Page ranking has nothing to do with spider visits. i.e. they are not directly correlated. spider visit != pageranking++
The fact that a spider is indexing your website is good. It means that the search engine will be indexing the content available on your website, and hopefully if you are changing it often enough it'll provide some good feedback on ranking relevance.
All Apache logs (if you're using apache and logging) should show spider results, and they usually identify themselves as robots, which should help you track them down. I will try to find an example and post it here shortly.
Posted is an example of what you would expect a robot/spider visit to look like in an Apache log :
220.127.116.11 - - [29/Jul/2007:22:54:18 +0100] "GET /robots.txt HTTP/1.1" 404 - "-" "Baiduspider+(+http://www.baidu.com/search/spider.htm)"
n.b. the 404 number you see in the middle indicates that it was unable to find the robots.txt file.