Oulema melanopus

The vernacular name Cereal Leaf Beetle is sometimes applied to this species, but also to the very similar Oulema duftschmidi. At least half of the individuals I've found at home have been found on house windows during the day.

Separating melanopus from duftschmidi relies on the relative measurements of the length and width of the elytra and the total length of the beetle, but these can be difficult to gauge accurately. Segment 5 of the antennae is also supposed to be longer on melanopus but perhaps the most obvious difference is in the male genitalia; differences in female genitalia are also present. This individual showed the short blunt median lobe that is characteristic of male melanopus genitalia.

male Oulema melanopus showing the median lobe of its aedeagus (2 orientations), North Elmham (Norfolk, UK), 27th March 2020

I was pretty confident about this female's ID based on the ratios but checked the spermatheca to make sure. The key thing here is the short spermathecal duct which in the first photo of the spermatheca below is the membranous bit protruding left from the uppermost blob. It was clearly short (would be long on duftschmidi) but could I be sure it was the duct I was looking at, not the point of attachment where the duct had broken off? Another difference is that on duftschmidi the duct emerges from the tip (it looks like it does in the first photo) whereas it emerges subapically on melanopus. In this case different orientations of the spermatheca (the last two photos) showed that this duct did in fact emerge subapically.

female Oulema melanopus showing its spermatheca, North Elmham (Norfolk, UK), 27th March 2020

This one was less convincing on measurements, at least from live photos, with at least one ratio suggestive of duftschmidi, but I suspected the beetle wasn't completely flat to the camera so making the elytra slightly shortened. The spermatheca seems to confirm that it is indeed another melanopus.

female Oulema melanopus showing its spermatheca (sideways oriented to show the position of the spermathecal duct), North Elmham (Norfolk, UK), 10th April 2020

This one was another male, so straightforward once I'd removed its median lobe.

male Oulema melanopus showing the median lobe of its aedeagus (2 orientations), North Elmham (Norfolk, UK), 19th July 2020

These two were a mating pair. I find getting an accurate measurement of the elytra length and width and especially overall beetle length quite difficult, and perhaps as a result not all of the ratios matched melanopus. I think measuring the 5th antenna segment is perhaps safer - the proportions certainly clearly pointed to melanopus on both of these anyway and the genitalia confirmed. The male is shown first with its genitalia and then the female. The first genitalia photo is the intact aedaegus which, so far as I know, is not useful for ID - you have to open this up to extract the median lobe.

male Oulema melanopus showing 5th segment of antenna, aedeagus and median lobe and female Oulema melanopus showing 5th segment of antenna and spermatheca, Wendling Beck Environment Project (Norfolk, UK), 26th May 2023

female Oulema melanopus showing the spermatheca, North Elmham (Norfolk, UK), 9th August 2021

female Oulema melanopus, North Elmham (Norfolk, UK), 13th August 2021

This one was before I knew about all of the genitalic differences so I relied on the external proportions.

Oulema melanopus, North Elmham (Norfolk, UK), 13th April 2016

This was one of the first two Oulema I attempted to identify, and did so from the photos. For reasons that are not now altogether clear, but were presumably based on measurements of the elytral proportions, I considered it to be Oulema duftschmidi. Looking at it again now, with the benefit of having now examined another half dozen Oulema (all of which were melanopus) I am rather doubtful about this ID. The 5th antennal segment seems rather long which according to some references points to melanopus, although some photos of duftscmidi online seem to very similar and the UK Beetles website concludes about this character, "but we have not found this to be generally helpful." Given the challenges involved in interpreting measurements (ratios at least) from photos of live insects, I am now of the opinion that my original ID as duftschmidi is not reliable. Indeed I suspect, at least based on probability, that it was wrong abnd that it is another melanopus. I welcome feedback from anyone who thinks it can be safely identified (either way) from the photos, but otherwise I shall now treat this as unidentified.

Oulema sp., North Elmham (Norfolk, UK), 5th May 2017