EELATION TO AGRICULTURE 29 



ren, and it sometimes leads to the total subjection 

 of an economically harmful species of vegetable- 

 eaters. 



An excellent example of this means of limita- 

 tion is to be found in the case of the common 

 cabbage-butterfly. The crafty executioner is a 

 tiny, almost microscopic, ichneumon-fly which, 

 for lack of a common English name, must be 

 known by its generic title, Microgaster. This 

 insect hovers about in the air until it perceives 

 a cabbage-caterpillar at work upon a leaf. Down 

 it swoops, and the long ovipositor enters the body 

 of the unsuspecting victim. A considerable num- 

 ber of eggs are ejected from the ovipositor into 

 the body of the caterpillar, and the little ichneu- 

 mon-fly departs, satisfied that its chief mission in 

 life has been accomplished. 



Before long the eggs hatch and the larvae of 

 Microgaster, minute grub-like organisms, begin 

 to feast on the fatty layer beneath the skin. 

 Undismayed, the caterpillar continues to demolish 

 the cabbage, while the larvae grow. The day 

 finally arrives for the caterpillar to pupate, to 

 form its chrysalis. It grows sluggish of move- 

 ment in preparation for the event to come. 



But, unfortunately for their host, the ichneumon 

 larvae feel that they, too, must pupate. The 

 space is too small within the caterpillar to permit 

 the spinning of cocoons, so without ado they eat 

 their way through the skin to the open air, leaving 



