Holly blue

Sunbathing on this hazel leaf, this forget-me-not blue butterfly is a holly blue.  It is one of the commonest blue butterflies, the other being the common blue.  

They are easy to confuse but the holly blue appears earlier in the year and has a pale blue underside to the wings with tiny black spots.  A common blue  has a mosaic of grey, white, brown and orange on the underwing so that it is well camouflaged sitting on top of a cow parsley flower.

Common blue (RSPB)

Holly blues have two broods.  The springtime butterflies, which have overwintered as chrysalises hiding behind something like tree bark, lay their eggs on holly. 

 But when those eggs hatch into caterpillars and develop into chrysalises and adult butterflies, it will be IVY that they lay their eggs on.

How do they know what to do?