WHY CATERPILLARS DO NOT GROW SHORT AND FAT

The integument of a caterpillar consists of a cuticle that is structurally isotropic in the plane of the surface, overlaying an epidermis and basal lamina, Between ecdyses the cuticle stretches so that the proportions of caterpillars are about the same at the end of a stadium as at the beginning. This introduces a paradox, since under uniform internal hydrostatic pressure, the hoop stress is twice the axial stress. Stress in the integument of a caterpillar inflated by compressed hemolymph would be twice as great in the circumference as in the axis, causing it to become progressively fatter in proportion to its length as it grows. This does not happen because axial pleats allow caterpillar cuticle to extend more easily in the axis, allowing uniform growth in spite of the greater hoop stress from internal hydrostatic pressure.