Ichneumonoidea
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 Megarhyssa macrurus Female
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The Ichneumonoidea are insects classified in the hymenopteran suborder Apocrita. The superfamily is made up of the ichneumon wasps (often inaccurately called "ichneumon flies"; family Ichneumonidae) and the braconids (family Braconidae). Like other parasitic wasps, they were long placed in the "Parasitica", variously considered as an infraorder or an unranked clade, but actually not a monophyletic group.
The superfamily Ichneumonoidea has been estimated to contain well over 80,000 different species. The ichneumon wasps are more familiar to non-entomologists, being larger and about three times as diverse as the braconids.
They are solitary insects, and most are parasitoids—the larvae feeding on or in another insect which finally dies. As with all hymenopterans, ichneumons are closely related to ants and bees.
Members of the family Ichneumonidae are usually larger than members of the Braconidae, and are distinguished primarily by details of wing venation. Many species in both families utilize polydnaviruses to suppress the immune system of the host insect.
Some members use many different insects as a host, others are very specific in host choice. Various ichneumons are used successfully as biological control agents in controlling pests such as flies or beetles.
Ichneumon wasp species are highly diverse, ranging from 3 mm (1/8-inch) to 13 cm (5 inches) long. Most are slender, with the females of many species (particularly in the genus Megarhyssa) having an extremely long ovipositor for laying eggs. The female finds a host and lays an egg on, near, or inside the host's body. Upon hatching, the larval ichneumon feeds either externally or internally, killing the host when they themselves are ready to pupate. Despite looking formidable, the ovipositor does not deliver a sting like many wasps or bees. It can be used by the wasps to bore into and lay eggs inside rotten wood.
Charles Darwin found the grisly life cycles of ichneumons wasps incompatible with the central notion of natural theology which saw the study of nature as a way to demonstrate God's benevolence. In a letter to American botanist Asa Gray, Darwin wrote:
"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."[1]
