It's probably very complicated, but a mixture of what was available when any particular species was evolving and what chemicals in a particular plant provided disease protection or predator avoidance, etc. The specifics of the insect's digestive system would also come into play, as well as the texture of the plant material and the chewing ability of the caterpillar. I'm completely guessing, of course, but all of those issues would have been at least part of the process.

Martha

There are many, many people who love to watch butterflies as they flit through the air slowly and gracefully. If you never have, you certainly should! Butterflies provide beauty in an often ugly world. They give us insight into the world of nature and how wonderful and complex it is. But the butterfly is much more than that.
Butterflies are some of the most beautiful and interesting creatures on Earth. A butterfly garden is an easy way to see more butterflies and to help them, since many natural butterfly habitats have been lost to human activities like building homes, roads and farms. It is easy to increase the number and variety of butterflies in your yard.
Creating a butterfly garden should start with some serious research to learn which kinds of butterflies are native to your area. You can learn that from this book âÂÂAttract Butterflies To Your GardenâÂÂ.

Here are some points you should focus:

Make a list of all of the different kinds of butterflies you would like to attract, and then learn which flowers and plants they both feed on and lay eggs on.

Plant the âÂÂButterfly Host Plantsâ to lay their eggs on.

You can add some butterfly garden accessories like a Butterfly House, which has slots the ideal size for keeping birds out while giving butterflies protection from the wind and weather, and are beautiful garden decorations. You could offer an additional nectar source close by to supplement your flowers.

Once you have designed and started your butterfly garden, you can be proud that you have made a habitat for butterflies in your own yard, which helps with the conservation of the many species of quickly disappearing butterflies today. You will certainly want to place your favorite outdoor furniture near so that you can enjoy all of your visitors day after day.

Here is a link that might be useful: Attract Butterflies To Your Garden

I just wanted to say...that's an ad! And the person posting it JUST signed up today!

Docmom: thank you..your guess was better than mine.

Linda_tx8: I was so confused by that second post, I just thought the person didn't understand the context of my question lol. But I agree if they just signed up...and wrote that than its probably an add like you stated