Wesleyan Library offers a vast array of resources, and of course there is a whole world of resources beyond just what is available here, so think broadly and creatively when looking for resources to use for your senior thesis/essay:
Databases - Find sources of all types (books, articles, conference papers, video and sound recordings, images, manuscripts, etc, etc.). Browse the list by discipline, or by type of source (newspapers, primary sources, data and statistics, etc.)
Here are two databases likely to be of particular interest to thesis writers in general:
WorldCat - Catalog of books and other materials in libraries worldwide. Use interlibrary loan to request books etc. not listed in OneSearch.
Web of Science - Journal articles in the sciences, social sciences, and arts and humanities. If you have a good article, look it up here, then you can see who has cited it since it was published, or select "related records" to find other articles that cite some of the same sources the original article used.
Journal Titles A-Z - Search for the title of an academic journal, or magazine, newspaper, or other periodically published publication.
Collections - In addition to standard books, articles, etc., Wesleyan Library has several collections of a variety of contents, such as Archaeology and Anthropology, Art, and Music
Two extensive collections likely to be useful for a wide range of thesis/essay topics are:
Special Collections & Archives - The Wesleyan University archive, and collections of rare books, local history, unique manuscripts, artists' books, and much more.
Honors Theses - Honors theses from past years are included in WesScholar. To see examples of previous senior theses in your major, scroll down to "Discipline" on the left side of the page and select your discipline.