Courtship can be a complex ritual, and when parasitic wasps get together,they stroke each other's antennae; which made Ferdinando Bin from the University of Perugia, Italy, wonder whether the males' antennae were more than simply sensory organs. Were they communicating vital sex signals? Having discovered in 1986 that some parasitic wasp males carry glands on their antennae, Bin wondered whether they might be crucial for successful courting. But it took another 20 years before Bin and his team identified the best species to test out his theory: Trichopria drosophilae. Teaming up with Bin, Marzia Rosi and Nunzio Isidoro, Roberto Romani played cupid for pairs of T. drosophilae to see how courtship progressed when he had tampered with their antennae(p. 2486).

But first the team had to figure out a way to cover up the minute glands on the insect's antennae. Romani admits that when Isidoro came up with the idea of covering the gland in glue he was sceptical. `I thought the wasps would push it off with their legs,' he admits. But after carefully anaesthetising the insects and gently applying a dab of glue to the antennomere housing the gland, the males soon recovered and were raring to go.

Isolating courting couples with exposed antennae glands, Romani filmed the insects as they slowly stroked each other's antennae, mating successfully in 40 s. But it was a different matter when he paired a male with glued antennae with an expectant female. After 2 min of enthusiastic antennae rubbing, the female had had enough and threw the frustrated male off. Undeterred the male attempted courting again, but to no avail. The females failed to become receptive to the male's advances. `She wasn't picking up a signal,' says Romani `so she decided that he was either no good, or the wrong species.'

Next the team tested the secretions' effects by removing one of the male's and female's antennae to see if they needed to be rubbed together to get the message across. Leaving both partners with an antenna on the same side, the insects were able to rub the antennae together and mate. But when the male lost his right antenna and the female lost her left, they no longer came into contact and the courtship ritual ended in failure. Without access to secretions from the male's antenna glands, the females lost interest and were unable to court successfully.

Finally, the team tested whether the glue itself had interfered with the amorous male's advances. But applying glue further along the male's antennae didn't deter the females.

Having found that the parasitic wasp antennae are more than sensory organs,and key to the male's courtship ritual, Bin and his colleagues are keen to know whether other insects rely on their antennae to the same extent for successful courtship.

Romani, R., Rosi, M. C., Isidoro, N. and Bin, F.(
). The role of the antennae during courtship behaviour in the parasitic wasp Trichopria drosophilae.
J. Exp. Biol.