THE HYMENOPTERA. 
87 
and which, like men in armour, care little for the attacks of their 
fellows, succumb to the active lancer-like parasitic flies. The 
weevils, for instance, are long-lived insects, and are encased in 
armour, but their active enemy finds out the joints, and sticks in 
its ovipositor between the articulations of the body, and lays the 
egg, which grows like a canker within. 
The parasitic Hymenoptera are of all dimensions ; many are of 
considerable size, but the name of the minute insects is indeed 
legion. So small are some of their larvae that several can be 
accommodated within the body of an Aphis or plant louse, and 
even in an egg of an insect. The eggs of many Lepidoptera are 
destroyed by the larvae of the parasitic Hymenoptera that have 
become developed within them from the eggs introduced by the 
ovipositor. 
Almost all these parasites introduce their eggs under the skin 
of their victims, so that their larvae are never seen, but there are 
some kinds which only lay their eggs on the outside of caterpillars, 
and other insects. When the larvae of these last-mentioned para- 
sites are hatched they pinch up the skin with their mandibles, 
and force the front part of their heads within the victim’s body, 
and remain in that position. They then suck the juices of their 
prey, and the greater part of their body is exposed to the air. 
Since nearly all the parasitic Hymenoptera lead the same sort 
of life, we might expect not to find any great structural differences 
amongst them. There is, however, a considerable variety in the 
length of the ovipositor of the females. The females which deposit 
their eggs within the bodies of caterpillars or larvae that live in the 
open air have very small ovipositors ; those which attack larvae 
which are underground, or are protected by some shelter, have longer 
instruments ; and those that have toreach insects which live within 
the trunks of trees, have very large and greatly developed egg 
depositors ; so that the Ichneumons habits and prey may be 
guessed by the length of the ovipositor. 
Nature employs these parasites constantly when insect life is 
too luxuriant ; they stop the multiplication of a crowd of creatures 
that would do an infinitude of injury, and they limit the numbers and 
the geographical distribution of many species. When the insects 
which are most injurious to agriculturists are studied, the important 
