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The front part of the abdomen is narrowed toward the connection to the thorax, this is a characteristic of the Polistes family. The similar looking Vespula and Dolichovespula families have the front part of the abdomen large till the connexion to the thorax as if the abdomen was truncated.
The most common species in Europe is Polistes dominula but the keys are quite complicated and need many different views to determine which Polistes it is.
There are two very similar species of Melolontha: Melolontha melolontha (common European cockchafer) and Melolontha hippocastani (European forest cockchafer).
The only way to distinguish them is to look at the form of the pygidium (the last abdomen part) but in your photo it is not very clearly visible.
There are two species of Aculepeira that can be: Aculepeira ceropegia (from greek κηροπήγιο, candelabra the shape of the motif on the abdomen) and Aculepeira armida which is a little more orange.
Aculepeira armida is found only near Mediterranean and Aculepeira ceropegia is found all over Europe. There is a third species, Aculepeira carbonaria but it's found in higher altitudes and rocky areas so I don't believe it's the one in Spetses island.
From the light and shades, I can't say if it is the Aculepeira armida or the Aculepeira ceropegia you have found there.