You might want to write a metaphorical poem. For example, when I think of a caterpillar my first thought is that they turn into butterflies. So instead of writing directly and literally about caterpillars, you can write metaphorically about change.
A good point, Litteacher. Try to stay original and creative, as they are the two elements that make any intellectual property stand out.
Well, first of all you need to establish your theme, which you have: 'Caterpillers'. Are there any other limitations? Can you do free verse, haiku, acrostic etc.? If so, choose your favourite and write according to the rules of the poem type. There's little anyone can do to help you construct poetry, as it is something unique to the writer. Regardless whether you think it's rubbish, keep writing and writing, as there is most likely someone out there who'll say it's gold!