Sialis lutaria

This species is common in Norfolk, often found during the day-time near water between April and early June. I have also found them at night by torchlight, but never attracted to light.

Aldeflies cannot normally be identified from photos and it would be very difficult to see the relevant characters when examining a live specimen. Males need a look at the abdomen from the side and females need a look at the adomen from below. Details of how to identify them can be found in the FSC Aidgap key to lacewings and their allies by Colin Plant (1997).

Most of the insects I have found have been males, and these are reasonably easy to identify with a fresh specimen. If it has dried out then it may be necessary to macerate the abdomen, for example in potassium hydroxide solution, before the critical details are visible. Females need a good look at the underside of the abdomen. The only female alderfly I have found so far was in the jaws of a centipede and unfortunately the bit of the abdomen that is required for identification had already been eaten.

male Sialis lutaria showing end of abdomen, Wendling Beck Project (Norfolk, UK), 15th April 2024

The first female alderfly that I found was in the jaws of a centipede and unfortunately the bit of the abdomen that is required for identification had already been eaten. This was the only other female that I've found so far, found on a Commmon Reed stem. It was obviously larger than any of the male alderflies I've seen recently (the forewing was 15mm long). It had begun to dry out when I examined it, though by no means completely, and I was unable to see the required detail of the anal plate without clearing the abdomen. I suspect this wouldn't have been much easier to see if I'd examined it completely fresh. Even after macerating the abdomen in KOH solution I could only just make out the necessary detail. For a clear view I had to carefully extract all the gunk from inside the tip of the abdomen in order for the sclerotised plates on the sternites to be clearly discernible.

female Sialis lutaria showing anal plate before, during and after clearing, North Elmham railway (Norfolk, UK), 2nd May 2026

All of the following alderflies were examined, including those first photographed in the field.

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 8th April 2019

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 15th May 2019

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 27th May 2021

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 16th May 2021

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 21st April 2019

male Sialis lutaria, Wendling Beck Project (Norfolk, UK), 26th May 2023

male Sialis lutaria, Wendling Beck Project (Norfolk, UK), 29th April 2023

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 24th April 2020

male Sialis lutaria, Costessey (Norfolk, UK), 28th May 2016

male Sialis lutaria, North Elmham Cathedral Meadows (Norfolk, UK), 20th May 2021

2x male Sialis lutaria, Wendling Beck Project (Norfolk, UK), 14th May 2022

2x male Sialis lutaria, Wendling Beck Project (Norfolk, UK), 16th April 2022

male Sialis lutaria, Wendling Beck Project (Norfolk, UK), 26th April 2022