And in the end it's not the years in your life that count, it's the life in your years."
~Abraham Lincoln~
Added note
I made a discovery just now, after I had already written my post and published it. I learned that the caterpillars in my photos were actually ones that would turn into a Black Swallowtail Butterfly. They are very similar but this site will show the difference. To repeat myself, the caterpillar shown here will not turn into the Monarch Butterfly, as I had first thought, but will reveal itself as a Black Swallowtail. All the information I have written, however, is still for the Monarch caterpillar and that is very interesting. I decided not to rewrite my post but have added more to explain. My signature logo at the end shows the Monarch Caterpillar, and the first clue is its tentacles. Another realization was that at a glance they seem similar but if you keep looking, the differences show themselves quite clearly. Here is another link. Sorry about my confusion but I am actually happy I made the mistake as I learned another fascinating lesson in nature.

The Monarch caterpillars are partial to the milkweed plant. Milkweed contains a bitter chemical known as an alkaloid that keeps most insects from eating it, but which Monarch caterpillars just love. Consequently, Monarch caterpillars and adult butterflies contain the same alkaloid, and this is great for them because other predictors, most of them anyway, will find them too bitter to eat. This explains why Monarch caterpillars, instead of being camouflaged, are boldly marked. Even with their brightly colored 'coat', it announces to all would-be diners you can't eat me, I'm dangerous and I taste horrible.
This is a Monarch Butterfly.
Unfortunately, and as you may know already, the Monarch is now on the endangered species list. If you click on this link it tells us what we can do to help, like plant milkweed and other flowers that bloom in every season for instance.
Regarding the milkweed plant, Jenn Jilks of Cottage Country Reflections, made a very important point in her comment today. "To be clear, monarch require milkweed on which to lay their eggs. It's not a preference. They feed off many plants, however, on their way home." Thank you Jenn! I found more about this here.