CHERRY. LEAVES APHIDIUS. ITS HABITS. 135 



of sustenance which the worm requires for bringing it to maturity. 

 It is singular that the parent ichneumon-fly knows if two eggs 

 were deposited in the aphis the worms from them would die for 

 want of a due supply of food, and that by a mere touch with her 

 horns she is able to ascertain which individuals have already been 

 impregnated. Some of the species of Aphidius are larger than 

 others and their offspring consequently require a larger quantity 

 of food; but each parent has the instinct to select an aphis of 

 such size as will yield the precise amount of sustenance which its 

 young requires. 



By the time the worm has attained its growth the aphis becomes 

 so exhausted that it dies. If it should now drop from the leaf to 

 the ground it would be liable to be found and devoured by centi- 

 pedes or other insects which feed upon the carcases of animals of 

 this class, and thus the worm within it would be destroyed. Na- 

 ture has therefore so constituted the aphis that in these circum- 

 stances it dies without a struggle or a spasm, with its beak inserted 

 and its claws clinging to the surface of the leaf, standing with its 

 antenna? turned backwards and its whole aspect so life-like that 

 in the infancy of my studies I supposed these were one of the 

 varieties natural to the species with which they occurred. Their 

 bodies are remarkably plump and smooth, commonly clay colored 

 or the hue of brown paper, and the aphis-lions and other insects 

 which destroy the aphides appear to pass by those which have 

 these parasites within them. Hence where a leaf or twig has 

 recently been cleared of plant-lice by their enemies, several of 

 these ichneumonized individuals may frequently be found remain- 

 ing upon it, dead and unmolested. In other instances the whole 

 colony of aphides appears to be exterminated by these parasites 

 alone, the dead swollen bodies of their victims covering the sur- 

 face of the leaves or twigs as closely as they can stand. The 

 worm remains within the body of the dead aphis during its pupa 

 state. It then cuts a circular hole through the dry hard skin and 

 comes out in its winged and perfect form. 



These parasitic insects which feed internally upon the aphides 

 are as efficient in destroying them as the aphis-lions or any other 

 class of their enemies. And it is truly wonderful that whilst 



