Byturus ochraceus

There seem to be quite a range of characters used by different references for determining the two Byturus species, but most are rather subjective or difficult to use. The size of the eyes seems to be one of the most consistently used characters, but this isn't easy to judge (at least I don't find it so). I suspect it's one of those pairs where it becomes a lot easier with experience, but although I've seen a few Byturus now, I still have to look at them really carefully (although having said that, after spending a lot of time on the ochraceus below I just looked at my other photos of other Byturus and they seemed a lot easier now). If in doubt, and if it's a male, genitalia will clinch any problematic individuals.

This individual was netted in flight. I didn't find the ID at all easy, but with significant care taken all the characters for ochraceus checked out. I will discuss the various characters I looked at in order to identify it:

Size: at around 4.1mm this fell within the given range for ochraceus and outside - but only just outside - the smaller tomentosus. A bit too close to call, but pointing towards ochraceus.

Eye size: This seems to be the most consistently used character for all the references I checked, but I don't always find it easy to judge. Duff provides sketches showing both and other references include photos, and I felt that my specimen matched the larger-eyed ochraceus best.

Eye size in comarison to frons width: some references (Mike Hackston's key and UK Beetles) attempt to make the eye size feature less subjective by comparing the longitudinal eye diameter with half the frons width (clearly greater than half the frons width in ochraceus and usually clearly smaller on tomentosus (though Hackston allows for tomentosus to be "at most half the distance across the frons between the eyes"). This helps, though working out precisely which diameter of the eye is longitudinal when the head-body line is curved can be tricky (probably won't make much difference so long as it's roughly longitudinal), and working out precisely where to measure the width of the frons isn't necessarily obvious (I think the shortest distance between the eyes is probably the one to use), makes it a little hard to use. Anyway, this one had eye diameter a little more than half the width of the frons, so that supported ochraceus too, though not by such a large margin that I was comfrotable that the word "clearly" applied.

Pronotum shape: Different references describe this in different ways, but I found the most helpful one being Duff in relation to the explanate sides continuing to the fore-angles on ochraceus (as they did on my specimen) but fading out (Duff uses "evanescent") on tomentosus.

Elytral punctures vs. pronotum: Mike Hackston's key (based on the key by Lompe, in German) says for tomentosus, "Elytra finely and very densely punctured, only slightly more strongly punctured than the pronotum" and strongly and densely punctured, much more strongly than the pronotum in ochraceus. I found this hard to see.

Elytral puncture arrangement: Hackston (and Lompe) also say the elytral punctures are "almost arranged in rows, forming a suggestion of longitudinal lines" in tomentosus and "without a trace of arrangement into lines" on ochraceus. But Hackston also provides a really detailed morphological illustrated description of ochraceus and in this he says the elytral punctures are "arranged in ten bands with spaces of variable width between them" (and illustrates this with lines drawn on to show the arrangement of these bands). At first this sounded like a contradiction, but comparing with my specimen I realised that it is not. The bands of larger punctures on ochraceus are evident (Hackston suggests clearing the elytra in potassium hydroxide to view - I found it adequate to examine the elytra from underneath through a bottom-lit microscope), but these bands are about 3 punctures wide and within the band these punctures are arranged randomly with no suggestion of lines.

Elyral shape: Some references describe a difference in the shape of the elytra - more parallel in ochraceus and widened towards the apex in tomentosus. You can see this in some images but it's pretty subtle.

Elytral margin: UK Beetles also notes that the elytral margin is only visible behind the humerus on ochraceus but "generally visible from above" on tomentosus. The elytral margin was clearly visible behind the humerus on mine, but not if examined from precisely above. In order to see both margins (i.e. on each side of the beetle) at the same time I had to make sure the beetle was perfectly perpendicular to my line of sight, and then the margins did in deed disappear behind the humerus.

Darkness: some references highlight differences in darkness of the two species. Without being able to compare specimens, at least without lots of experience, this is pretty limited in usefulness and certainly didn't help me.

Aedeagus: Mike Hackston provides a detailed description of ochraceus genitalia, but doesn't say if tomentosus is any different. Lompe clears this up nicely with figures and photos for both species, which are clearly different. Any doubt about mine being ochraceus was swiftly eliminated after checking this.

male Byturus ochraceus showing its head and pronotum, front edge of the elytra, whole elytra from below and aedeagus (with 8th segment and close up of tip), Wendling Beck Environment Project (Norfolk, UK), 14th May 2022

Having spent a fair bit of time on the one above, the following ones which I had previously identified look relatively easy. Some of them weren't at the time though.

male Byturus ochraceus, North Elmham Cathedral Meadows (Norfolk, UK), 25th May 2020

male Byturus ochraceus, North Elmham Cathedral Meadows (Norfolk, UK), 8th May 2020

male Byturus ochraceus, North Elmham Cathedral Meadows (Norfolk, UK), 9th May 2018

male Byturus ochraceus, Honeypot Wood (Norfolk, UK), 22nd April 2017

male Byturus ochraceus, Great Ryburgh (Norfolk, UK), 14th May 2017

male Byturus ochraceus, North Elmham Cathedral Meadows (Norfolk, UK), 25th May 2018