Dark-knobbed Splay Erioptera lutea

This came to light at the edge of wet woodland with lakes and ponds. It's wing was 5.5mm long. I keyed it using Stubbs but wasn't quite sure if the abdomen was dark enough to key it as dark. Taking it to be pale it went to squalida and so far as I could see on the dried specimen, the genitalia matched that species pretty well. However I felt it was more likely I should call the abdomen dark, in which case it went to the much commoner lutea. I wasn't convinced the genitalia matched that species though, but the halteres were dark-knobbed which is characteristic of lutea. After macerating the abdomen in KOH it became very clear that the genitalia did in fact match lutea and not squalida.

male Dark-knobbed Splay Erioptera lutea showing wing and genitalia (before and after clearing), Wendling Beck Environment Project (Norfolk, UK), 17th September 2024

This female was caught at the same light as the male above, but being considerably larger (wing length 6.3mm) I wasn't sure if it would be the same species. This time the abdomen really did look yellow, so keyed to nielsini. However that didn't seem very likely, and if I ignored the yellow abdomen it went to lutea which I had just identified from the same trap. Moreover the knobs of the halteres were dark which seemed to support an ID of lutea. The books didn't show the shape of the cerci or sternal valves for lutea though, and although dark knobs on the halteres are characteristic of lutea, the books are not explicit that this is not also present on other species such as nielsini. Any niggling doubt was removed by searching for photos of both species online - photos of nielsini do not show dark knobs on the haltere and a photo of the female terminalia of lutea matched mine exactly.

female Dark-knobbed Splay Erioptera lutea showing wing, haltere, abdomen and terminalia, Wendling Beck Environment Project (Norfolk, UK), 17th September 2024