Genetics of Wood Nymph Spots?

[TDWolfe]

I'm very excited to have found this group!  I've taken an extended maternity
leave from my work in cell biology and now that I have time to take lots of
country walks with my babies, I've rediscovered my childhood love of
butterflies.  But pardon me if my questions are kind of weird...that's what too
many hours at the bench will do ;).  

Anyway, I'm enchanted by the common wood nymph.  I've seen several this week
and I'm intrigued by the variability of their spotting.  How are discrete spots
like those of the wood nymphs passed on?  In people, traits with a wide range
of variability (height for instance) are usually thought of as being coded by a
large number of genes, with the offspring usually achieving a height somewhere
between that allowed by the genes of each of the parents.  (At least this is my
understanding, I'm not a geneticist!)  But butterfly spots don't fall on a
smooth continuum...you have 1, or 2, or 3, but not 2.74 spots.  So how does
this work?

I have another question about mimicry, but this post is already long enough!  

Pat