Update: Criss-Cross Caterpillars The appearance of Xs and Ys in
the striping pattern of monarch caterpillars occurs due to a
misalignment of segments. These are referred to as teratogens or
as teratogenic forms. Most of these forms are developmental in
origin but a few may be genetic. We usually spot these in the
third instar but it may be that the misalignment just becomes
more apparent as the caterpillar gets larger. Most of these do
not survive to the adult stage; however, several years ago one
of these "criss-cross caterpillars" appeared in our culture,
emerged as a female monarch, mated, and laid eggs. The
mysterious "X" appeared on only one of her offspring. Photos by
Monarch Watch (2002). See photos on this
website:http://www.facebook.com/media/set/?set= ... 114&type=1