CREEPY CRAWLIES in Greece
Insects - The same insects as northern europe can be found in Greece only BIGGER. As previously mentioned mosquitoes are the most frequently encountered critter but you may occasionally come across some mammouth centipedes as well as scorpions. It's a good idea to check your boots before putting them on, particularly if you are camping as both can sting.
Reptiles - You will see numerous lizards, skinks & ghekos, the latter distinguishable by their bulbous eyes and big feet and are usually bright green. There are snakes in Greece, though like the lizards are very shy and you won't get very close to them (not that you would want to).
Sea Creatures - The only real concerns while swimming are urchins, which are easy to see and painful if stepped on but not poisonous and jellyfish which are not so easy to spot and some can sting. Thankfully neither will cause permanent injury.
After stepping on a Weaver Fish on Paros I now always wear sandals or sea shoes while in the sea as the physical pain is even worse than the financial cost of the Doctor.