Daddy Long Legs

Daddy Long Legs are a significant midgame threat. They appear as floating bulbous masses covered in X's with long tentacles, and will devour any moving creature they can, including slugcat. Daddy Long Legs are all blind, however, and will not aggressively pursue the player unless some stimulus clues them to their location. While brown ones can only feel their way around, blue ones can hear slugcat if the player is not careful.

If the player is grabbed by a tentacle, slugcat will be unable to struggle free. The player can free themselves by throwing a rock or a spear at the tentacle grabbing them. If the player is unable to free themselves, the Daddy Long Legs will slowly drag its prey to its core and swallow it whole. Daddy Long Legs can be killed by dealing significant damage to the core, but this is a difficult prospect since its tentacles will tend to get in the way. Sneaking by or outrunning these threats is typically the optimal solution.

Subtypes
There are five distinct varieties of Daddy Long Legs:

Brown Daddy Long Legs
Also known as "Brother Long Legs", these less threatening varieties are found in central Garbage Wastes and will likely be the first variety new players see. They are slower, smaller, and possess fewer tentacles than their blue brothers. They appear to emerge from a central pool of water, but have no difficulty navigating through midair. They are completely blind and have no sense of hearing, simply wandering aimlessly through the air until they touch something, which makes avoiding them relatively easy, but their long reach and high health pool still makes them a formidable threat. Since they can't hear like the blue ones they have a complete dependence on the sense of touch, so they will keep moving and they will patrol the place touching every surface.

Blue Daddy Long Legs
The main threat, Blue Daddy Long Legs are larger and more aggressive than their brown counterparts. They are found in The Exterior and in Five Pebbles, and will likely be a significant obstacle to the player in these locations. Unlike their brown brothers, these have a keen sense of hearing. Should the player land on the ground after a jump or throw something at a wall, a blue flash will be given off by its core indicating it heard the noise, and it will chase the noise down. The blue ones are very  lazy , they can stay without moving for 2-5 minutes straight if they don't touch or hear something, and if they do hear something then sometimes they will just  send  a tentacle to the place where the sound came from without moving the core.

Disguised Blue Daddy Long Legs
This is just a regular Daddy Long Legs, but instead of being blue it has the Brother Long Legs texture. It's basically a Brother Long Legs that is not deaf and is super strong. They can be found in the garbage wastes region if you play as the hunter. They are very dangerous because they look like a harmless Brother Long Legs but they have the strength, speed and ability to hear of a Daddy Long Legs.

Proto Daddy Long Legs
Found exclusively in Five Pebbles: Unfortunate Development, these stationary threats cover certain walls in the area. Although not mobile, they may still possess short ensnaring tentacles, and will devour slugcat should they stray too close. Escape is not a likely option once trapped by these, so the player should make every effort to avoid them. However, they do offer one advantage to slugcat: the long, connecting tentacles between patches of Proto Daddy Long Legs. These, whether in gravity or not, are not dangerous and offer something for slugcat to climb on to when nothing else is available. They can also be used to propel slugcat through zero-gravity when the slugcat is moving slowly through the air. These tentacles make traversing Five Pebbles: Unfortunate Development much easier. Proto daddy long legs are not considered creatures, and as such cannot be killed.

Proto Brother Long Legs
Similar to the Proto Daddy long legs these are brown and they can be found in the garbage wastes region only if you play as the hunter. They are weaker than the blue ones.

Tips

 * When a Daddy Long Legs hears a noise, the X symbols on its core will light up slightly and momentarily project blue lines towards the source of the sound. This is useful for figuring out where it is likely moving towards.
 * Explosive spears present a somewhat reliable way of killing Daddy Long Legs. A direct hit to their core from an explosive spear will kill them instantly.
 * Throwing a rock at a tentacle will stun it briefly, while throwing a spear at it will permanently disable it. Either attack will alert the Daddy Long Legs to your presence, however, so the player must be prepared to move quickly if they do so.
 * Throwing a rock at a wall is an easy way to create a distraction. The noise is typically loud enough that the Daddy Long Legs will go to that location and search around it for a short time. Be wary not to hit a tentacle when doing so, however, as this may simply cause the Daddy Long Legs to find you more quickly!
 * If all of a Daddy Long Legs' tentacles are disabled via spear, it will fall to the ground and writhe uselessly. Be aware that its core can still eat you.
 * Climbing or grabbing poles, walking on flat terrain, and crawling through tunnels will not create any noise. Landing on the ground from even small jumps will create noise, as will most thrown objects.
 * Especially loud noises such as explosions appear to temporarily deafen a daddy long legs.
 * Despite their large size, Daddy Long Legs are actually able to traverse tunnels, although this thankfully seems to be a rare occurrence, and instead they will usually just give up chasing the player when escaping through a tunnel.

Trivia

 * Lizards, scavengers, and other beasts seem to regard the Daddy Long Legs as significant threats, and will tend to run away from them.
 * Daddy Long Legs were apparently created by Five Pebbles, before growing quickly out of hand following an unfortunate incident.
 * In hard mode, Garbage Wastes suffers from a slightly larger infestation, including a few brown proto daddy long legs found mostly on the ceiling near their spawning pool, and a much larger brown brother long legs (that resembles a daddy long legs except brown in color) that can hear you.
 * Developer commentary on the design of these creatures and monster kelp can be found here.
 * The sandbox unlock for blue Daddy Long Legs is found on Monk and Survivor difficulties in Unfortunate Development. Brown Daddy Long Legs can be unlocked on normal difficulty by diving into their spawning pool in Garbage Wastes.