Well that's a surprise. Don't understand why Tesla would decide to offer a new 21" wheel style, and only available after you buy the car (it's not shown in the Design Studio).
The name "Arachnid" strikes me as odd: the wheel has five pairs of spokes, for a total of 10. All arachnids (spiders) have 8 legs.
As to the aesthetics of the wheel, not to my taste.