AWS SWF (simple work flow) starts only on existence of SQS messages, is that possible?

My Objective:

I have a very simple workflow I would like to implement. Given an ID, create a file in a s3 bucket called ID.txt only if that file does not already exist.

What I have tried so far:

I have read the doco and walked through some examples using aws-ruby-flow.
Makes total sense so far.

What I still don't get:

My desire is to have the workflow process SQS messages that contain s. What I don't grasp is how to kick off the workflow when queued messages appear.

Question

Do I need to implement my own polling service somewhere that triggers the workflow whenever it finds messages in SQS?