Whilst sampling invertebrates at Cribbinsgate Lake, Flatford I separately caught two specimens of the Screech Beetle (Hygrobia hermanni) in an area of submerged leaves at the lake margin. I put them into the same collecting pot and on returning to add some more invertebrates from the next net sweep I noticed that the two beetles had joined together.

The male had settled on top of the female gripping the lateral margins of her elytra with his tarsi and claws. Even though this meant that the male was therefore unable to assist with maintaining position and the female had to do all the swimming, he seemed to be able to direct her to some degree. They would pop up periodically to restore the air bubble at the end of the abdomen which acts as an oxygen supply.

At least the air bubble on the male was restored; the female appeared to have no attached air supply, which I thought interesting. Her hind abdominal segments were more parted than the males, perhaps stopping her from carrying a bubble. There could still have been an air supply trapped beneath her elytra but positioned beneath the male this would have been impossible to replenish at the surface.

I could see as the aedaegus (penis) was extended from the male down into the female at the end of her abdomen. The aedaegus was sharp and, in fact, quite surprisingly long as the distance between the beetle’s abdomens is fairly large in this position. Coupling in terrestrial insects is often achieved by bring the two ends of the abdomens together, ‘tail to tail’. Presumably, especially in flowing water, the mating position described above is more stable although it may require a longer and more flexible aedaegus and parameres (claspers).

After a short while a film started to be extruded from the female. It was translucent as it first left her abdomen but became more opaque and whiter as it spent longer in the water. The first part of this egg mass to be produced was the whitest and the last part remained more translucent.

Then, eventually, the male retracted. They held together for a little while longer and then they separated. The female, who seemed to have done more swimming, with less oxygen, sank to the bottom and rested. The male swam off. The eggs had dropped off. I think either the male or more likely the female cut off or separated the egg mass or perhaps the female abdominal segments caused it to be ejected as they closed together.

      Christine McClure