Tijdschrift voor Entomologie

Tijdschrift voor Entomologie is the Journal published by the Netherlands Entomological Society. The society has a bit of a heritage---it was founded in 1845, and has remained in action since. The Journal itself is has open access to PDFs of all papers published from 1998 until two years ago. Volumes published before 1923 are available at the Biodiversity Heritage Library.

This clearly illustrates what I think is a growing trend. Literature published since 2000 can generally be easily found online. The same goes for literature older than 1925---there's a good chance it's on BHL. However, it's actually fairly difficult to find the literature published in the period in between (i.e. 1925--2000). No doubt this will change as the sliding window of Copyright slowly moves over this time period. In addition, I think that some societies and publications are starting to fill this gap of their own goodwill, which is excellent also.