I was quite surprised to find this large (4 cm) beetle in my moth trap!
This beetle lives in aquatic environments like lakes and ponds and is among the largest aquatic insects.
Adults can reach up to 5 cm in length and 2 cm in width.
It occurs in most of Europe and parts of Asia but it is not very common.
And yes, they are often found in moth traps since they are attracted to light.

Caught in my self made moth trap.
In my yard, Hilversum, The Netherlands, 8 April 2019