839 
No. 145. | 
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 
antennse 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 iu its winged and perfect form. 
These parasitic insects which feed internally upon the aphides 
are as ethcient in destroying them as the aphis lions or any other 
class of their enemies. And it is truly wonderful that whilst 
