Carcelia iliaca, the odd one out

With the spread of the invasive Oak Processionary Moth (Thaumetopoea processionea) there has been a lot of interest in its parasitoids. Both tachinid parasitoids are specific to OPM and do not attack any other species so they make interesting species in their own rights and very well worth looking out for.

Carcelia iliaca is unusual because, although it has big eyes, it doesn’t have the huge eyes that all other UK Carcelia have. In the usual keys (e.g. Belshaw, 1993) Carcelia and Senometopia are brought out using the relative height of the gena to the eye and most species will pass the test but if you compare the 2 images above you will see that Carcelia iliaca causes problems:

• Eye very large – approximately 12 times the height of the gena … Carcelia & Senometopia
• Eye not more than 8 times the height of the gena … NOT Carcelia & Senometopia

It’s actually a nice example of how the “easy” features that we try to use are not always the ones that apply best to the full spectrum of morphology in the group. In the case of the Carcelia there is a lovely feature we could use – the presence of bristles on the dorsoventral surface of the hind coxa – but as you can imagine, that’s very hard to see!

This photo, cropped from the above full lateral shot, shows the feature nicely but most specimens are not as obliging and you need to usually look along the underside of the abdomen, between the hind legs.

So, we still use the gena/eye ratio in the key for most UK species but just bring Carcelia iliaca out earlier and in a different path.