The spider lair is a large structure made of spider silk (a solid block with a high strength and a silk-like texture) and filled with cobwebs, filled with spiders and cave spiders, and a few other spider variants. It also contains a large spider with a large abdomen. If you attack that spider, all spiders will be hostile to you for 40 days. The extra variants are:
The camel spider:
The camel spider is sand coloured, has 2 eyes, has long legs and has a set of jaws that open vertically. It can swim in sand. When it is hostile it raises its forelegs and opens its jaws.
The whip spider:
The whip spider has a raised abdomen, claws, very long forward-pointing forelegs with elbows, and a long whip on the end of the abdomen. It attacks with its whip. It can jump very far.
The harvest spider:
The harvest spider has no waist, is white, and has extremely long elbowed legs. It attacks by kicking you.