THE SOCIAL INSECTS 



importance in the economy of the nest. The variation can 

 be referred back still earlier to what happens to the grubs. 

 Cumber, by weighing grubs or pupae extracted from different 

 positions in the groups, showed that those in the middle are 

 heavier and those from the periphery lighter. In fact, as 

 Sladen first suggested and Cumber later confirmed, the 

 variation in the size of the workers depends mainly on the 

 competition of the grubs in one pocket for their common 

 supply of food. Grubs which happen to be near the middle 

 of the pocket grow large, while those at each end are smaller. 



The variation in size of worker so produced is the basis of 

 the later economy of the nest. It seems that once workers 

 are sufficiently numerous, a division of labour is established 

 between them based on the size-difference which itself 

 depends on the earlier competition for food. Weighing 

 samples of workers throughout the season shows that there 

 is no increase in the average weight of the whole set of 

 workers, but when the colony is at its largest, the large bees 

 do most of the foraging; the small ones mostly stay at home 

 to act as nurses. According to very recent work (1952) of 

 Mrs. Brian, small workers do not begin to forage till 15 

 days after leaving the cocoon, whereas large ones begin at 

 5 days. Small bees forage mainly for nectar while large bees 

 also collect pollen. Cumber found a further minor division 

 of labour between the foragers, for by measuring the length 

 of the tongue of humble bees caught on different flowers he 

 showed that the largest workers of the species, with the 

 longest tongues, went to flowers with long nectaries, whereas 

 the smaller, shorter-tongued foragers went to flowers in which 

 the nectar was more readily accessible. 



In a humble bee such as Bombus agrorum there is a com- 

 plete overlap in size between the young queens and the largest 

 workers. There is, however, an essential difference in 



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