Transmission and scanning electron microscopic observations on antennal apical pegs in the wasp species Pimplinae (Insecta: Hymenoptera)

Pimplinae are parasitoids belonging to the family Ichneumonidae that attack and develops inside hidden host; female wasps evolved a peculiar host recognition strategy, that involves the use of self-produced vibrations which are transmitted through the antennae on the substrate and perceived back as an echo using the leg subgenual organ. In this study we investigated, using both scanning and transmission electron microscopy, the antennal tips of a few Pimplinae. In all the investigated species, the antennal tips present peculiar apical pegs with different shape and number, often defining a flattened sole devoid of other antennal structures, such as sensilla. These pegs are present in both sexes with different number and development, are inserted on the antennal wall through an inflexible socket and present a cuticular shaft with cuticle of different thickness. We never found the presence of sensory neurons or glandular epithelium associated with these pegs. Because of their peculiar morphological features, we hypothesize for the antennal apical pegs a role in the context of host searching behavior (in the case of the female through the vibrational sounding strategy), as well as during mating behavior.