Tachina fera vs magnicornis vs tetramera

The recent arrival of Tachina magnicornis in Suffolk and then later in Kent has created a quite tricky problem for us at the recording scheme. It has always been very easy to identify T. fera in photos and to distinguish them from vaguely similar species like Nemoraea pellucida or Peleteria rubescens, but now we have a genuinely very similar species to contend with. Furthermore, Christer is convinced that they have a third species called T. tetramera in Scandinavia but there it seems to be a montane species so we are not expecting it here.

David Basham has very kindly helped out here by providing a lot of specimens of T. magnicornis and together we have gone through them and compared to a selection of specimens sent to me by Christer Bergstrom. All 3x species are a little bit variable and they seem to vary across their geographic ranges and between the 2 broods, but we have managed to work out some stable features of at least the males that seem to be very reliable.

Basically:

• if the frons is normal / narrow then it’s still T. fera
• if it’s wide then it is either T. magnicornis or T. tetramera
  • If it is a male then
    • if it has short fore claws then it is T. magnicornis
    • if they are long (like T. fera) then it is T. tetramera
  • if it is female then
    • we haven’t found any reliable features to split females of T. magnicornis & tetramera.

Male cercus seen laterally

Male fore claws