Thank you! I actually ended up scraping the images from the site using wget and reuploaded to the Internet Archive just recently:
https://archive.org/details/InsectLiterature. However your upload is very appreciated because it retains Google's superior text layer and the Internet Archive does some preprocessing which ruins the images.
Regarding copyright: if the work is legitimately public domain, then no one can restrict your use of it unless their copy is a derivative work. It could just be their standard disclaimer.[0] The site and PDF both explicitly note that the work is in the public domain in the US. It was published before 1923, and the primary author died more than 100 years ago.[1] However I can't track down when the translator died but I trust that HathiTrust did their due diligence. A caveat is whether it is public domain outside of the US; I'm not a lawyer and I don't know the copyright laws of other countries so I couldn't tell you.[2] That's probably why the site also restricts access to US IPs. However it is public domain in the US and as long you host the book on servers in the US, you're likely fine.
Quite a situation though eh, how we're both paranoid about infringing on the copyright of two long-dead guys? As you might tell though, I edit Wikipedia and am quite conscious of respecting copyright and always make sure to do my own research in addition to copyright claims made. I can understand though if you're not comfortable hosting it on a dropbox account linked with you so it's fine if you want to take it down.
Edit: I've found it: Hearn lived 1850-1904 and Otani lived 1875-1933, so the book is in the public domain in both the U.S. and Japan! http://iss.ndl.go.jp/books/R100000039-I000689989-00
[0]: Google has been sued before so I bet they can't be too careful: https://en.wikipedia.org/wiki/Authors_Guild,_Inc._v._Google,.... Note also that it only says "requests" and not stronger language like "prohibited".
[1]: According to this: https://en.wikisource.org/wiki/Help:Public_domain#Published_..., works published outside the US pre-1923 are public domain if the author died by 1945.
[2]: Though according to this: https://commons.wikimedia.org/wiki/Commons:Copyright_rules_b..., in Japan, "works enter the public domain 50 years after the death of the creator (there being multiple creators, the creator who dies last)". So that all ties it back to when Otani died.