Distinguishing Cylindromyia spp.

We now have 4 species of Cylindromyia in the UK with more and more photos of them cropping upp on social media so it seemed like a good reason to go through my specimens here and work out the best ways to distinguish them in photos. In the keys one of the most important features is the absence (brassicaria) / presence (all other species) of posteroventral bristles on the hind tibia but this is very difficult to see in most photos.

The easiest species by far to distinguish is bicolor, which has an orange-red abdomen without any black on the tip. The other species have orange abdomens with black on T1+2 and on the tip.

This next section just works on the other species:

• Pre-sutural thoracic dusting:
  • interrupta: 2 broad black bars
  • auriceps: 2 fine central lines
  • brassicaria: 2 fine central lines
• Scutellar bristles:
  • interrupta: 2 pairs (basal, subapical), no apicals
  • auriceps: 2 pairs (subapical, apical crossed)
  • brassicaria: 3 pairs (parallel/converging basals, subapicals, apicals crossed)
• Abdominal black central stripe: (be careful – T2 is elongated!)
  • interrupta: usually (but not always) complete but sometimes with a narrowing on the margin of T2
  • auriceps: usually (but not always) complete but sometimes with a narrowing on the margin of T2
  • brassicaria: extends to the margin of T2 (T3 lacks stripe) but sometimes a faint stripe on T3
• Bristles on Tergite 3: (be careful – T2 is elongated!)
  • interrupta: median marginals AND median discals (sometimes 2 pairs) – the same on T2
  • auriceps: median marginals AND median discals (sometimes 2 pairs) – same on T2
  • brassicaria: median marginals BUT NO median discals – same on T2

So, to conclude:

• interrupta is smaller and has 2 broad pre-sutural thoracic bars and lacks apical scutellars, but these are very hard to see anyway because they are black and are located above a black part of the abdomen. Current UK distribution is: south-east England (east Dorset to north Kent); Mansfield to Doncaster; around west Norfolk north of Bury St. Edmunds and a few scattered records elsewhere.
• brassicaria never has media discal bristles on T3, just marginals and it usually has an orange T3 but sometimes there is a central stripe but where it does occur it’s often fainter. It will also have basal scutellar bristles which are usually parallel or slightly converging – it also has crossed apicals and subapicals. Current distribution is: Cornwall to Dorset coasts, from the Lizard to Lyme Regis.
• auriceps will always have median discal bristles on T3 along with the usual median marginals AND should have a stripe on the same tergite but the latter feature isn’t stable. It will lack any basal scutellar bristles but has crossed apicals and subapicals. Current distribution is: Box Hill, Surrey – just one record.