The Occasional Hymenoptera: Diplazontinae antennae curling

Diplazontinae is a subfamily of parasitoid wasps of the family Ichneumonidae, the ichneumon wasps (which are also almost all parasitoid). Parasitoid insects are parasites during their laval stage and become free-living when adult. The ovipositors of the females of this group are adapted for piercing so that eggs can be laid into the body of the host (often under bark, which the ovipositor drills through). A gallery of photographs of the Giant Ichneumon Wasp (Megarhyssa macrurus) and a video can be seen here. The diplazontines deposit their eggs into the eggs or larvae of hoverflies (members of the family Syrphidae), and when the diplazontine larva emerges, it eats its way through the larva of the hoverfly.

Ichneumon wasps in general use their antennae in several ways. The females use them to tap on bark to locate the larva of suitable hosts for their eggs. Males use them to locate females for mating. Most diplazontine wasps use their antennae also for a courtship behavior called “antennae coiling,” and it is displayed either as “single coiling” or “double coiling.” The function of this behavior is to bring male antennal glands into intimate contact with the female’s receptors.

In a study in the open access BMC Evolutionary Biology (abstract and pfd file of the full article — both are still provisional, however), Seraina Klopfstein from the Natural History Museum of Bern, Switzerland, and others, describe their work on museum specimens of 56 different species of diplazontines and “reconstruct the evolutionary history of antennal coiling and associated morphological modifications to study the mode of evolution of this complex character system.” They find that there exists “a large variation in shape, location and ultra-structure of male-specific modifications on the antennae” and show that “the possession of antennal modifications is highly correlated with antennal coiling behaviour.”

They tested the coiling by removing the specimens from their alcohol solution and transferring the amputated antennae into water. The change in solution cause some antennae to curl. There was general sexual dimorphism — the antennae of females only bent slightly at the middle, whereas the male antennae showed a wide range of reactions from “even curve of the antenna (no coiling …), to a single, tight coil in the middle of the antenna (single-coiling …) and two consecutive turns (double-coiling …).” (Photographs of specimens, both male and female, showing each of the three reactions is found as Figure 2 of the article.) Based on the correlations between antennae modifications and curling behavior the authors reconstruct an ancestral reconstruction. (A chart of the reconstructed states of antennae coiling is found in Figure 3 of the article.) The authors show that both antennal modifications and antennal courtship are highly congruent with the molecular phylogeny, implying a comparatively low speed of evolution. Antennal coiling as a courtship character is lost on two independent occasions, once in a single genus, once in one of the three genus-groups. No instance is found were antennal coiling once lost is reacquired.


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