Sex is literally a matter of life and death. If an animal doesn't manage to produce offspring, its genes will die with it. So they have evolved to be very good at courting and mating.
Males are often the ones doing the pursuing, so they have evolved all sorts of ways to attract females. Male birds often have stunningly beautiful feathers – think of a peacock's tail – or the ability to sing elaborate songs.
The same principles apply to insects. That is why male Roesel's bush-crickets have evolved their amazing courtship apparatus.
They have a pair of curved rods called titillators pointing out of their genital openings. They look a bit like coat hooks.
Until recently no one knew what they were for. But it now seems they stimulate a sensitive area of the female's genitals.
The new results have been published in the journal Arthropod Structure and Development.
Gerlind Lehmann of the Humboldt-University Berlin in Germany and her colleagues examined the titillators under microscopes. Then they watched bush-crickets mating, and used CT scans to figure out what the titillators were doing during mating.
They found that the male repeatedly, and rhythmically, inserted the titillators into the female's genital opening.
The titillators were gently drumming on a plate inside the female's genitals. This plate was covered in different kinds of sensory cells, suggesting that the female could detect the titillators' drumming.
This suggests that the titillators are somehow helping the male keep the female interested, much as mating humans perform foreplay on each other's genitals to stimulate sexual desire.
In unpublished experiments, Lehmann tried shortening the titillators.
The females became more likely to resist the male's attempts at mating, and males were less likely to succeed in transferring sperm.
The highly sensitive spot inside the female's genitals may sound rather similar to the G-spot, a region inside the human vagina that is sensitive to sexual stimulation. But the two may be very different.
In particular, it's unclear whether the female insects experience any pleasure from the titillators. Insects such as bush-crickets may not be conscious enough to experience sensations such as pleasure.
Instead the action of the titillators might do something more functional, such as suppressing her instinct to fight the male off.