Dear Urban Legends:
The stories of the spiders and such reminded me of a story I was told in Boy Scouts about using a river as a bathroom. Who knows how true this is (I think they told it more to scare us but some may have believed it), but our Scoutmasters always insisted that there were small, parasitic fish in some rivers (particularly the Amazon) that could swim up the urine stream and get inside you.
Also, another variation on the maggots in the brain story was one I heard about people inhaling items through their nose, such as string, and bringing it through their mouths to "brain floss." One person in a group that was doing this bet that he could pass a fly the same way. Unfortunately, the fly didn't come out. Soon the symptoms came and the diagnosis was maggots on the brain.
Much as it may sound like nothing but an old scoutmaster's tale, the parasitic fish you refer to does exist. Actual close encounters with the candiru (Vandellia cirrhosa) must be fairly rare, judging from the dearth of medical literature on it, but it's still high up on my own personal list of Scary Things to Avoid.
The candiru is a member of the catfish family and can be found in lakes and streams of the Amazon region. It's tiny only about an inch long, if that and, according to Enyclopedia Britannica, "has been known to enter the urethras of bathers and swimming animals." That's hardly the worst of it.
"Once in the passage," the Britannica continues, "it erects the short spines on its gill covers and may thereby cause inflammation, hemorrhage, and even death to the victim." The little bugger can only be removed by surgery.
Why, you ask, would a fish want to swim up there in the first place? Audubon magazine explains:
Candirus primarily set up house inside larger fish, where they feast on the host's blood. They are attracted to nitrogen, which usually leads them to a gill chamber, but apparently they can't distinguish between one nitrogen-emitting orifice and another: They have been known to follow a stream of urine right to its source.
As to your "brain floss" scenario it's charming, but I doubt I'm alone in thinking that anyone who would purposely thread a fly through his nose must have maggots in his head to begin with. Intracerebral myiasis maggot infestation of the human brain is a rare but very real medical condition. My hunch is that doctors would recommend against flossing your brain with insects of any kind.