Brown and blue, part two

During my flower-filled and mosquito-ridden walk in Poland, I saw a number of "blue" butterflies that were in fact mostly brown.  I found these butterflies congregating around several species of plants (mostly legumes) that appeared to be the hosts for the butterflies' larvae.

A female blue butterfly (either Plebejus argus or P. idas) sitting on a potential host plant.

However, the brown "blues" were not the only butterflies attracted to these plants.  There were also many butterflies with a similar size and wing pattern -- but notably bluer wings.

The male blue is similar to the female on the underside of the wings.

When the wings are closed (as above), the difference is relatively minor; yet, when the wings are open, there is a striking difference between the brown (see here) and the blue types.

Unlike the females, the males have very blue tops to their wings.

What is the relationship between the brown and blue butterflies?  Despite their distinct colorations, the butterflies belong to the same species, either Plebejus argus or P. idas.  These species, and several other related species, are sexually dimorphic -- meaning that the females and males differ in their appearances.  In this case, the females are brown and the males are blue.  A possible explanation for this pattern is that the females are under pressure to be inconspicuous to avoid the notice of predators, whereas the males are under pressure to be showy to attract the interest of potential mates.