There are three insects in SLASH'EM, all represented by a, which possess the ability to multiply without the aid of water or other external sources. These are the giant tick, the giant flea, and the giant louse. All normally only appear in the Lawful Quest, and none is particularly much of a threat, even in the large groups they can create.
Oddly enough, a giant tick can grow up into a giant flea, and a giant flea into a giant louse, even though in real life they are all different species.
The mechanics of their multiplication is similar to gremlins, in that each subsequent monster is created with half the hit points of the previous one. However, the maximum HP remain the same. None of these insects can use their multiplication attack when their HP is at 1. However, as they heal, they can start to multiply again. In this manner, they can slowly multiply indefinitely after a quick initial burst of multiplication. Note that the multiplication only occurs if the monster attacks you. Pets, monsters attacking pets, and conflicted monsters will not create more insects.
The new insect created from multiplication will have the same name (if any) as the old one. This can occasionally produce amusing results, particularly when polymorphing shopkeepers.
Since all the monsters created will have 1 HP for a while after the initial spasm of multiplication, it makes these insects almost a non-threat. None of them can deal any appreciable amount of damage, and one hit from anything will kill them. The biggest danger is that they might heal and start reproducing again; for this reason, it is worth destroying them as soon as you see them simply to avoid having them fill up the level and impeding your movement.
Someone looking to permanently banish the Riders by filling up the Astral Plane might consider reverse genociding giant lice. (Optionally, a wand or spell of (extra) healing will make it go faster.) This is one of the easiest ways to fill the level up. Just be sure to have a way to kill them quickly enough to move at all, and either have enough armor to survive their attacks or preferably a permanent source of Elbereth.
Lacking hands, these insects will not produce death drops. Thus, there is little point in farming them, except possibly as a source of corpses to sacrifice at an altar.
Note that a chameleon which polymorphs into such an insect *will* gain its multiplication ability: it will produce other chameleons which are temporarily morphed into giant fleas, but which can then morph into anything else. This can be exceedingly dangerous.
- mon.c in SLASH'EM 0.0.7E7F2, line 2095: All monsters with no hands never produce death drops in SLASH'EM, likely to prevent things like pudding farming