Pond bugs hooked in sex struggle

Pond skater males have evolved elaborate antennae due to sexual struggles with females, say researchers. Females of the water-skimming bugs are known to vigorously resist the advances of males deemed to be poor mates. With the use of high-speed video, scientists in Canada were able to analyse how males responded. They found that males used hook-like antennae to pin down and restrain females and mate successfully. Professor Locke Rowe from the University of Toronto suggested that the males have evolved their antennae to perfectly match the contours of a female's head. The study, undertaken with Dr Abderrahman Khila and Professor Ehab Abouheif from McGill University in Montreal, was published in the journal Science. Rheumatobates rileyi are true bugs found in the ponds and streams of Canada where they are called water striders. As a group they are known for their "war of the sexes" behaviour when it comes to mating. "Females store sperm, so after a single mating, further mating is superfluous and costly. Costly because carrying the male decreases their feeding success and increases their vulnerability," explained Prof Rowe. "In light of this, when males and females meet there is often a struggle over mating." To understand how males' physical attributes helped them to mate successfully despite female resistance, Prof Rowe and colleagues used high-speed video to record the insects' sexual encounters. They also flash-froze some of their subjects and analysed them using an electron microscope. At this level of detail, researchers could see how different parts of the males' antennae perfectly restricted female heads, allowing them to leverage their body on top of the female and mate. "We weren't looking at simple modifications, but a sensory device (the antennae) that had evolved in males into a spectacularly modified grasping device - large, muscular, and fitted with hooks and spikes exquisitely adapted for grasping - a long way from an insect 'feeler'," said Prof Rowe. As part of their experiment, the researchers also identified that the antennae development was controlled by a single gene. By manipulating this gene in larvae, scientists created specimens with varying stages of development and studied their reproductive success. "We were surprised that each small step in modifying these antennae resulted in an increase in the mating success of these males," said Prof Rowe. "The study gives us new insight into the evolution of novelty, and novelty is one of the most spectacular outcomes of the evolutionary process."