SpiderPet reads the text of the pages you start it on, inside your browser, and passes what it finds to the SpiderPet app on your own computer (through Firefox's native messaging). Nothing is sent to the developer, and there are no analytics.

When "Online checks" is on in the SpiderPet app, the app looks up each single find (a DOI, ISBN, PubMed/PMC/arXiv id, a citation title or a few search words from a sentence) in these public services: Crossref, OpenLibrary, Google Books, NCBI PubMed, arXiv and Wikipedia. The rest of the page is never sent. Turn Online checks off and nothing leaves your computer.

The app keeps its library of finds in Documents\SpiderPet and its settings in %APPDATA%\SpiderPet on your computer. Delete those folders to remove everything.