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