Sudden influx of hit's on site / how do search engines trawl

[#1]

I havehttp://www.carmylechurch.co.uk which is run on joomla cms. last night the joomla site stats reported upwards of 170 visitors online at one time. It's quite a high number for the site and I was wondering if it could be down to search engines trawling.
I have installed a more advanced stats package today to try and monitor things more closely.
if a search engine is trawling a site, does it start at the front page and then work its way through all the local links or does it bombard the site with requests quickly and move on? The 2nd theory would explain the sudden increase in stats which lasted for some time.
Regards,
podman

[#2]

Go and check out the Google Webmaster Tools
Once you register your site (pretty easy to do) it will show you various bits of info regarding how Google sees and indexes your site.  I've attached a screen grab of one of the reports that it shows. (gwt.jpg)
Essentially, your site could be indexed by several 'bots' simultaneously, which may or may not follow a particular pattern of 'link clicking'.  The might start at a particular page on your site after coming in from an incoming link, for example.
Another useful toolset is Google Analytics which allows you to see exactly who is visiting your site, and where they've come from.
Attachments ga-1 and ga-2 shows a screengrab of some of the info that analytics can show you.
B.