According to the San Diego Zoo, there are more than 165,000 different species of butterflies and moths. All of these insects start as caterpillars. They range in size from the Queen Alexandra's Birdwing Butterfly, which boasts a wingspan of 11 inches, to the Pygmy Blue Butterfly, which measures just half an inch. The caterpillars of different species develop in a variety of sizes and colors depending on their species and environment. Since some caterpillars are destructive to plants, identifying the species is a vital step to protecting your landscaping.
Take a picture. The photo gives you a static reference to the caterpillar or butterfly. Keep a small digital camera with you when working outside. Snap a picture of interesting caterpillars. Quick moving butterflies may be more difficult to capture with a camera. Consider pulling out your cell phone and taking a quick shot.
Check with your local Agricultural Extension. Many Agricultural Extensions have data on local butterflies and caterpillars. Share your picture, or describe the butterfly or caterpillar you encountered. They can also tell you if the caterpillar is destructive or safe for your plants.
Look online. There are a wide variety of butterfly and caterpillar websites. The Smithsonian institute hosts the Discover Life website. There you answer a series of questions to help you identify the species of caterpillar in your life. You can then register your sighting with the Institute, which keeps track of caterpillar sightings all over the world.
Share your picture with an etymologist. Many local colleges or universities employ professors that study all types of insects. Montana State University hosts a butterfly website that accepts pictures from all over the world. A free membership allows you to upload your photo for identification.