1970] 
Matthews — Microstigmus 
121 
teeth shorter than in some of the other species. Transverse keel 
of pronotum very strong but surface otherwise smooth. Mesoscutum 
shining with some feeble subcoriaceous sculpture in front half, sur- 
face with rather close, short stout pale bristles, recurved backwards. 
Scutellum strongly raised from in front and behind to a sharp 
transverse keel. Metanotum with a well-marked transverse keel just 
behind the front margin, a central longitudinal keel with a weaker 
keel on each side. Propodeum with a number of strong keels, mostly 
transverse or longitudinal, defining about twelve large rectangular 
or polygonal areas. Mesopleuron generally smooth but with a system 
of irregular keels, especially one curving well in front of mid coxa, 
two irregularly sinuous ones leading upwards from near it, and a 
keel defining a prepectal area, distinctly below but only feebly above. 
Male. Rather smaller and paler than the female. Gaster without 
dark marks. Face from a little way below median ocellus, most of 
scape, much of side of pronotum, front part of mesopleuron, much 
of femora, white. Head wider, oculo-malar space longer, but not 
quite quadrate. Antennae very similar though with one more seg- 
ment, segment 3 if anything shorter. Distal part of gaster somewhat 
distorted but tergite 7 apparently not emarginate, with rather long 
bristles on each side; sternites with much longer apical bristles than 
female; sternite 7 triangular but with a short, straight apical 
truncation, a little upcurved, with numerous long bristles beneath. 
Holotype 9 , cT paratypes, Costa Rica, Prov. Puntarenas, Osa 
peninsula, 2 y 2 miles S.W. Rincon, o8°42' N, 83°29' W, 16 Feb., 
1969, ex nest (R. Andrews) . One paratype from nest in same 
locality 26 Feb. 1968 ( D . H. Janzen ). Holotype and two para- 
types deposited in U. S. National Museum, Type No. 70759; other 
paratype in British Museum. 
Discussion. There are more species of Microstigmus than is 
usually supposed ; as a result several references to the type of the 
genus, M. theridii Ducke, 1907, cannot be accepted without reserve. 
The British Museum, however, has a female from Ducke’s original 
series from Tefe. 
Pate (1937) gives a key to the genera of Pemphredonini in- 
cluding the Neotropical Microstigmus. A key to the described 
species was given by Richards (1932) ; since then, only M. comes 
Krombein (1967) has been described. The species of Microstigmus 
fall into two groups; those like the present one in which the third 
antennal segment is less than twice as long as broad and segments 
