Enochrus melanocephalus

One of my first attempts at identifying an Enochrus led to an ID of melanocephalus, but upon reviewing my Hydrophilidae records recently it was immediately clear that this was a misidentification. So it was good to find a real one 2-3 months later. The very black legs and underparts set this apart from other Enochrus I've seen, although it was the proportional length of the maxillary palp segments that clinched the ID.

Enochrus melanocephalus showing scutellum, maxillary palp and underparts, Wendling Beck Project (Norfolk, UK), 9th August 2023

This came to light a couple of fields away from the one above. Although I suspected it was Enochrus I keyed it to make sure, but as the apical maxillary palpomere was longer than the subapical one, it keyed to Hydrobius. That was clearly incorrect, and as the apical palpomere was only very slightly longer than the subapical one I tried the alternative option and that led to Enochrus as expected. It was the same character that then firmly placed it at melanocephalus, the other Enochrus all having shorter apical palpomeres.

Enochrus melanocephalus showing hind tarsus, maxillary palp & antenna and sutural striae, Wendling Beck Project (Norfolk, UK), 16th June 2025

This came to light in the same field as the first. Again I felt the apical palpomere was marginally longer than the penultimate one but it failed to key if I took it to be longer, but otherwise keyed to Enochrus mealnocephalus as before.

Enochrus melanocephalus showing underside and palps from below, Wendling Beck Project (Norfolk, UK), 25th June 2025

This one keyed to mealnocephalus straightforwardly.

Enochrus melanocephalus showing palps, palps and atennae and body from below, Wendling Beck Project (Norfolk, UK), 28th May 2026

Using Duff, this one did not key to mealnocephalus for maxillary palpomere 4 was evidently shorter than palpomere 3. As such it keyed past melanocephalus and on to Enochrus nigritus, although I couldn't see the notch in the apical sternite until I cleared the abdomen in potassium hydroxide solution. However, having now recorded a few melanocephalus, I was struck by how much this one's habitus resembled melanocephalus. This prompted me to look up photos of nigritus to see if this species also had entirely black legs. The one I'd had before did, but most other photos of nigritus that I could find had distinctly paler tarsi at least.

So, the question now was, could this be melaocephalus despite the palpomere 4 being so short? Duff's description of nigritus suggested that the upperparts should be darker than this, so I was increasingly coming round to the view that this was not nigritus. I found photos of nigritus online which showed paler tarsi and this pretty much put the nail in the coffin for any thoughts of it being nigritus.

Although strongly suspecting melaocephalus, merely eliminating nigritus was not sufficient to call it melaocephalus. The aedeagus ruled out some species but is quite similar to nigritus and I couldn't find any photo or illustration of melanocephalus aedeagus, so this was of limited help, and so I turned to Hackston. Hackston provides an additonal difference between melanocephalus and other Enochrus: "Elytra with eight rows of coarser punctures towards the rear but these do not extend into the front half, fading well before the middle." At first, looking at the elytra in situ, I was unable to see this. Removing an elytron and looking at it back-lit I could make out some pattern that could have been what Hackston was referring to, but it didn't seem clear that they were punctures. I removed the other elytron and cleared it in potassium hydroxide, and this made it much clearer. The pattern was pretty much exactly as described for melanocephalus and wrong for the other species.

male Enochrus melanocephalus showing apex of elytra, body from below, maxillary palp, apicaal sternite (before and after clearing), aedeagus and elytra (one dry and one cleared), Wendling Beck Project (Norfolk, UK), 24th May 2026

The above experience prompted me to look again at my 2023 record of Enochrus nigritus. That had seemed to key to nigritus using Duff, and like this one I hadn't been convinced about whether I could see a notch in the hind margin of the last sternite until I'd put it in potassium hydroxide solution. I had then keyed it using Hackston and did come across some issues with calling it nigritus, but felt that on balance using a combaination of characters, I was happy calling it nigritus - especially as the aedeagus matched that species.

However, this was a similarly pale beetle with black legs, and in view of my latest experience, I am convinced that it was not nigritus, but was presumably another melanocephalus.

male Enochrus melanocephalus showing head from front, tip of last sternite (before and after soaking in potassium hydroxide). maxillary palp (and antenna) and genitalia, Wendling Beck Project (Norfolk, UK), 11th June 2023