LIMITATION OF FAMILIES 127 



enemies. The gall-flies are provided with a delicate ovipositor ,. 

 and by means of this the females pierce the tissues of plants to 

 deposit their eggs. Some of them, however, use their weapons to- 

 insert their own eggs into the actual eggs, or more often the soft larvae, 

 of other insects, and the young when they hatch are thus provided 

 with a living prey. The ichneumon-flies have similar habits. 

 They prey chiefly on the caterpillars of butterflies and moths, and 

 when they have found a suitable victim, which may be many 

 hundred times larger than themselves, swoop on its back, pierce 

 the body with the ovipositor and leave their eggs in it. The larvae 

 thus hatch out in a favourable and protected position and eventually 

 devour their unwilling host. In many cases their ravages are so- 

 timed that the caterpillar is not killed before it has pupated, and 

 its parasites then go through their own pupation within the chrysalid. 

 Those who breed butterflies and moths have to take sedulous 

 precautions to keep off ichneumon-flies from the eggs and cater- 

 pillars they are rearing, and none the less often find that at the 

 time when the butterfly should appear there comes out only a 

 swarm of little flies. 



The females of the gaudy little ruby-flies haunt places occupied 

 by solitary wasps. When one has discovered a cell with a young^ 

 wasp larva in it, together with the store of caterpillars that 

 the wasp has placed for the benefit of its own grub, she places 

 a few of her eggs in it, and the larva devours not only the 

 wasp-grub but the caterpillars stored for the latter. These 

 ruby-flies which have thus learned to provide so well for their 

 young lay very few eggs, and of those that are laid usually only one 

 hatches out. 



The larvae of bees are soft, legless grubs, and are placed in cells 

 constructed by the mothers themselves in the case of the solitary 

 bees, or by the arrested females known as workers in the social 

 bees, whilst in some parasitic bees the mothers deposit the eggs in 

 cells constructed by other bees, and the parasitic grubs hatch out 

 more quickly and devour the food prepared for their host. In the 

 solitary bees each cell is packed with a mixture of honey and pollen 

 collected by the mother ; in the social bees the food is collected by 

 the workers, who feed and tend the young. 



The female solitary wasps construct a cell for each egg, in which 

 they store from eight to a dozen caterpillars, which are paralysed 

 by the sting and so remain fresh and alive until the wasp-grub is 

 ready to devour them. Among the social wasps, each colony or 



