The adaptive significance of host location by vibrational sounding in parasitoid wasps

Vibrational sounding, which is a form of echolocation, is a means of host l ocation by some parasitoid wasps. The wasp taps the substrate (wood, stem o r soil) and detects the position of a potential host through the returning 'echoes'. The deployment of vibrational sounding is inferred through the fo rm of the subgenual organ in the female tibia in combination with the prese nce of modifications to the female antenna used for tapping the substrate. Vibrational sounding and its associated modifications were found in two fam ilies. The use of vibrational sounding by parasitoid wasps was positively c orrelated with the depth of the host in the substrate relative to the size of the parasitoid. There were also significant correlations between the use of vibrational sounding; and parasitism of immobile and concealed hosts an d between vibrational sounding and idiobiosis. The data suggested that vibr ational sounding evolved under a variety of ecological conditions, being em ployed in the location of wood-boring, stem-boring, soil-dwelling and cocoo ned hosts and stem-nesting aculeates, often in situations in which the host does not produce vibrations itself.