|Diphyus sp., Rhône (France)|
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). Over 24,000 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 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.
In the 19th century, experts wondered how a loving God could have created wasps such as these. Charles Darwin named them as an example of animal life that, to his mind, could not have been created by an all-powerful, good God.
Etymology and history
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.
A female Xanthopimpla punctata. The ovipositor is longer and more slender than the stingers of aculeate wasps
Echthrus reluctator female
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).
Ichneumonids are found on all continents with the exception of Antarctica. They inhabit virtually all terrestrial habitats, wherever there are suitable invertebrate hosts.
The distribution of ichneumonid species richness is subject to ongoing debate. 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. Recently this belief has been questioned, after the discovery of numerous new tropical species.
Reproduction and diet
Some ichneumonid species lay their eggs in the ground, but most inject them either directly into their host's body or on its surface. After hatching, the ichneumonid larva consumes its still living host. The most common hosts are larvae or pupae of Lepidoptera, Coleoptera and Hymenoptera. For example, a species of ichneumonid has been found to lay eggs in African sugarcane borer larva, a moth common in sub-Saharan Africa. Ichneumonids are also considered a primary enemy of the arctic woolly bear moth. Some species in the subfamily Pimplinae also parasitise spiders. Hyperparasitoids such as Mesochorinae oviposit inside the larvae of other ichneumonoids. The hosts of many species are unknown; host information has been summed up by e.g. Aubert, Perkins. and Townes.
Ichneumonids use both idiobiont and koinobiont strategies. Idiobionts paralyze their host and prevent it from moving or growing. Koinobionts allow their host to continue to grow and develop. In both strategies, the host typically dies after some weeks, after which the ichneumonid larva emerges and pupates.
Adult ichneumonids feed on a diversity of foods, including plant sap, nectar and other insects. They spend much of their active time searching, either for hosts (female ichneumonids) or for emerging females (male ichneumonids).
The predation pressure exerted by ichneumonids can be tremendous, and they are often one of the major regulators of invertebrate populations. It is quite common for 10-20% or more of a host's population to be parasitised (though reported parasitism rates often include non-ichneumonid parasitoids).
Zatypota albicoxa laying its egg on a spider
A larva of Acrodactyla quadrisculpta parasitising a live spider
Taxonomy and systematics
The taxonomy of the ichneumonids is still poorly known. The family is highly diverse, containing 24,000 described species. Approximately 60,000 species are estimated to exist worldwide, though some estimates place this number at over 100,000. They are severely undersampled, and studies of their diversity typically produce very high numbers of species which are represented by only a single individual. Due to the high diversity, the existence of numerous small and hard to identify species, and the majority of species being undiscovered, it has proven difficult to resolve the phylogeny of the ichneumonids. Even the relationships between subfamilies are unclear. 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.
Extensive catalogues of the ichneumonids include those by Aubert, Gauld, Perkins, and Townes. Due to the taxonomic difficulties involved, however, their classifications and terminology are often confusingly contradictory. Several prominent authors have gone as far as to publish major reviews that defy the International Code of Zoological Nomenclature.
The large number of species in Ichneumonidae may be due to the evolution of parasitoidism in hymenoptera, which occurred approximately 247 million years ago. Ichneumonidae is the basal branch of Apocrita, the lineage in which parasitoidism in hymenoptera evolved, and some ichneumonids are thought to have been in stasis for millions of years and closely resemble the common ancestor in which parasitoidism evolved. This common ancestor was likely an Ectoparasitoid woodwasp that parasitized wood-boring beetle larvae in trees. The family has existed since at least the Jurassic (ca. 150 mya), but may have appeared some time before. It diversified during the Oligocene.
- Anomaloninae (= Anomalinae)
- Brachycyrtinae (sometimes included in Labeninae)
- Campopleginae (= Porizontinae)
- Cryptinae (= Gelinae, Hemitelinae, Phygadeuontinae)
- Ctenopelmatinae (= Scolobatinae)
- Cylloceriinae (sometimes included in Microleptinae)
- Diacritinae (sometimes included in Pimplinae)
- Eucerotinae (sometimes included in Tryphoninae)
- Hybrizontinae (= Paxylommatinae) (sometimes placed in own family)
- Labeninae (= Labiinae)
- Labenopimplinae (extinct)
- Lycorininae (sometimes included in Banchinae)
- Neorhacodinae (sometimes included in Banchinae)
- Orthocentrinae (sometimes included in Microleptinae)
- Pimplinae (= Ephialtinae)
- Poemeniinae (sometimes included in Pimplinae)
- Rhyssinae (sometimes included in Pimplinae)
- Sisyrostolinae (sometimes included in Phrudinae)
- Stilbopinae (sometimes included in Banchinae)
- Tatogastrinae (sometimes included in Microleptinae)
Famous ichneumonologists include:
- Jacques Aubert
- Carl Gustav Alexander Brischke
- Peter Cameron
- Arnold Förster
- Johann Ludwig Christian Gravenhorst
- Alexander Henry Haliday
- Gerd Heinrich
- August Emil Holmgren
- Joseph Kriechbaumer
- Thomas Ansell Marshall
- Henry Townes
- Constantin Wesmael
Darwin and the Ichneumonidae
The apparent cruelty of the ichneumonids troubled philosophers, naturalists, and theologians in the 19th century, who found the parasitoid life style inconsistent with the notion of a world created by a loving and benevolent God. 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. There seems to me too much misery in the world. 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.
Megarhyssa greenei female
Morphology of the head and its processes: (А) head capsule; (В) antenna; (С) mandible
Morphology of the thorax (D)
Morphology of the abdomen and processes of the thorax: (E) front wing; (F) leg III; (G) abdomen of female
- Aristotle (2015) [343 BC]. The Complete Aristotle. Booklassic. p. section 87, chapter 20. ISBN 978-963-525-370-8.
The wasps that are nicknamed 'the ichneumons' (or hunters), less in size, by the way, than the ordinary wasp, kill spiders and carry off the dead bodies to a wall or some such place with a hole in it; this hole they smear over with mud and lay their grubs inside it, and from the grubs come the hunter-wasps. ... The eagle and the snake are enemies, for the eagle lives on snakes; so are the ichneumon and the venom-spider, for the ichneumon preys upon the latter.
- Sharkey, M.J. (1993), Family Braconidae, pp. 362-394. In: Goulet, H. and J. Huber (eds.). Hymenoptera of the world, an identification guide to families, Agriculture Canada Research Branch Monograph No. 1894E.
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- Veijalainen, A.; Wahlberg, N.; Broad, G. R.; Erwin, T. L.; Longino, J. T.; Saaksjarvi, I. E. (2012). "Unprecedented ichneumonid parasitoid wasp diversity in tropical forests". Proceedings of the Royal Society B: Biological Sciences. 279 (1748): 4694. doi:10.1098/rspb.2012.1664. PMC .
- Quicke, D. L. J. (2012). "We Know Too Little about Parasitoid Wasp Distributions to Draw Any Conclusions about Latitudinal Trends in Species Richness, Body Size and Biology". PLoS ONE. 7 (2): e32101. doi:10.1371/journal.pone.0032101. PMC . PMID 22355411.
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- Genera Ichneumonorum Nearctica. Morphology of Ichneumonidae