104 INSECT ARTIZANS AND THEIR WORK 



(O. detritus), and blocked it with, cocoons containing 

 living grubs of Solenius vagus. As these were not 

 of their own race, the Osmia had no compunction 

 in destroying them. Although living, they were of 

 no more value in their estimation than their dead 

 kindred. 



Cemonus unicolor, a little black wasp only a 

 quarter of an inch long, also forms its nests in these 

 dead bramble-stems, which, by the way, offer a 

 very convenient means of making acquaintance 

 with the nesting habits and general history of some 

 of our hymenopterous insects. The dead stems that 

 have had their broken ends plastered up by the 

 mother bee have only to be collected and stored 

 in a suitable gauze-covered box until spring, when 

 the bees will emerge and their identity can be 

 established. A careful paring of the wood along 

 one side of the stem will then reveal the structure 

 of the contained cells. This species provisions her 

 cells with Aphides, the detested " green fly " of the 

 gardener. Sometimes, instead of bramble-stems she 

 makes use of the bullet-galls of the oak, enlarging 

 the borings through which the rightful inhabitants 

 have emerged. A very similar but slightly larger 

 black wasp, Pemphredon lugubris, bores into the 

 soft wood of decayed beeches. Both these species 

 occur in Britain. 



Several of the burrowing wasps of the genus 

 Crabro, of which we have thirty British representa- 

 tives, make their burrows in bramble-stems and 

 similar material. The grubs before pupation spin 



