P-Trap Requirements on Clothes Washer Drain?

[Mitch H.]

I recently had to do some research (read: cutting into the wall) on my clothes washer drain.  I found that there is no p-trap, either in the wall or below grade in the crawl space.  The washer plumbing drains straight down in the crawlspace, and then does a 90 degree turn into the main line.  

What would the plumbers here recommend in this situation and what might be the code requirements?