1970] 
Matthews — Microstigmus 
125 
certain, although the presence of three females in one nest seems 
indicative of some degree of social cooperation, which may perhaps 
be similar to that of M. comes. 
Prey. Use of Thysanoptera as prey has not previously been re- 
ported for the genus Microstigmus ; those species for which prey 
records are known (M. comes, M. theridii) use Collembola for 
provisioning their cells. However, it seems possible that M. wagneri 
and M. guianae may also be found to utilize thrips, for they share 
similar morphological characteristics with M. thripoctenus. Other 
Sphecidae known to prey on thrips include members of the genera 
Spilomena and Ammoplanus , pemphredonines which, so far as known, 
nest in various pre-formed cavities. 
All of the over 300 thysanopterans in the Andrews nest were 
immature individuals, belonging to apparently 4 species (differences 
may be in stadium rather than taxon). The largest individual mea- 
sured nearly 1 mm long, but most were about 0.5 mm in length. 
With but two exceptions, the 82 thrips from the Janzen nest were 
also immatures, belonging to 3 or more apparent species. Kellie 
O’Neill of the U.S. National Museum determined the Janzen nest 
prey as species of Thripidae and identified the two adult females as 
Leucothrips sp. and Bradinothrips n. sp. In her letter, she states 
that the prey “belong to groups of minute, pale, solitary leaf feeders 
that jump readily and are difficult for humans to capture.” Indeed, 
she added, Bradinothrips was previously known only from a unique 
individual collected in British Guiana. 
Within the M. thripoctenus nests, the prey appeared to be piled 
loosely in the cell bottoms rather than packed in compact masses 
like the Collembola of M. comes. Since several specimens were dis- 
lodged from their cells during shipment, exact prey counts per cell 
were not possible. However, the 1 7 1 individuals that remained in 
one cell of the Andrews nest seems an extraordinary number; by 
contrast, the most Collembola present in any of 22 M. comes cells 
examined was 58. 
Summary 
The new sphecid wasp, Microstigmus thripoctenus O. W. 
Richards, is described and its relationships to other species of 
Microstigmus discussed. Its 11-12 mm long bag-like nest is sus- 
pended on a straight pedicel from the underside of leaves in the 
Costa Rican rain forest. Constructed of punky wood chips bound 
together in a silken matrix, the nest has a hood-like overhang above 
