Search engine crawler (spider also called as robot) is a software program that crawl your website pages, records the changes in your web pages and stores them in its database, which is useful for giving you results against your search queries.
Visits of a spider to any website is depends upon relevancy of the website i.e. content of the page, linking structure, link popularity, link saturation, updates in page etc. Each search engine has its own spiders (software programs). And they are having their own individual technology of crawling websites. For example Google’s spider name is Googlebot, Yahoo! Slurp, MSN’s MSNBot etc.
Google crawls every website as per the relevancy given. And this relevancy depends upon link popularity, age of the domain and page rank assigned by google from 01 to 10. Google crawl some websites daily and some website weekly. It may crawl some website pages monthly also. It is totally depends upon your website contents and linking structure.
I advice you all that design your website visitor friendly as well as crawler friendly by writing unique contents and sophisticated linking structure. This will help your website to achieve boost in search engine ranking and promote your company’s products on internet.