bugs in my drain

Hi,

My bathroom is torn up for renovation. I have my toilet drain plugged up with a rag wrapped in plastic bags. I pulled the rag out today as it was in the way of some tile I was laying, and got a bigggg surprise, a number of them actually.

There were 4 or 5 huge cockroaches, I guess you might want to call them Palmetto bugs but as an ex NY'er they are just big ass roaches to me.

I've had this plug out a number of times to dump stuff and never seen these things before. I live in a neighborhood serviced by a sewer system, so I don't think there are any openings anywhere near me where things would have just crawled in. The only thing I can imagine is that they dropped down one of the PVC vent stacks on the roof. Does this seem likely? There is nothing at all covering the stacks. Is there supposed to be something? I've searched for vent stack covers and turned up diddly. Are there not some kind of pre-made screens sold for this application?

Thanks
Eric