SOCIAL LIFE 219 



among the " sub-social " groups ; the members of such a 

 flock keep near their companions, but in their activities 

 they show no such mutual co-operation as characterises a 

 true insect community. 



Examples of insects which live in societies made up of 

 an assemblage of many families are afforded by several 

 groups of beetles. The family life of the Passalidae was 

 described in the last chapter (p. 213), and the conditions 

 of their existence — parents and offspring feeding in galleries 

 excavated in timber — afford a starting-point for the more 

 distinctly social habits of the *' ambrosia " beetles, which 

 are akin to the destructive " bark-beetles " of our forests. 

 Most of these Scolytidae (or Ipidae) eat bark or v/ood, both 

 in their larval and adult stages, but the " ambrosia " beetles 

 have developed a more elaborate method of feeding and a 

 simple type of social life. The habits of various European 

 and North American species of Gnathotrichus, Xyleborus 

 and Platypus have been described by H. G. Hubbard 

 (1897) and others whose accounts are summarised by 

 Wheeler (1923). While most of the Scolytidae make 

 galleries at the inner surface of the bark, the ambrosia 

 beetles burrow deeply into the wood ; it is not wood, 

 however, on which they feed, but a fungus — the " ambrosia " 

 — specially cultivated on a '' bed " prepared from woody 

 material which has passed undigested through the insects' 

 food canals. Beetles of both sexes work together, but 

 most of the parental care devolves on the mother, who 

 excavates a series of circular pits along the tunnel, laying 

 an egg in each and depositing fragments from the " bed " 

 with some growing fungus as food for the grubs when 

 hatched. Each grub as it grows increases the size of its 

 '* cradle " (Fig. 58, G', /) by biting and swallowing wood 

 which is not digested but, being ejected in pellets from 

 the intestine, is removed by the mother and used for 

 fungus-bed. " The mouth of each cradle is closed with a 

 plug of the food fungus, and as fast as this is consumed it 

 is renewed with fresh material." The females of Platypus 

 lay eggs in groups of ten or twelve at intervals along the 



