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The cited earlier post in fact used the length of the abdomen primarily to indicate the sex of the individual, rather than the species. Then the combination of its sex (male) and its coloration (brown) determined the species (Brown Argus).
for putting me right I had misunderstood. A female Brown Argus would, then, have a short abdomen and not be so straight forward.
Howardian Local Nature Reserve
Yes, the females of these species can be trickier to tell apart, though even the brownest Common Blue females usually have at least a few blue scales somewhere.
Failing that, the positioning of spots on the underside is the best way of distinguishing them.
Lat/Lng: 51.617656823731, -1.1127895116806
OS grid ref: SU615913
Small damp nature reserve owned by Benson Parish Council and managed by Benson Environmental Study Team