Light trapping is an incredibly efficient way of finding water beetles. This is due to them mistaking light sources for reflections in water bodies and actively heading towards them during flight. There are lots of groups that can be classified as water beetles and there are far to many to list them all here. Below is a list of the most commonly found beetles attracted to light.
Alternatively there are Mikes insect key with pictures, however you will need to know the family of beetles that it belongs to first
Long filliform (thread like) antenna
racing green coloured body with lighter yellow markings
Elytra may have deep ridges in females, or lightly punctured in males
February - December
Large ovate beetles with lighter bands on the pronotal margins.
There are 6 similar species in the UK.
I recommend using
Dytiscus species, require a photo of the underside with a clear focus on the hind coxal processes as the shape and colour is important for species level identification. In addition, a good dorsal photo that has the front and hind margins of the pronotum, as the relative thickness of the bands can also be a good ID characteristic. This is good practice for all water beetles if you are not sure!
An ideal underside photo showing the yellow underside colour and hind coxal process shape (Right)
indicative of Dytiscus circumflexus. Credit Gary Lowe
Dytiscus circumcinctus Credit Mike Southall
15.0 - 17.0 mm
head black with lighter labrum and markings above eyes
dark transverse reticulation on a light brown/olive coloured elytra
legs orangy brown
All year round
Transverse reticulation on the elytra is a key ID characteristic for this species and can be viewed easilly using a 10x hand lens
Transverse reticulation. Bill Unwin 2017
Narrow oval body
Pronotum and Elytra with two very well marked pale stripes (pictured)
Bronzy head with a paler front (Labrum)
Antennaepalpes pale brown/orange
Front legs usually paler than the mid and hind legs
Jan - Nov
Superficially this looks similar the the much rarer, larger and heavier set Cybister lateralimarginalis, Which has only been found 4 times in Britain, although found more frequently in continental Europe
The great silver diving beetle, one of our largest beetle species in the UK.
Instantly recognisable from the other oval shaped diving beetles due to its short, distinctly clubbed antennae, elongate shape and size.
Body Black upper side with green/purple lustre
Short clubbed antennae
long red maxillary palpi (twice as long as antennae)
red antennae and maxillary palps
Local in SW & E England & S Wales