A Bunch of Apocrites

An unidentified male of Megalyridae, a family of 'evaniomorphs' parasitic on wood-boring beetles, from here.

During the late nineteenth century, many women attempted to achieve a 'wasp waist', using corsets to tighten their waist to as narrow a diameter as possible. The style was so-called, of course, because of its resemblance to the body of a wasp, with a sharp constriction dividing the body. However, this feature is not universal among wasps: rather, it characterises a distinct clade within the wasps, the Apocrita.

Basal members of the Hymenoptera possess a broad junction between thorax and abdomen like that seen in other insects. In apocritan wasps, the first segment of the abdomen became incorporated into the body of the thorax (where it is referred to as the propodeum) and the characteristic wasp waist developed at the front of the second abdominal segment. Because the major divisions of the body in Apocrita therefore do not correspond directly to the thorax and abdomen of other insects, workers on Apocrita instead refer to the mesosoma and metasoma (or 'altitrunk' and 'gaster'). So narrow is the connection between mesosoma and metasoma, in fact, that members of the Apocrita are incapable of taking solid food: only liquids can pass through the waist. This limitation is believed to have later been significant in the development of the social wasps and ants: because mature ants cannot themselves eat solids, they must feed any solid food they collect to their larvae. The larvae then regurgitate the semi-digested food in a liquid form that the adults can handle. This dependance on their larvae induced the formation of stable colonies. Mature wasps that do not form colonies feed on naturally-occurring liquids such as nectar.

An unidentified wasp of the Stephanidae ovipositing, from Singapore Nature.

Ancestrally, the Apocrita are a lineage of larval parasites, and the majority of species remain so today. The wide distribution of parasites of wood-boring beetles among basal apocritans, and in their sister group the Orussidae among the non-waisted wasps, suggests that this was probably the original lifestyle for the apocritans (Grimaldi & Engel 2005). Living Apocrita can be divided between five main groups: the Stephanidae, the Aculeata (stinging wasps, including all the social forms such as ants and bees), the Ichneumonoidea (ichneumons and braconids), the Proctotrupomorpha, and the Evaniomorpha (though the monophyly of the latter group is debatable). The Stephanidae are a family of long slender beetle parasites that are most diverse in tropical parts of the world.

An evaniid of the genus Hyptia, from Kurt Schaefer.

The evaniomorphs have been suggested to form a group on the basis of the form of the inner articulation of the coxa (the basal segment) of the middle pair of legs, but the polarity of this feature is debatable (Ronquist 1999). The type superfamily, the Evanioidea, includes a group of families characterised by having the articulation of the metasoma to the mesosoma positioned high up on the propodeum rather than low down as in most other wasps. The hatchet wasps of the Evaniidae have a particularly distinctive body form: the mesosoma is boxy, often almost square in side view; the first segment of the metasoma is developed into a long and narrow petiole; and the remainder of the metasoma is relatively small and hangs off the petiole like the head of the eponymous hatchet. Evaniids are parasites of cockroaches, laying their eggs on the cockroaches' egg cases.

Female trigonalyid of the genus Trigonalys, photographed by Simon van Noort. Note the hooked end to the metasoma; when ovipositing, the female will stand on one side of a leaf and hook her metasoma around to lay her eggs on the other side of the leaf.

Females of another evaniomorph family, the Trigonalyidae, lay large numbers of eggs inserted into incisions on a plant leaf. When a piece of leaf containing a trigonalyid egg is eaten by a caterpillar, the egg hatches out and the trigonalyid larva emerges, then burrows into the body of the caterpillar. However, the larva's target is not the caterpillar itself. Instead, the trigonalyid is looking for the parasitic larva of another wasp that may be inside the caterpillar: it is what is called a hyperparasite (that is, a parasite of a parasite). Trigonalyids are also known as parasites of the larvae of social wasps: when the social wasp feeds its larvae on a caterpillar containing a trigonalyid, the trigonalyid may infect the larva to which it is fed (Grimaldi & Engel 2005).

REFERENCES

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press.

Ronquist, F. 1999. Evolution of the Hymenoptera (Insecta): the state of the art. Zoologica Scripta 28: 3-11.