Great Diving Beetle. Dytiscus marginalis)
The Great Diving Beetle is quite common in the scrapes at Dun Coillich where the larvae have been found by young visitors doing pond dips.
The adult female is pictured in the left hand photo. The female has ridges running down the elytra and the male has smooth elytra. The larva is pictured in the photo on the right. Its sickle shaped mandibles are clearly visible. Both the adult and the larva are voracious predators of small invertebrates, tadpoles and even small fish. They may sometimes turn cannibal. They impale their prey with their sharp mouthparts and then inject enzymes which liquefy the contents so that they can suck the resultant 'soup' back as semi-predigested food.
When the larva is ready to pupate it crawls into damp soil. The emerging adult can fly and will lift its elytra (wing cases) to release the wings. They fly at night and are attracted to the reflection of moonlight from water. Sometimes they make a mistake and can land on a wet road. They can certainly get to inappropriate water such as bucket left out with water in it.
A sweep of the nest in the Dun Coillich scrapes is likely to catch the larvae which may vary greatly in size depending on the stage that they have reached in the life cycle. The adult beetle can be three centimetres long. The adult is an adept swimmer and has fringes on its legs to give it greater thrust against the water. It comes to the surface in order to trap air beneath its elytra and this acts as an store when it is submerged. The larva takes in air to its tracheal system of air tubes through the tip of its abdomen where there are fringes which allow it to hang from the surface tension.