252 PARTHENOGENESIS VII 



which the board was hung, and sometimes adding to 

 it themselves additional strength. By degrees such 

 movable nests could be lowered a little bit each day 

 from an inconveniently high position, or taking the 

 nest in the night under a cover whilst all the wasps 

 were in it, it could be removed from a distant locality 

 to the Professor's garden. In such cases a certain pro- 

 portion always came to grief by the desertion of the 

 colony ; and the queen was then sometimes found at 

 work on the old site constructing a new nest. Al- 

 though strangers are not admitted in a well-regulated 

 Polistes nest, yet by carelessness or desertion the brood 

 of one colony will sometimes be exposed to the attacks 

 of the workers of another, who then make use of the 

 unfortunate larvae to feed their own young. It fre- 

 quently happens that workers who have once indulged 

 in this kind of thing, become what are called " robber 

 wasps," utterly demoralised, and actually undo the 

 whole labour of a colony by dragging out the grubs 

 which they were lately so carefully tending in common 

 with their fellows, to feed the still younger larvae. 

 When this condition of things has once begun in a 

 colony it soon goes to ruin, and hence it is necessary 

 to destroy any deserted Polistes nests in the neighbour- 

 hood of those under observation, lest by entering the 

 former the members of the latter should get the bad 

 habit of pulling grubs out of their cells, and proceed 

 to do the same in their own nests. 



The rain is a very constant source of destruction to 

 Polistes colonies, drowning the young by saturating 

 the cells with moisture. Light rain will not, however, 



