For many years Linnaemya picta had been confused with rossica because early workers (e.g. Fonseca) hadn’t tried using the European keys. Since the ‘discovery’ of picta in Kent it has spread across southern England as far as Bedfordshire, Oxfordshire & Berkshire and so now we have another fairly common black-legged species to confuse with tessellans. I have occasionally been confused myself by 2 couplets in the Linnaemya keys and so I have posted here some photos of the various species and illustrated the features that you need to look out for.
Firstly, both picta & tessellans have dark hairs on the gena but picta has bristles mixed in with them. The bristles should be fairly obvious because they arise from sockets, while the hairs go directly into the integument.
Secondly, both picta & rossica have a scattering of small black bristlets on top of the head, behind the post-ocular row. But rossica is distinctive in having many more bristles in multiple rows and at least a few of them are as large as medium-sized post-ocular bristles. This feature is actually very easy to find but can cause confusion if you only have a picta and don’t know whether your bristles are big enough.