Spiders move very quickly and will go in all directions randomly, making them unpredictable. However, if they have a direct line of sight on Isaac, and are close enough, they will move towards him.
Due to their small size, they can even fit between the corners of some rocks. As such, it's best to stay as far from them as possible.
Besides naturally spawning in some rooms, Spiders are also spawned when fighting Sacks, Widow, The Husk, The Wretched, Super Sloth (2 at once) and when destroying the smallest part of the Teratoma (also 2 at once). Outside of combat, two spiders can occasionally spawn from a Red Chest and will always be spawned if you blow up a Devil Beggar.
B. Spiders (Butt Spiders) look like regular spiders, but with a large black abdomen attached. They behave in the same way as the regular spiders, having the same movement speed and erratic behavior. However, they have almost double the health of regular spiders, and on death, they will split into two small spiders that have decreased health. The smaller Spiders will not spawn on a B. Spider's death if they are killed with a large explosion.
Trites are even larger spiders, with an upside down face. They possess large fangs and four eyes that are leaking blood. They are functionally identical to the F. Hoppers, randomly jumping great distances around the room, although they will move towards the player if they're too close.
It is entirely possible to move safely beneath them while they're airborne. They can still receive damage from tears while in the air, although it's best to target them just after they landed, because they will wait about 1.5 seconds before the next jump.
They are commonly found in the Cellar and Catacombs, and rarely in Necropolis. Some Hoppers are sometimes replaced with these during Challenge Room fights or Greed encounters. They're also spawned by The Wretched during her boss fight.
- All spiders only have 4 legs. Edmund has acknowledged this by saying he doesn't know how spiders work. http://www.formspring.me/EdmundM/q/347549200755476067
- A Trite's Face resembles Isaac's appearance when upgraded with Mutant Spider, only turned upside down.
- If Isaac gets killed by a Trite, Isaac's Last Will will state that it was a Hopper instead. This hints the Trite has the same coding as a flaming hopper.
- The name Trite was first used by a monster from the Doom franchise.
- Both Wrath of the Lamb's and Doom's Trite are inspired by a monster of John Carpenter' "The Thing". This was confirmed by Edmund on formspring. (Link needed)
- Spiders don't have a "passive" variant like flies do (the black fly).