How to set a different session timeout for spiders?

[AceNZ]

I would like to set a different session timeout for spiders than for logged-in users. Spiders don't usually accept cookies, so it can quickly look like you have hundreds of them simultaneously accessing the site, when in fact it's just a single spider making a lot of requests. Reducing the session timeout will help minimize that effect.

I suspect that spiders aren't identified until the who is online page is displayed -- so it would actually be OK to make this change for all guests.... Ideally, the code would detect whether an incoming request included a session cookie, and set the timeout based on that.

Does anyone know where I would look in the code to make such a change?