The first speargun I ever used was a single stage speargun my grandpa had made - the shaft had a hole in it & the trigger was directly connected to a pin which would pull out and release the shaft. No power & wildly inaccurate... but I was hooked!
The first time out, I shot 2 mamo (Abudefduf abdominalis) - roughly 3-4" a piece. 
Since then, I've moved on to tinkering with all the latest modern spearguns... but single stage triggers always have a special place in my heart and the nostalgia makes me smile. 
The most common of the single stage trigger spearguns are the old Polynesian thumb trigger guns. On a January 2008 dive, I came across a couple divers from Palau that had two adaptations of these guns. Homemade guns, they had obviously taken much pride in making them - one thumb activated & one converted into a bottom trigger. Both had makeshift line releases, held together with innertube rubber, & varnished/epoxied. They were equipped with one hefty set of bands and a very long & thin custom 3/16" shaft.
With their permission, I took a few pictures. Found them while cleaning up my computer & thought I'd share.
Gun 1
Traditional thumb trigger speargun - innertube rubber takes place of a spring to keep tension on shaft:
Flipped upside down to show how the simple line release pin is actuated:
Gun 2
A bit fancier gun - under-handle trigger, with innertube & string texture grip:
Picture showing the line release setup & the way the trigger is operated:
Top of gun:
Stiff bands with independent wire wishbones:
Ornamental band attachment & line anchor:
