in Tropical Insect 
Ecology offered by the Organization for Tropical Studies during 
February and March, 1967. I thank Drs. H. E. Evans, E. O. 
Wilson, M. J. West Eberhard and C. D. Michener for much con- 
