Wanted to add this but don't know to which order it belongs. It's a mud dauber type wasp and I believe Sceliphron curvatum of the Sceliphron genus and the Sphecidae family. I have found up to 9 nests behind on small picture. The nests contain paralyzed spiders and in the case of one opened what I take to be a immature wasp.
Solitary wasp that builds nests behind pictures in my house
we would love to be able to add your images to BugFiles, but to do so we would need to be certain of its identity. Sceliphron curvatum is certainly found in France, but it is impossible to see in your photo the distinctive yellow markings on the thorax and abdomen: http://en.wikipedia.org/wiki/File:Sceliphron_curvatum-pjt6.jpg The 'immature' wasp you found inside one is not a Sphecid wasp. It does not have the long slender waist section of the body.. It must be another prey species.
I tried searching for other Specid wasps found in France. Isodontia mexicana looks quite like yours, but descripions of its nesting habits seem to rule it out: http://en.wikipedia.org/wiki/Isodontia_mexicana
Unless you can obtain a firm identification, I am afraid we shall have to omit adding your pictures,
Hello kennedyh, many thanks for taking the trouble to reply to my entry and sincere apologies for not coming back to you sooner. Sorry I couldn't get a better of that wasp but the little blighters don't stay put long enough and it's now the end of season so don't see any more.
Incidently I found 27 nests behind one picture so they can be prolific.
Just out of intererest I attach a pic of one very dead little spider found in one nest.
Best wishes and thanks again.