This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
These are found all over the kingdom, and are chiefly engaged in destroying destructive caterpillars of various kinds. Some deposit their eggs in the caterpillars or the pupae. The Ichneumon maggots feed upon the soft parts until the caterpillar or chrysalis is about to undergo a change. This, however, it cannot effect, owing to the injuries received, and it consequently dies. Fig. 160 shows on the left how a large caterpillar is overpowered with the maggots of the Ichneumon Flies; while on the right a caterpillar is covered with their cocoons. Amongst the prey of these Ichneumon maggots may be mentioned the Cabbage Butterfly, the caterpillar of the Death's Head Moth, the various kinds of aphides, wire-worms, and no doubt other grubs. There are so many kinds of Ichneumon Flies, all helpful to the gardener, that it becomes a difficult problem to know them from the enemies. Fig. 159 shows an Ichneumon Fly highly magnified, and it will be observed that it bears a resemblance to the wasps and bees.
Fig. 158. - Hawkflies.
1, Scaeva balteata; 2, Larva with greenfly in its jaws; 3, Pupa. 4, Scaeva Pyrastri; 5, Larva; 6, Pupa. 7, Scaeva Ribesii.
Fig. 159. - Ichneumon Fly (magnified).