Common Damsel Bug Nabis rugosus

Despite apparently being common, it took me over 9 years to find this species after my interest in the family was piqued by finding a new species for the UK (Nabis capsiformis).

These two came to light but their identification was far from straightforward. I initially used the excellent British Bugs website and quickly narrowed it down to two (or possibly three) species: Common Damsel Bug Nabis rugosus, Heath Damsel Bug Nabis ericetorum or possibly Nabis brevis. A cursory glance at the photos (and others from elsewhere) suggested it might be ericetorum as these mostly shared the rusty orange tones present on both of my bugs, whereas the rugosus were generally paler brown. But there was no mention in the text of this being relevant and indeed other sources confirm that rugosus can show similar colouration. The main difference between rugosus and ericetorum seemed to be the relative length of the second antennal segment (A2) compared to the width of the pronotum. For the female, they were the same length, to within hundredths of millimeters, and that should put it as ericetorum. A2 was very slightly longer than the pronotum width, but so close that I found it hard to believe it mattered. However on the male A2 was a bit more substantially longer than the pronotum width. Still close enough that I could believe it was close enough to be considered the same, but sufficiently longer to give me good reason to doubt this.

At this point I was favouring ericetorum but was far from sure. I turned to Kirby and this suggests that the reddish-brown colouration is linked to proximity to heather/heathland in both species, so that didn't help (these were not on heathland and I don't know of any heather very nearby). It says that ericetorum is narrower and more parallel-sided than Common, which swung the pendulum more towards rugosus, although there seems to be quite a bit of variation in shape judging from photos online.

Looking at the male parameres, I struggled to see what the differences were. The text said they project laterally more on rugosus vs. ericetorum, but I'm not clear what this means. At first I thought it was talking about the shape of the paramere itself, incorporating more of a lateral projection on rugosus, but I couldn't reconcile this with the diagrams. Then I wondered if it simply means the parameres stick out more from the abdomen on rugosus, but this was now impossible to assess as I had pulled them out for a better view (in hindsight I should have done this to just one, leaving the other in situ). Kirby also describes and illustrates a difference in the shape of the genital segment. Mine was a little damaged but I couldn't really reconcile it with either diagram. I wondered if it meant an internal genital capsule that had to be removed from the segment, but nothing I extracted resembled either diagram at all.

After getting stuck with the male I turned to the female, and at first this didn't seem any easier. I struggled to reconcile the structure with either diagram at first and was almost ready to give up or seek assistance. Then I remembered that I had previously seen ericetorum, so checked my photos of that to see if I had examined its genitalia and if so, compare to this. Thankfully I had - it was a female, and the genitalia looked significantly different. In particular it had a broad sclerotised border that closely matched Kirby's diagram. I was now confident I was looking at the right bit of anatomy, and now knew which way up it should be! Mine lacked this broad border but had a narrow enclosed structure at what was now the top and, I then realised, if I tilted it slightly it became oval shaped pretty much as shown in Kirby's diagram for rugosus.

The genitalia also helped reassure me that I had correctly eliminated brevis. So, the female was rugosus and, as it was the female that was least convincingly rugosus on the relative length of A2, I think it is safe to assume the male was too.

This is the female.

female Common Damsel Bug Nabis rugosus showing lengths of second antennal segments, width & length of pronotum, underside of abdomen tip, cleared genitalia (2 views), Wendling Beck Environment Project (Norfolk, UK), 30th October 2023

This is the male. The specimen had become mouldy since capture and broke up on examination - in the end I cleared the whole insect in potassium hydroxide in order to clean off the mould and get a good look.

male Common Damsel Bug Nabis rugosus showing lengths of second antennal segment, width & length of pronotum, genital segment with parameres extended, genital segment after clearing with one paramere removed, one paramere (2 photos, attached and detached), the other paramere (3 photos at differing angles) and internal genital structures, Wendling Beck Environment Project (Norfolk, UK), 30th October 2023