Although the Ichneumonidae are perhaps the most purely carnivorous of all the great families of Hymenoptera, there is nevertheless reason for supposing that some of them can be nourished with vegetable substances during a part at any rate of the larval existence, Giraud and Cameron[[474]] having recorded observations that lead to the conclusion that some species of the genus Pimpla may inhabit galls and live on the substance, or juices thereof.
Fig. 364.—Agriotypus armatus, female. Britain. (After Curtis.)
Over 1200 species of Ichneumonidae are known to inhabit Britain, and there can be no doubt that this number will be increased as a result of further observation. Unfortunately no general work has yet been published on this department of our fauna, and the literature is very scattered.[[475]] The species of North America have not received so much investigation as those of Europe, and the Ichneumon fauna of the tropics remains almost uninvestigated. Six sub-families are recognised: Agriotypides, Ichneumonides, Cryptides, Tryphonides, Pimplides, Ophionides. Of these the first is the most remarkable, as it consists of an Insect having aquatic habits. It has for long been known that the unique species Agriotypus armatus, a rare Insect in our islands, is in the habit of going under water and remaining there for a considerable period, and it has now been satisfactorily ascertained that it does this for the purpose of laying its eggs in the larvae of Trichoptera.[[476]] The resultant larva lives inside the cases of species of Silo, Goëra, etc. It undergoes a sort of hypermetamorphosis, as its shape before assuming the pupal condition is very different to what it was previously. It changes to a pupa inside the case of the Trichopteron in a cocoon attached to the walls of the case. Previous to making this, however, the Agriotypus forms a curious, elongate, string-like process attached to the anterior extremity of its cocoon. The use of this is unknown. Full information as to the life-history of this aquatic Hymenopterous larva, especially as to its respiratory functions, would be of great interest. The affinities of this remarkable Insect are still doubtful. It may probably prove to be between Proctotrypidae and Ichneumonidae.
Fig. 365.—Metamorphosis of Agriotypus. (After Klapálek.) A, Larva; B, sub-nymph; C, case of the Silo with the string of attachment formed by Agriotypus; D, section of the case: v1, operculum of case; v2, cocoon; ag, pupa of Agriotypus; e, exuvia of Agriotypus; w2, wall of cocoon; s, remains of Silo; w1, closure of case.
Remains of Insects that may be referred with more or less certainty to Ichneumonidae have been found in some abundance in various tertiary strata both in Europe and North America, but nothing indicative of the existence of the family has yet been found in the older rocks.
Fam. V. Braconidae—Supplementary Ichneumon-flies.
Antennae with many (nearly always more than fifteen) joints, not geniculate. Wings with a moderate number of cells, which on the anal part of the front wing are more or less imperfect, the anal (i.e. the second posterior) cell being separated from the cubital cells by a large space in which there is no cross-nervure. Abdomen with but little mobility between the segments; the suture between the second and third usually absent, or obsolete. Larvae living parasitically in—possibly exceptionally outside—the bodies of larvae or pupae of Insects.
