That's a tough task you're working on- good luck! Opuntias are extremely variable, they tend to hybridize a lot, and it's a huge genus, all of which makes your project that much more challenging. Desert-tropicals.com lists 6 different varieties for O. engelmanii. As for O. ficus-indica, I'm not sure of how many varieties there are. Rivenrock.com lists 3 varieties for Indian Fig, but I'm not sure how valid those varieties are. I often see Opuntia ellisiana being sold as O. ficus-indica.

I guess I would recommend posting pics of your plants for identification. There are many amazing cactophiles on this forum who can probably id your plants from photos.

cd

You might go to cactiguide.com and post pics in the ID section. Some very good eyes on that board.

thanks for the advice, cactus dude and fred grow! The hybridization factor does make it tougher to arrive at a positive id. I was going to post pics on this forum, but they are large plants that have been placed in a 7x14 arrangement for ovipostition trials. Therefore, all my pics are of just one plant and the forum only allows one image per post. I will try cactiguide.com. '
Thanks again!