It appears that they do "spit" out flies pretty fast. Generally they've been in groups of 3 at my guys but the attack's easily avoidable if your guys have got any speed whatsoever.
There's no need to even risk your runners stopping and firing if you've got Snipers, just put them on a line of sight out of range of the Bellows and use your runners as the snipers "eyes".
And this can be done without the Enhanced Plasma Rifles I don't have yet
The only times I've ever had problems with Deathbellows are when:
1) A Chryssalis suddenly appears and zaps my guys within Deathbellows range....god I hate them
2) I first saw them and thought they were as pathetic as most of the other transgenants...(oops ok ??? )
3) Crossfire....which generally doens't happen if you run back the way you came away from the Deathbellows (unless you wre running from something else of course :devil: )