There are four types of Paederus beetle in the UK. This was Paederus littoralis because it was larger than 7.5mm (around 10-11mm) and had black mandibles. Littoral means shore: and these beetles can be found in marginal areas of wetland. They live in decaying plant matter and leaf litter but climb up plant stems and foliage to feed on smaller insects like aphids and mites. These, they grip with their mandibles and pre-digest before consuming them (like houseflies). They also exude a defensive toxin, so any direct contact can result in skin irritation or dermatitis.