The name/version of the genome given by the source is included in the full name at all usegalaxy.* public servers. Only https://usegalaxy.org.au has Tophat2 indexes for: Tribolium castaneum: triCas2. The source is UCSC (based on the Baylor assembly): http://hgdownload.cse.ucsc.edu/goldenPath/triCas2/bigZips/
The version/source for indexes can vary by Galaxy server and the administrators have control over index creation and naming. So, if you are working somewhere else and the version is not included in the index name, contact those administrators directly for help.
Thanks! Jen, Galaxy team