[2] However, this likely represents less than a quarter of their true richness as reliable estimates are lacking, along with much of the most basic knowledge about their ecology, distribution, and evolution.

[3] Ichneumonid wasps, with very few exceptions, attack the immature stages of holometabolous insects and spiders, eventually killing their hosts.

[4] They play an important role as regulators of insect populations, both in natural and semi-natural systems, making them promising agents for biological control.

The name is derived from Latin 'ichneumon', from Ancient Greek ἰχνεύμων (ikhneúmōn, "tracker"), from ἴχνος (íkhnos, "track, footstep").

Long believed to be rare in the tropics, and at its most species rich in the temperate region, the family became a classic textbook example of an 'exceptional' latitudinal diversity gradient.

They are severely undersampled, and studies of their diversity typically produce very high numbers of species which are represented by only a single individual.

The sheer diversity also means DNA sequence data is only available for a tiny fraction of the species, and detailed cladistic studies require major computing capacity.

[27][28][29][30][31][32] The large number of species in Ichneumonidae may be due to the evolution of parasitoidism in Hymenoptera, which occurred approximately 247 million years ago.

[35] However, this has been disputed as early fossil ichneumonoids (such as Praeichneumonidae, Tanychorinae, Eoichneumoninae, and Protorhyssalinae) have ovipositors that are only a few milimeters long.

In 2019, combined morphological and molecular phylogenetic analysis of the family resulted in the following 41 subfamilies being recognized, in addition to the extinct Labenopimplinae.

[41] Famous ichneumonologists include: The perceived cruelty of the ichneumonids troubled philosophers, naturalists, and theologians in the 19th century, who found the parasitoid life cycle inconsistent with the notion of a world created by a loving and benevolent God.

[42] Charles Darwin found the example of the Ichneumonidae so troubling that it contributed to his increasing doubts about the nature and existence of a Creator.

In an 1860 letter to the American naturalist Asa Gray, Darwin wrote: I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us.

I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.