A group of insect that keeps a close eye on the bee hotels are the Carrot wasps (Gasteruptiidae), of which several species can be found on them.
This wasp is a female Gasteruption erythrostomum. This species has a middlelong ovipositor a little shorter than the abdomen, a finely coarse appearance and a longer nose. Also the hindtibia and the point of the ovipositor are colored in vanilly white spots1.
Carrot wasps are parasites and the females have an ovipositor to lay eggs in bee broods. G. erythrostomum is a parasite on at least Chelostoma campanularum and Chelostoma rapunculi 2.
The thickend hind legs are used to clean the ovipositor and put it back into the shaft for protection. The red color, that fits nicely with the metal-like appearance, is variable and not a good determination characteristic3.
I really like these wasps as their appearance has something alien. They have a striking posture when flying in which the hind legs are extended behind the body and the other legs are kept close to the body. This in cobination with the erect abdomen and the dark metallic grey color makes look like a humanoid alien in a fancy spacesuit. Unfortunately I didn’t succeed in getting a good picture of this posture yet… the photograph below gives a good impression though.
References1 Kees van Achterberg, 2013. "de nederlandse hongerwespen (hymenoptera: evanioidea: gasteruptiidae)"