Head capsule width was a reliable indicator of larval instar in a strain of Michigan codling moths, Cydia pomonella. Head capsules were 0.33, 0.50, 0.82, 1.18 and 1.55 mm in width from first to fifth instar respectively. Development as measured by days and degree days was much more variable than head capsule width in estimating larval instars.
Weitzner, Peter and Whalon, Mark E.
"Head Capsule Widths as an Indicator of the Larval Instar of Codling Moth (Lepidoptera: Olethreutidae),"
The Great Lakes Entomologist, vol 20
Available at: https://scholar.valpo.edu/tgle/vol20/iss3/8