Life cycle

Identification of caterpillars isn't always easy but this one is fairly distinctive.  It is the fifth instar (final developmental stage) of a comma caterpillar (Polygonia c-album).  I came across it last month in a patch of nettles, its favourite food plant.  

By now it will be a pupa, emerging in the next couple of weeks as a comma butterfly.

These photos show well the white mark on the underside of the wing that gives it its name.

Comma butterflies overwinter as hibernating adults which lay eggs in the spring for a new crop of caterpillars.  There are two generations per year.