Spiders regularly return to the web pages they index to look for changes.
When changes occur, the index is updated to reflect the new information. They search the databases of multiple sets of individual search engines simultaneously, from a single site and using the same interface. The searches they return are drawn from the top of search engine lists, and therefore tend to be more relevant. Editors review and select sites for inclusion in their directories on the basis of previously determined selection criteria. There are general directories, academic directories, commercial directories, and portals.