Cut the avocados in half, remove the pits, and scoop the flesh into a mixing bowl.
Add the lime juice and salt, then mash with a fork until mostly smooth but still slightly chunky.
Stir in the red onion, tomato, and cilantro until evenly mixed.
Spread the guacamole onto a serving plate in an even layer.
Spoon the sour cream into a small zip-top bag, snip off one corner, and pipe three or four circles on top of the guacamole. Drag a toothpick from the center outward through the circles to create a web pattern.
Slice four olives in half lengthwise for the spider bodies. Cut the remaining olives into thin strips to use as legs. Arrange the olive pieces on the guacamole to form spiders.