I'm gonna put this in here because I can't find a specific Bestiary board for new monsters.
Shrewflies: Lvl 1A, Base Move (Flying) 150', AT 1, MS/AQ VF/VF, Hits 5A
Attacks: Bite, Max "Tiny", +50. Add +10 to OB and increase Max result by 1 step for every 100 shrewflies attacking the same target in a given round.
# Enc.: 1 or 5,000 - 50,000
Shrewflies are a species of biting fly, like horseflies. But shrewflies are nearly the size of hummingbirds. They inhabit foothills and high plains, where most of the larva lie dormant for nearly the entire year. Occasionally you'll see a few around concentrations of livestock, but only a few. That is, for most of the year.
For the first two weeks in June, shrewflies swarm. This is when they look for mates outside their swarm, and lay eggs. They lay their eggs in rotting meat when they can, but any rotting material will do. They are entirely focused on eating, mating and laying eggs, as they will all die in the next two weeks. During this time they will strip any food in any form utterly bare, but will always take meat first if they have the choice. Two weeks later, the females who have mated lay their eggs, and all the adults die within a day or two. The grubs will mostly live underground until next summer. If a grub is exposed to the surface and can't burrow back under, it will mature to adulthood within 2 days, and it will die in 2 weeks.
Bats are one of their very few natural enemies. Most are not agile enough flyers and skilled enough hunters to treat a swarm of shrewflies the way other predators would treat a herd of grazing animals.
Basically a cross between locusts and piranha.