232 THE BIOLOGY OF INSECTS 



drone, through which the inherited characters are trans- 

 mitted. 



Most insect communities harbour a number of other 

 insects which Hve during a part or the whole of their lives 

 as '* messmates," ** guests," or parasites. The large popu- 

 lation of a wasps' nest or a bee-hive may contribute to 

 render such association easy and profitable to the invaders 

 if not to the ** hosts." It is possible only to refer to some 

 of these invaders that are definitely related to the social 

 life of the host-insects. The large two- winged (Syrphid) 

 flies of the genus Volucella are conspicuous British insects, 

 whose curiously formed maggots, provided with elongate 

 processes on the body segments, live, in the case of Vohicella 

 inanis in the nests of social wasps, and in the case of V. 

 bombylans in those of bumble-bees. The former fly has a 

 yellow-banded abdomen like a wasp, and the latter is hairy 

 like a bumble-bee. Both enter freely into the nests of their 

 respective hosts in order to lay their eggs, and the maggots, 

 after hatching, live on the combs, where they appear to act 

 as scavengers by devouring refuse, thus performing a 

 definite service to the community. There is no evidence 

 in support of statements that have been made to the effect 

 that the Volucella maggots devour the grubs of the wasps 

 and bees ; the position in the nest w^here the syrphid larvae 

 live indicates that they feed on refuse. It is possible that 

 the bee or wasp-like aspect of the Volucella may serve as 

 a protection against an attack by the workers of the com- 

 munity. Sladen has noticed that the female Volucella 

 bombylans continues to lay eggs after receiving fatal injury, 

 and has suggested that this pow^r of partial survival may 

 enable an invading fly, detected and stung by a bumble- 

 bee, to ensure, notwithstanding her own speedy death, the 

 development of her offspring. 



It is well known that a very large proportion of the nests 

 of solitary wasps and bees are invaded by females of other 

 wasps and bees for the purpose of egg-laying, and the grubs, 

 as soon as hatched, begin to feed on the store provided by 

 the rightful owner for her own offspring, which may conse- 



