Sylvicola fenestralis

This is one of 3 similar species that are sometimes known as Window Gnats. Some sources imply that this is the commoner species but others suggest that in fact Sylvicola cinctus is much the commoner species and that many examples of fenestralis are misidentified. Certainly my old (unsubmitted) records of "Sylvia fenestralis" were not examined critically and so I have deleted them from my database. But this one was examined properly and I *think* it does stand up as fenestralis - but as always I welcome correction if I've slipped up.

Sylvicola fenestralis is said to be larger and more strongly marked (on the wings) than cinctus, so with mine having a wing-length of 6.5mm and a strong pattern on the wings this seemed to be a good sign for fenestralis. Mine also shows the 3 dark lines on the thorax which are more typical for fenestralis, but my references suggest that examination of the genitalia is essential for a concrete ID. Mike Hackston's key doesn't include female genitalic differences, so, with this being a female, I couldn't get any further with that. Steven Falk's Flickr site included photos of female cinctus so I figured they must be resolvable too. After a bit of searching I found this key which referred to differences in the female's subgenital plate (with links to illustrations). I needed to dissect and clear the abdomen to find the subgenital plate and while doing so I came across another useful key which included photos of the subgenital plate for all 3 species, published in 2014 by Geir Söli and Eirik Rindal (here).

This key highlighted two differences between the genitalia of fenestralis and both cinctus and zetterstedti. The genital fork of fenestralis lacks the pair of median sclerotised rods present in the other two species. Mine lacks these rods, so that seems to confirm it as fenestralis. But the shape of the hypoproct also differs, and that isn't so clear. My first issue was working out what the hypoproct is! Well there's a bit - and only one bit - in the photos of the 3 species that conforms to the descriptions of the hypoproct given in the key, so I figure it's that bit - a plate that's distal to the subgenital plate. It's supposed to have a distinct median incision on cinctus and zetterstedti and be medially prolonged but without a distinct median incision on fenestralis. Here we have a chink in my identification so far, because comparing my specimen with the photos in this key, the median incision seems more distinct than on fenestralis, if perhaps not quite as distinct as shown for cinctus. Oriented so that the subgenital plate is flat to the camera (as seems to be the case in the photos in the key) the hypoproct doesn't seem to be as medially prolonged as shown for fenestralis either - overall the shape of my fly's hypoproct looks closer to the photo of cinctus, leading me to wonder whether it's possible for the scleritised rods to somehow disappear on cinctus (and my specimen). But tilting the hypoproct back a bit I can see that in fact it is quite strongly medially prolonged. So I think it's good enough, in combination with the other characters, to call it fenestralis - but if others with more experience of Sylvicola need to correct me then I welcome them getting in touch.

female Sylvicola fenestralis showing wing, abdomen tip, hypoproct and subgenital plate (two views) and close-up of tilted hypoproct, North Elmham (Norfolk, UK), 9th May 2021