Spider Jockey

A spider jockey is the rare appearance of a spider being ridden by a skeleton.

Spiders and skeletons
A spider has a 1% chance to spawn with a skeleton rider. This means that a spider spawner, spawn egg, or the command can very rarely spawn a spider jockey.

Spider jockeys spawned in narrow enclosures can cause the skeleton rider to die of suffocation, due to the spider scaling the walls.

Using the summon command: will yield a spider jockey.

On hard mode, you have a better chance of spawning a spider jockey.

Strays, wither skeletons and cave spiders
A cave spider can spawn as a jockey, just as a spider can. If either type of spider is spawned in the Nether and occurs as a jockey, it has an 80% chance that its skeleton rider will be a wither skeleton. If either type of spider is spawned in a snowy biome with a view of the sky, and occurs as a jockey, it has an 80% chance that its skeleton rider will be a stray.

Baby zombies
Baby zombies can mount spiders and cave spiders. Specifically, baby zombies have a 15% chance to want to be a jockey, and if they do, then upon nearing the player and before attacking, they check for one of the following and mount it: adult chickens, adult ocelots, adult wolves, adult zombies, cows, pigs, sheep, spiders and cave spiders.

Drops
The skeleton and the spider each have separate drops when that part of the spider jockey is killed.

Their drops are no different than if the skeleton or spider were standing alone.

Behavior
The spider controls the movement, so a spider jockey will wander aimlessly in the day and only pursue the player at night or in dimly lit areas when the spider is hunting. The two creatures attack and take damage individually (i.e. if the spider is killed, the skeleton will continue to attack and move, and vice-versa). If a spider jockey jumps into water, the skeleton will jump off the spider.

The skeleton will turn and fire at the player as soon as it sees them no matter the time of day (although the skeleton will quickly burn in sunlight, leaving the spider unharmed). A spider jockey can attack two different players in SMP - this is caused by one player damaging one half of the spider jockey and not the other. If the skeleton shoots another hostile mob, the mob will focus its attack on the skeleton but not the spider being ridden, even after the skeleton is killed. If the skeleton on the spider jockey is shot by another skeleton, it will jump off and leave the spider to engage the skeleton that shot it.

A spider jockey can kill itself - sometimes the skeleton arrows will injure the spider and other times the spider's pouncing will run the skeleton into its own arrow.

When a spider jockey rides a minecart, the minecart will automatically accelerate.