In one species of scorpionfly, males have evolved a cunning technique to catch mates and prolong the process of copulation.

Scorpionflies are insects that have a scorpion-like stinger on the tips of their abdomens – although they do not have pincers. In the genus Dicerapanorpa, the males also have a pair of stumpy "anal horns" a little forward of the stinger.

It now seems that these horns help the male grasp a desirable female, preventing her from resisting him or from getting away.

To find out what the horns are for, Wen Zhong and his colleagues at Northwest A&F University in Shangling, China, reared Dicerapanorpa magna in the laboratory. They observed two groups of 80 males courting females. One group had their horns covered in adhesive and clay to stop them working.

In the group with functioning horns, 78 out of 80 males managed to mate, but in the other group only 37 succeeded. That suggests the anal horns are an important tool for initiating mating and controlling the females.

Romantic scorpionflies

Zhong found that scorpionfly courtship is a long-drawn-out affair.

First, the male releases sex pheromones to attract a female, which may be lurking nearby. The male also shows off his wings by vibrating them, and moves his abdomen up and down. This stage can last for hours.

When a female approaches, the male uses his anal horns to seize her abdomen, locking her into his embrace for 70 to 140 minutes.

At this point the female twists her body to shake him off, vibrating her wings rapidly. So to get her back to the task at hand, the male presents her with a "nuptial gift" which he secretes from his salivary gland.

The mating process pauses for a moment while she tastes and eats the gift. The male then restarts copulation while she continues to eat. He seizes her again, both with his anal horns and with an additional clamp-like structure called the notal organ. This way he can prolong the mating process even after she has eaten her gift.

The males' horns and notal organs are so effective that they do not always bother with a nuptial gift. In about half the matings, the male managed to hold onto the female despite her struggles, and so did not present her with any tasty saliva.

But despite the males' coercive ways, the females do have some control over who fathers their offspring. Another study has shown that females can choose to receive more sperm from males that give gifts.

Leif Engqvist at the University of Bern in Switzerland, has worked extensively with other species of scorpionfly but says the horns were a new feature he had not previously seen.

"So many different grasping devices have evolved within scorpionflies in the context of sexual conflict," says Engqvist.