Dragonfly:
Insects frequently become eaten by dermestid beetles within a year of mounting. They may be controlled with an application of moth crystals behind the mount. No, it probably is not conservation approved but it works. Place the crystals in a small plastic bag with some holes poked in it to release the preventataive. Another tried and true method is to place some cyanide crystals in the bag; it will work equally as well but do not inhale the fumes (I suspect that cynanide crystals may be a bit hard to get).
It is not necessary to spray the dragonfly with anything to hold it together; if you keep the beetles away, it should last a long time. Mountings of this type are ephemeral at the best and 100 years may be a bit much to expect.
Jack Cee