I agree with Roger ... with the tachinids we have often discussed how many field-identifiable tachs there are in the UK and it always comes down to a very small % of the 270-odd species you could possibly find here.
One of the main issues is that most photographers are happy to accept a name with a much lower degree of certainty than, for instance, a recording scheme would. Looking at a photo you can always say what the fly *looks* like ... but seldom can you actually say what it *is* because the characters that really define the species are usually so small.
A classic example of this is the very common species
Phania funesta, which is very easy to ID from photos ... except that it can only be split from its very rare cousin
P.thoracica by examining a specimen under a microscope. If we take loads of photo-records then we would be meerly guessing (even if we were correct 99% of the time) and it would artificially bias the recordset towards the common species and we wouldn't notice a potential increase in the population of
P.thoracica.
Also, recently Dipterists like Ivan Perry have discovered quite a few new species by closely examining specimens that would otherwise have been given the name of an existing UK species because they look very similar. Only when you run a lot of material through European keys do you come up with these newcomers - or in the case of
Linnaemya picta, find that a well-known colony has been misidentified for decades.
