Its hard to tell, maybe you are incorrectly substracting 1 when looping through your body parts in draw?
for (int i = 0; i < wormBodyTexture.Count; i++)
Have you considered making a simple BodyPart class, and making a list of those that would represent the worm. First object in the list would be the head and last one the tail. When the worm moves you would just remove the last item from the list and insert new one at the start of the list based on direction you are going. When you eat something you just dont remove the last item and thus the worm gets larger. For collision you can easily check if the new head position collides with any other BodyPart in the list or walls of the map. When drawing you would just loop through the list, drawing the first part with head texture, last with tail, and the rest with body texture. Later on when you have the basics down you could easily expand the class to do all kinds of neat stuff. I hope that wasn't too confusing. :>