A Spider in Antwar.io is a type of bug. It is a predator, which means it can hunt and kill Ants. They are one of the main sources of Super Food, the other being the Centipedes. The Spider also has two forms, Wolf Spider (yellowish brown) and Black Widow (dark brown), though each are the exact same statistics wise. The only difference are their game sprites.
The spider is a source of Super Food, which is essential to the success of a colony against other ants. When killed, they will drop six Meat and three Super Food. They don't supply as much Meat as the Spider Carcass, but their Super Food can give the colony an edge by allowing Soldiers and Majors to be created.
A Spider's 100 health makes it equally as tanky as a Worker. However, their 60 damage will kill a Nanitic in one hit and a Worker in two. This is why it is best to have at least six workers before going to war with a Spider; a Soldier and 2 Workers can kill it to. Their 5.25 makes them slower than Workers and Nanitics, but faster than Soldiers or Majors.
They spawn randomly around the map after five minutes. The delay is to allow the ant colonies time to grow, because just four Nanitics cannot kill a Spider. They spawn in all four Seasons except Winter. It should also be noted that they have a higher spawn rate than Centipedes early in the game.