GURPSよろず

　Some life forms or machines are small enough that they effectively have 体力 0. A living organism weighing 0.25 ounce (0.016 lb.) would have 体力 0.5, rounding up to 1. Anything smaller would usually be unable to take effective physical action, not to mention being tiny (SM -8 for life forms). They would be unlikely to make suitable player-character races or interesting adversaries.
　There’s a way around this: treating a swarm of tiny entities as a single character with a 種族テンプレート. The primary trait for this is 「負傷耐性」 (「集合体」) (p. 43). Other traits depend on the size of the swarming entities. This represents being a swarm all the time. The ability to become a swarm is 「変身」 (Shapeshifting, pp. 38-39).

● Small entities down to SM -8 (from 3.5” for typical animals to 1.8” for cubical or spherical objects) have the 特色 「群れの体」 (p. 12). They normally have 体力 0.5 (rounding up to 1) or better; individuals can perform some effective physical actions.

● Tiny entities down to SM -20 (the size of a flea or a sand grain) have Body of Swarm and the modifier 「L-浸潤」 (p. 43). Individuals have negligible 体力.

● Microscopic entities (such as single cells, nanites, or fog droplets) are too small to be seen or to perceive the macroscopic world; treat them as constituents of a single 集合体 being, without 「群れの体」.
　The number of small entities that make up a swarm can be estimated as the square of (HP of swarm / HP of separate bodies). The HP of separate bodies should be less than 1/3 of swarm HP; any larger and there would be too few to make up a “swarm.” Tiny and microscopic entities don’t need to be counted or assigned an exact size.
　With 「L-散在」 (p. 43), a swarm can disperse its component bodies over a wide area. While dispersed, it’s effectively insubstantial.