Calathus cinctus

I had not appreciated how similar this species can be to Calathus melanocephalus for some time and had therefore overlooked some examples as that species.

In late 2025 I examined three similar Calathus beetles that I found by suction or pitfall trap over a couple of days at two nearby sites. All three had reddish pronotum contrasting with blackish head and elytra and on that basis I intially identified all three as melanocephalus. For some reason I stalled over one of them, a niggling doubt prompting me to investigate further. I ran a photo past ObsID and it was 100% confident it was cinctus. The only differences between these two species given in Hackston's key are that the pronotum is reddish brown rather than reddish, which doesn't seem to me to be an easy call in most cases, and the head and elytra being a more sombre brown on cinctus vs. black on melanocephalus. On that basis I was not at all convinced that Obs ID was correct (it often isn't!), but I turned to Duff to see if that helped clear things up. Duff doesn't differentiate on pronotum colour (calling both rufous) but agrees that the head and elytra should be black on melanocephalus and dark brown on cinctus. Still thinking all three were correctly identified as melanocephalus I checked the elytral humeri as Duff also says these are denticulate on melanocephalus and toothless on cinctus. One of the three had clearly denticulate humeri but neither of the others did. Moreover I could now see that the two toothless individuals also had proportionately longer elytra and that, according to Duff, was another difference between cinctus and melanocephalus.

Now suspecting that ObsID might actually have been correct, I dissected all three and confirmed that the one with toothed humeri and shorter elytra was indeed melanocephalus, the one that had originally prompted me to check it was female so not genitalically confirmable, and the other one, below, was indeed a male cinctus. At this point I searched the internet for photos of Calathus cinctus and discovered that individuals with almost black heads and elytra seem to be quite frequent. Here is the male cinctus which had been found in a pitfall trap.

Calathus cinctus, Wendling Beck Environment Project (Norfolk, UK), 26th September 2025

Here are the median lobes and right parameres for the males of each species - in each case the above Calathus cinctus is on the left and the Calathus melanocephalus is on the right.

median lobes and right parameres of Calathus cinctus (left) and Calathus melanocephalus (right), Wendling Beck Environment Project (Norfolk, UK), 25th-26th September 2025

This was the female, found by suction.

female Calathus cinctus, Wendling Beck Environment Project (Norfolk, UK), 26th September 2025

Although this one predated the two above, I examined it after those and immediately suspected it was another cinctus. In this case the head and elytra were clearly brownish, however they do not appear particularly so in the live photo (admittedly poor quality) and it is possible that they faded after death (the contents of this pitfall trap had been a bit damp so I'd left them to dry out on my sunny windowsill for a while to prevent them from going mouldy). The elytral humeri lacked teeth and the genitalia clinched it.

male Calathus cinctus showing head, elytral humeri and median lobe with right paramere, Wendling Beck Environment Project (Norfolk, UK), 30th-31st July 2025

In view of the above experience I needed to review all of my previous records of Calathus melanocephalus, and it quickly became clear that there were more Calathus cinctus lurking among them - in fact I think I have had more cinctus than melanocephalus. One or two even showed browner-toned elytra and heads, although this was not as obvious as I had previously imagined it should be.

This one had come to light.

Calathus cinctus, Wendling Beck Environment Project (Norfolk, UK), 14th July 2024

These were in a pitfall trap.

5x Calathus cinctus, Wendling Beck Environment Project (Norfolk, UK), 18th to 27th November 2022

Calathus cinctus, Wendling Beck Environment Project (Norfolk, UK), 16th February 2023

Calathus cinctus, North Elmham (Norfolk, UK), 6th September 2023

Calathus cinctus, North Elmham (Norfolk, UK), 31st August 2021