The ichneumons are one of the best-known groups of parasitoid wasps. The most familiar ichneumons are relatively large for parasitoid wasps, and sometimes even for wasps in general. This can make them somewhat intimidating in appearance, especially considering the likelihood of the long ovipositor of a female being mistaken for a sting by those not in the know. However, not all ichneumons are giants. The photo above shows a tiny ichneumon of the genus Gelis, females of which are wingless and bear a distinct superficial resemblance to ants. This resemblance is likely to afford them some protection from potential predators, and at least one Gelis species, G. agilis, has been shown to release a chemical when threatened very similar to the alarm pheromones of the black garden ant Lasius niger (Malcicka et al. 2015). On the other hand, one might be tempted to wonder if this mimicry may sometimes serve a more nefarious purpose: another species, G. apterus, has been recorded as a parasitoid of the ant-eating spider Zodarion styliferum (Korenko et al. 2013). However, G. apterus has not been recorded to use its ant appearance to lure its host; instead, the female ichneumon uses its ovipositor to pierce the igloo-like silken retreat that the spider occupies during the day. Other species of Gelis are known to be parasitoids of moth cocoons rather than spiders (Gauld 1984), so Gelis' status as an ant-mimic and its choice of host may be simple coincidence.
Gelis belongs to a world-wide tribe of ichneumons known as the Phygadeuontini (sometimes referred to in older sources as the Gelini), a diverse group including well over 100 genera. Most, but not all, phygadeuontins are also among the smaller ichneumons. The range of hosts attacked by the group is equally diverse, including (among others) moths and lacewing pupae, and spider egg sacs, while some are hyperparasitoids on the pupae of other parasitoid wasps (Gauld 1984). Species of the genus Phygadeuon include parasitoids of wood-burrowing beetles that use the enlarged ends of their antennae to tap at wood in search of hollow burrows within. Some phygadeuontins are external parasitoids, while others are endoparasitoids. The larvae of Gelis apterus can even be regarded as true predators, as they attack not the eggs of their host but its newly-hatched spiderlings (Korenko et al. 2013). A common theme between these diverse hosts, though, is the production by most of them of silken cocoons or other protective structures that the female phygadeuontin is able to pierce with her ovipositor.
Gauld, I. D. 1984. An Introduction to the Ichneumonidae of Australia. British Museum (Natural History).
Korenko, S., S. Schmidt, M. Schwarz, G. A. P. Gibson, & S. Pekár. 2013. Hymenopteran parasitoids of the ant-eating spider Zodarion styliferum (Simon) (Araneae, Zodariidae). Zookeys 262: 1–15.
Malcicka, M., T. M. Bezemer, B. Visser, M. Bloemberg, C. J. P. Snart, I. C. W. Hardy & J. A. Harvey. 2015. Multi-trait mimicry of ants by a parasitoid wasp. Scientific Reports 5: 8043. doi:10.1038/srep08043.