Please forgive my ignorance, but what the hell is a SPIDER.

You are forgiven

The spiders you are talking about Diesel are mostly from the search engines, they come into the site to read the pages, that's how sites get on them.

There are other "spiders on the web" too, they all basically just sniff over the site, see what's going on.

But it goes both ways, now the site can sniff them back 

Oh, right, and there was I thinking that were horrible black things that can do 0-60 in a nano second, that climb your walls completely uninvited, that you have to deal with else your partner will leave you.

Stupid me, can't even get my lyrics right.

The spiders in question are a sub class of bot (robot). Bots being automated data send/recieve across the net.

Being automated web crawlers they get the name spider. Human naming conventions are weird.

There are other types of nasty bots, esp. zombie bots used in DDoS attacks. (distributed denial of service).

What can I say? The whole thing is definitely daft. 

What can I say? The whole thing is definitely daft.  
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Thanks Snowcrash, that did make me smile and Thank-you all for your explanations.

Am I alone in thinking that it might be appropriate for me to change my user name to Daft.