PARASITES THAT ABB FREE WHEN OLD. 169 



It is not only caterpillars which are assailed by- 

 mortal enemies ; the eggs themselves are watched by some 

 hymenoptera, which pierce the shell, and lay within it 

 their own eggs. When the larvae are hatched, the yolk 

 and the young tissues of the legitimate proprietor serve 

 as rations for the usurper. 



In this manner, the Ophionewri live, in their larva 

 state, in the egg of the Pieris brassica, the cabbage butter- 

 fly so abundant in our gardens ; without this police 

 establishment they would multiply immoderately, and 

 our kitchen gardens would suffer still more from the 

 ravages of these caterpillars. 



It is in vain for insects to lay their eggs in the 

 middle of fruits, .or in the substance of a leaf or a 

 branch ; there will be always some hymenopterous insect 

 which, guided by its marvellous instinct, will pierce them 

 with its ovipositor, and reach them without their even 

 perceiving it. 



In the substance of those beautiful leaves of the 

 water-lily which cover our ponds in summer, we often 

 see a charming insect, known by the name of Agrion 

 virgo, or damsel dragon-fly, a name given to it on 

 account of its graceful attitudes and its elegant appear- 

 ance. We observe this insect deposit its eggs with 

 great prudence, fully persuaded that they are safe in 

 the midst of the water ; but the poor neuroptera reckons 

 without its host. An hymenopterous insect, named 

 Polynema, is there, watching every movement of the 

 Agrion ; and as soon as the latter has laid an egg, the 

 Polynema darts down like a bird of prey on its victim, 

 pierces it, and deposits its own egg in the interior. The 

 egg of the wounded agrion will hatch a polynema. The 



