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Thank you both Tony and Jari.

I'll try the acetone as soon as I get the opportunity. For the conservation of dragonflies I think the excercise was difficult, but I personally know Domanget Jean-Louis (president of French society odonatologie) and he told me he had to leave for a day odonates after capture for the tip, and in this case it keeps better odonates. I feel that this s'appplique to many insects. I am passionate apoïdes and wasps and I think the conservation easier ;).