292 THE WONDER OF LIFE 



ranked the ' Inquiline ' gall-insects, which are not them- 

 selves gall-producers, but utilize what others make. 



Another half -and-between case is that of a moth, Galleria 

 melonella, whose caterpillars feed entirely on beeswax. 

 The female lays her eggs on pieces of wax or wood within 

 the hive ; the minute grub-like caterpillars emerge in 

 about eight days and make themselves a shelter of silk 

 which protects them from the stings of the bees. They 

 feed chiefly on old honeycomb. As they grow they en- 

 large their shelter into a gallery which opens on the surface 

 of a comb. When they reach their limit of growth they 

 make cocoons on the wall of the hive near the entrance, 

 and pass out as moths in a fortnight or so. 



Commensalism. This term, which is just the same 

 as companionship, ' eating at the same table ', may be use- 

 fully restricted to external associations which are beneficial 

 on one side at least. When the benefit is two-sided, the 

 term * mutualism ' or * commensal mutualism ' may be used. 



Many associations remind one of the beggar at the rich 

 man's gate a small creature living on the crumbs from 

 its larger host's table. But it is difficult to draw the line 

 between cases where the benefit is all on one side and those 

 where some degree of reciprocity obtains. Thus Miss 

 Winifred Coward has described a peculiar little hydroid, 

 Ptilocodium repens, which grows among the polyps of a 

 Pennatulid, Ptilosarcus, from the Timor Sea. The two 

 kinds of animals live literally ' cheek by jowl ', and as the 

 hydroid has numerous defensive polyps, out-numbering 

 the nutritive ones (which, it is interesting to notice, have 

 degenerate tentacles), it may be that it confers some pro- 

 tective advantage on the Pennatulid on whose food-supply 

 it levies toll. 



