sex determination in caterpillars

[Priya54]

I am not a butterfly or moth expert and I have several questions about
male/female caterpillars.  I am guessing that caterpillars are sexually
dimorphic, so:

A.  How can one determine the sex of a caterpillar?

B.  How or when is the sex of a caterpillar determined (for example, is it
environmental sex determination?)

C.  Why don't caterpillars mate?  What would happen if they did?  

D.  Can caterpillars change their sex?

Finally, an unrelated question, can a caterpillar die of old age?
Thank you!