Horseflies. Trivial to some perhaps, but to me a truly deadly enemy. I find it difficult to express the extent of my hatred for these damned things. Once one's homed in on you it will not leave you alone until you kill it. If two attack you at once you risk dislocating limbs or beating yourself senseless. If more than two attack you at once you might as well kill yourself and get it over with.
They do have one chink in their armour, which I have discovered while fishing. If you sit under a large fishing umbrella they will always settle on the inside of the fabric, where the best method of picking them off is to fire the unloaded pouch of a catapult at them. If you're walking, a large, hand-held umbrella might work, but you might feel a bit silly (not that you'd care).
They are silent and tend to circle around your head before choosing their spot. They are incredibly difficult to swipe with your hand, but can sometimes survive a direct hit. If you miss, they will simply fly around you and come straight back from another angle. They will never go away and will just follow you until you kill them. Just as bad are the ones that land silently and unseen on bare legs. You often don't feel it until it's well embedded, but by that time you are doomed to suffer a bite than can swell to the size of a bantam's egg.
They are most active in hot and humid weather – just like it is at the moment.