Ichneumonidae

The Ichneumonidae are a parasitoid wasp family within the order Hymenoptera. Unlike other parasites, parasitoids kill their hosts. Ichneumonids are important parasitoids of other invertebrates; common hosts are larvae and pupae of Coleoptera (beetles), Hymenoptera (wasps and relatives), and Lepidoptera (moths and butterflies). About 25,000 species have been described worldwide. Estimates of the total species range from 60,000 to over 100,000 – more than any other hymenopteran family. The most commonly recognized wasps are boldly colored social wasps whose females have venomous stings, such as in North America are called yellowjackets. They are in a separate clade: Aculeata. In contrast, ichneumonids have ovipositors instead of stingers, and they are all solitary. They use their ovipositors to lay eggs on or in the body of their prey, and the eggs hatch into carnivorous larvae that eat and kill the host. The distribution of the ichneumonids was traditionally considered an exception to the common latitudinal gradient in species diversity, since the family was thought to be at its most species-rich in the temperate zone instead of the tropics, but numerous new tropical species have now been discovered. Insects in the family Ichneumonidae are commonly called ichneumon wasps or ichneumonids. Less exact terms are ichneumon flies (they are not closely related to true flies), or scorpion wasps due to the extreme lengthening and curving of the abdomen (scorpions are arachnids). The name is derived from Latin 'ichneumon', from Ancient Greek ἰχνεύμων (ikhneúmōn, 'tracker'), from ἴχνος (íkhnos, 'track, footstep'). The name first appeared in Aristotle's 'History of Animals', c. 343 BC. Aristotle noted that the ichneumon preys upon spiders, is a wasp smaller than ordinary wasps, and carries its prey to a hole which they lay their larvae inside, and that they seal the hole with mud. Adult ichneumonids superficially resemble other wasps. They have a slender waist, two pairs of wings, a pair of large compound eyes on the side of the head and three ocelli on top of the head. Their size varies considerably from a few millimetres to seven or more centimetres. The ichneumonids have more antennal segments than typical, aculeate wasps (Aculeata: Vespoidea and Apoidea): ichneumonids typically possess 16 or more, while most other wasps have 13 or fewer. Unlike the aculeate wasps, which sting in defense and do not pass their eggs along the stinger, ichneumonid females have an ovipositor (homologous to the stinger) which they use to lay eggs inside or on their host. Ichneumonids generally inject venom along with the egg, but only larger species (some in the genera Netelia and Ophion) with relatively short ovipositors use the ovipositor in defense. Males do not possess stingers or ovipositors in either lineage. Ichneumonids are distinguished from their sister group Braconidae mainly on the basis of wing venation. The fore wing of 95% of ichneumonids has vein 2m-cu, which is absent in braconids. Vein 1rs-m of the fore wing is absent in all ichneumonids, but is present in 85% of braconids. In the hind wing of ichneumonids, vein rs-m joins Rs apical to (or rarely opposite) the split between veins Rs and R1. In braconids, vein rs-m joins basal to this split. The taxa also differ in the structure of the metasoma: about 90% of ichneumonids have a flexible suture between tergites 2 and 3, whereas these tergites are fused in braconids (though the suture is secondarily flexible in Aphidiinae).