They are search engine bots.
Here's a definition from the internet:
A spider is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index. The major search engines on the Web all have such a program, which is also known as a "crawler" or a "bot."
If you check on "Who's Online" you'll see that the spiders that go through GB are Google, Bing, Yahoo, etc...