Factorio version 0.16.51/0.17

worm-autoplace.lua

-- data for 0.16.51, functionality in 0.17 explained in comment below
["big-worm-turret"] = {
      autoplace = {
        control = "enemy-base",
        force = "enemy",
        max_probability = 0.2,
        order = "b[enemy]-b[worm]",
        peaks = {
          {
            influence = 0,
            richness_influence = 40,
            tier_from_start_max_range = 68,
            tier_from_start_optimal = 34,
            tier_from_start_top_property_limit = 34
          },
          {
            influence = -1,
            starting_area_weight_max_range = 2,
            starting_area_weight_optimal = 1,
            starting_area_weight_range = 0
          },
          {
            influence = 0.1,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5
          },
          {
            influence = 0.2,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5,
            tier_from_start_max_range = 68,
            tier_from_start_optimal = 34,
            tier_from_start_top_property_limit = 34
          },
          {
            influence = 0.005
          }
        },
        random_probability_penalty = 0.05,
        richness_base = 0,
        richness_multiplier = 1,
        sharpness = 0.4
      },
      build_base_evolution_requirement = 0.5
},
["medium-worm-turret"] = {
      autoplace = {
        control = "enemy-base",
        force = "enemy",
        max_probability = 0.2,
        order = "b[enemy]-b[worm]",
        peaks = {
          {
            influence = 0,
            richness_influence = 40,
            tier_from_start_max_range = 52.399999999999999,
            tier_from_start_optimal = 26.199999999999999,
            tier_from_start_top_property_limit = 26.199999999999999
          },
          {
            influence = -1,
            starting_area_weight_max_range = 2,
            starting_area_weight_optimal = 1,
            starting_area_weight_range = 0
          },
          {
            influence = 0.1,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5
          },
          {
            influence = 0.2,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5,
            tier_from_start_max_range = 52.399999999999999,
            tier_from_start_optimal = 26.199999999999999,
            tier_from_start_top_property_limit = 26.199999999999999
          },
          {
            influence = 0.002
          }
        },
        random_probability_penalty = 0.05,
        richness_base = 0,
        richness_multiplier = 1,
        sharpness = 0.4
      },
      build_base_evolution_requirement = 0.3
},
["small-worm-turret"] = {
      autoplace = {
        control = "enemy-base",
        force = "enemy",
        max_probability = 0.2,
        order = "b[enemy]-b[worm]",
        peaks = {
          {
            influence = 0,
            richness_influence = 40,
            tier_from_start_max_range = 42,
            tier_from_start_optimal = 21,
            tier_from_start_top_property_limit = 21
          },
          {
            influence = -1,
            starting_area_weight_max_range = 2,
            starting_area_weight_optimal = 1,
            starting_area_weight_range = 0
          },
          {
            influence = 0.1,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5
          },
          {
            influence = 0.2,
            noise_layer = "enemy-base",
            noise_octaves_difference = -1.7,
            noise_persistence = 0.5,
            tier_from_start_max_range = 42,
            tier_from_start_optimal = 21,
            tier_from_start_top_property_limit = 21
          },
          {
            influence = 0
          }
        },
        random_probability_penalty = 0.05,
        richness_base = 0,
        richness_multiplier = 1,
        sharpness = 0.4
      }
}

Q:
What are the rules governing how worms are spawned and upgraded during enemy expansion? What does richness determine?
A:

• Enemy expansion randomly chooses an entity that has the enemy-base autoplace control and that matches the evolution requirement. No richness involved there.
  • Worms and spawners have the enemy base autoplace control.
  • build_base_evolution_requirement = 0.5 is the evolution requirement of the big worm, so it can only be created if the evolution is above 0.5. This property be found on medium and big worm.
• Upgrading of existing structures like worms isn't a thing. Once a spawner or worm is placed, it's there to stay until the player kills it.

Q:
What about map generation? We don't see big worms sometimes in biter nests near the starting area. How is that controlled if not by richness?
A:
See how tier_from_start is different for the different worms? That right there controls it. That and the last influence on the small worm are the only differences of the autoplace between the worms, further enforcing that richness does not affect the worms.

With 0.17, the above autoplace controls are outdated, but the general principle still stands:

• Medium worms need an evolution factor of 0.3 to be spawned during expansion. Big worms need a factor of 0.5 and behemoth worms a factor of 0.9.
• The new autoplace controls use a "distance_factor" for the spawning, this is 0 for the small worm, 2 for the medium worm, 5 for the big worm and 8 for the behemoth worm. Just like in 0.16, this is the only difference between the different tiers.