968 



AN INTRODUCTION TO ENTOMOLOGY 



Tribe PSENINI 



The Psenini are small sphecoid-wasps in which the base of the 

 abdomen is slender, formin<^ a petiole much like that of the Sphecinae, 

 but differing in being flattened and usually furrowed 

 above (Fig. 1202), and these wasps are much smaller 

 than the true thread-waisted wasps. The antennae 

 are inserted at the level of the middle of the eyes; 

 and there are three complete submarginal cells in 

 the fore wings. These wasps make their burrows 

 either in sand or in the pith of brambles, and pro- 

 vision them with aphids or other small Homoptera. 



Fig. 1202.— A Tribe PEMPHREDONINI 



psenid. 



In this tribe the antenuce are inserted low on the 

 face, near the level of the base of the eyes; and there are at most two 

 submarginal cells present, sometimes a trace of vein r-\-m and Rg 



Fig. 1203. — Wings of Stigmus podagricus. (From Bradley). 



incompletely indicates a third (Fig. 1203). The ab- 

 domen is sometimes sessile. 



The pemphredonids usually burrow in the pith of 

 dry branches and provision their burrows with plant- 

 lice. A very common species in the East is Stigmus 

 fraternus. This insect measures 5 mm. in length, and 

 makes tortuous burrows in the pith of sumac (Fig. 

 1204). Other common members of the tribe are larger. 

 Some species of Xylocelia (Diodontus) have been found 

 to burrow in the ground. 



This tribe was monographed by Fox ('92). 



Fig. 1204. — 

 Nest of Stig- 

 musfraternus. 



