As you may already know from reading How Butterflies Work or Where do butterflies get their striking colors?, butterfly wings are made of very thin layers of a hardened protein called chitin. (Your hair and nails are also made out of this protein.) On top of these chitin layers are thousands of tiny scales that serve several purposes, which may vary for different species of butterflies.
Besides being responsible for the magnificent colors characteristic of butterflies, scales also protect and insulate the insects and aid in the flow of air along their wings as they fly. Scales also may help the butterfly to soak up the heat that flying requires. Since butterflies are cold-blooded, they rely on external sources of heat to bring their core temperature to a high enough level for their bodies to function. Preliminary research shows that even tiny changes in scale thickness can have a big impact on how well the scales absorb heat [source: Chiang].
Going back to our original question, if when you touch a butterfly's wing, enough scales came off to negatively affect its heat absorption, it could conceivably lead to its death. If you're wondering how you can tell that you brushed any scales off, just take a look at your fingers. That light dusting you see? Those are scales -- and they rub off easily -- partly to allow the butterfly to escape from predators in a tight situation. Unfortunately, that escape could ultimately lead to the same outcome as getting caught in the first place.
Beyond rubbing scales off, you could also break a butterfly's wing if you handle it roughly. The upper wings, called the forewings, and the lower wings, called the hindwings, are both very fragile. Although they are strong enough to support the butterfly's body in the air, they are also flexible to enable flight. Although you can't see them, a system of miniscule veins runs through the wings and if the vein on the forewing gets broken, the butterfly will usually die [source: Opler].
So while death isn't necessarily imminent for a butterfly if you happen to touch its wings, the odds aren't on its side either. The moral of this story is to keep all hands and feet inside the vehicle and leave the butterflies alone. For more on butterflies, try some of the links on the next page.