SOLITARY AND SOCIAL WASPS 



Since the length of the developmental period, and prob- 

 ably also the egg-laying rate of the queen, depend on tem- 

 perature, the control of temperature is very important. It 

 is well known that the developmental period is shorter in 

 large colonies than in small ones. If the nest gets too hot 

 some of the wasps stand at the entrance and fan with 

 their wings. This has been shown under experimental 

 conditions to be moderately effective in the hornet. When 

 a nest was heated with an electric heater the wasps began 

 actively fanning at yj° C. This kept the temperature 

 steady for a few minutes. When, in spite of fanning, it 

 began to get hotter, they flew off and abandoned their 

 brood. 



We may now consider the behaviour of the colony in 

 two of its most distinctively social aspects, the relation of 

 the workers to the grubs and the division of labour amongst 

 the workers. The larger wasp grubs are able to call attention 

 to themselves by bending their heads backwards and scraping 

 the wall of the cell with their mandibles. When a number 

 of them do this together quite a distinct rustling sound may 

 be heard. It is possible that they do this more often when they 

 are hungry. But they have another and probably more 

 important way of attracting the workers. The labial gland, 

 which when they are full-grown produces silk for the cocoon, 

 produces earlier in life a sweetish secretion. This appears as 

 a little drop on the lower lip, and can be elicited by stroking 

 the grub's head with a straw. The workers lick up the 

 secretion whenever they feed the larvae. It has been sug- 

 gested that this mutual exchange of food is a very important 

 factor in maintaining social ties, but there is very little evi- 

 dence that the liquid obtained from the grubs is of much 

 consequence to the workers. It may be that the process is 

 really of more importance in nest-hygiene, the workers 



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