>the question is then, if the peritrophic membrane is or is not a mucus?

Juanse, as best I can tell, the PM in bees is composed of layers of
chitinous gut epithelial cells bound with protein.  These are continually
shed into the gut lumen, and allow for a countercurrent exchange of
digestive enzymes and nutrients.

As far as a mucus layer, there is one between the intestinal villi.
-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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