THE ICHNEUMON FLIES. 99 
shelter to the caterpillar. Nothing is easier than to 
procure the insects from the cocoons, as the yellow mass 
needs only to be put into a box, with a piece of gauze 
tied over it by way of a cover. Nearly every cocoon 
will produce its ichnéumon, and as the little creatures 
are not strong-jawed enough to bite through the gauze, 
they can all be secured. 
There are many species of Microgaster; but those 
which have been mentioned are the most important, and 
make the most interesting habitations. 
The large oval cocoon was brought from New South 
Wales, and is evidently the produce of some lepidop- 
terous insect, probably a moth allied to the silkworm. 
Upon the larva which constructed the cocoon an ich- 
neumon has laid her eggs, and the consequence has been 
that the caterpillar has been unable to change into the 
pupal condition, but has succumbed to the parasites 
which infested it. These insects are not of minute 
dimensions, like the Microgaster, but are tolerably large, 
and in consequence can be but few in number. The 
cells are very irregular in shape, and are not rounded 
like those of many Ichneumonide, but have angular 
edges. 
In this, and in one or two other examples which are 
shown in the illustration, the reader will note a pecu- 
liarity in the development of the parasite. The Micro- 
gaster larvee emerge from the caterpillar just before 
it undergoes its change into the pupal condition, and 
effectually prevent that change by killing the creature 
in which they had been nurtured. But in many instances 
the ichneumon larva delays its escape until the cater- 
pillar has completed its cocoon, and in some cases waits 
until the change into the pupal state has been achieved. 
