Beginner question: How can measure object length?

[Jonathan Rosenthal]

Hello all,
    I'd like to use ImageJ to measure the length of some caterpillars  
in digital images. When I clicked on "Analyze" and then "Set  
Measurements" it seemed that all the options were for area, none for  
length. I'm guessing there has to be some way to measure length - if  
you know, please tell me.

Thanks very much,

Jonathan Rosenthal
Visiting Scholar
Dept. of Biology
Vassar College