It has been proposed several times to put up water barrels under the gutters to collect water for plants.
Just putting it up formally.
The most logical(?) method to keep the bugs at bay is to use a water trap contraption (its not a true water trap, but its the only phrase I could think of).
So, the top of the barrel is in fact covered with the appropriate lid. A small opening is cut in the top to allow the gutter pipe to drop into the barrel.
The pipe is sealed to the barrel (nothing fancy here).
Next a short, small diameter second pipe escapes the top of the barrel to allow air out. It only need be a 4cm pipe.
The top will also need some sort of overflow, such that when the barrel is full, water goes off the side, just as if the barrel did not exist. This can be accomplished with a "T" directly above the barrel on the gutter pipe.
Note that the air escape pipe must be taller than this "T".
Finally, a standard hose tap is fit to the bottom of the barrel.
*elevate the barrel at least 6 inches off the ground, to assure easy access to the tap, and to provide some pressure in the line, even when the barrel is low.
*installing a 'level glass' is very easy. On the side of the barrel near the top and the bottom, fit right angle FTP to Hose barbs (on one side its a threaded pipe fitting, on the other its a hose barb connector.) Fit a clear rubber hose between the two barbs. The water level is easy to determine.
The Earthship Community folks in New Mexico have a cool way of building rainwater caches out of concrete and used cans/bottles.
Might be a nice way to recycle all of those beverage containers cluttering the shower off the kitchen!