First instar Limenitis weidemeyeri latifascia caterpillar on its perch.

First instar admiral and viceroy caterpillars do not measure length of day to determine whether or not they will construct a hibernaculum and hibernate at third instar.  This measurement takes place as soon as they molt to second instar.  Therefore, if you ever find a first instar viceroy or admiral caterpillar, it is advisable to set it up in a semi-closed terrarium and then expose that to 24 hours of light so that you can emerge an adult within 30 days or so as opposed to wintering the larva and waiting for the next spring.