104 OCCASIONAL PAPERS. 



in not getting any moth to emerge from some of 

 your cocoons. On breaking them open you will 

 find a cluster of long black cases inside. These are 

 cocoons out of which have escaped ichneumon flies 

 the flies and their cocoons are both on the table. 

 You ask how came they there ? And where is the 

 chrysalis ? All caterpillars are liable to the attacks 

 of these creatures, the office of which appears to be 

 to assist in preserving the balance of nature by 

 checking the undue increase of vegetable-feeding 

 caterpillars; in this of course they act in concert 

 with the birds. You will generally find that in those 

 years when caterpillars are more than ordinarily 

 abundant, when the gardener finds his cabbage 

 plants worked into verdant antimacassars of every 

 complicated pattern, that then ichneumon flies are 

 also abundant, and that comparatively few of the 

 caterpillars arrive at butterfly-hood. I once counted 

 nearly 100 chrysalises of the Cabbage White Butter- 

 fly in an outhouse, and with the exception of not 

 more than half a dozen, each one had beside it a 

 cluster of small yellow cocoons those of an ichneu- 

 mon fly. This fly darts down on the unfortunate 

 caterpillar's back, bores a hole in it and lays therein 

 an egg ; this egg hatches and the small larva feeds upon 

 the bodily substance of its huge brother, carefully 

 avoiding, it is said, the vital parts, " growing with his 

 growth and strengthening with his strength." 



It would not appear as if much inconvenience re- 

 sulted, for the unwilling landlord eats and goes about 

 his allotted work with all unconcern, and often has 

 strength sufficient at last to manufacture the tomb 

 out of which he is destined never to arise. 



