Maguuma Spiders often descend to the battle from trees. Because of that they may cause trouble by suddenly increasing the number of enemies. They can also inflict Poisoned and Crippled conditions.
Spider Queen is the queen of Maguuma Spiders, players can see her in the second cutscene of The Wilds mission. It is trying to devour the very last remaining Captured Chosen when a Shining Blade Scout comes in and eventually they kill each other. As the Spider Queen dies, many more spiders will begin appearing everywhere around to avenge her death.
As she is killed during a cutscene, she will never drop any items for the party.
Notes
The Spider Queen is very likely also responsible for killing all the Chosen who are already dead when the players arrive.
Since she only appears in a cutscene where she is killed, players can't actually interact with her in any way except for exploiting her corpse.
Strangely, the Spider Queen actually has a lower level than all other regular Maguuma Spiders (as discovered in Hard Mode).
Anomaly! Due to her use of Apply Poison in Hard Mode, the cutscene will appear strange as the camera changes view too quickly while the Queen is still casting her preparation.
Apply Poison (Hard Mode only)