Spider

Spiders are arachnid enemies that can be commonly found in most chapters. They move 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. They are also able to fit through small diagonal gaps.

Big Spider
Big 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 nearly triple the health of regular Spiders, and upon death, they will split into two regular Spiders. They will not split if they are killed by an explosion.

Swarm Spider
Swarm Spiders act much like normal Spiders but are much smaller and tend to come in much larger groups. They also hop and flash a bright yellow.

They are spawned by Eggy once it takes enough damage.

Strider
Striders are a type of spider that can only spawn in and. They glide on the water towards Isaac in short bursts and can swim across flooded gaps.

Trivia

 * The original Spider and Big Spider are first added to the original The Binding of Isaac game's DLC - Wrath of the Lamb.
 * Striders are named after the family Gerridae, also known as water striders.