We have investigated ultrastructural changes in the integuments of larval–adult and larval–pupal intermediates produced by exogenous application of juvenile hormone (JH) analogues in Pyrrhocoris apterus (Hemiptera), and Galleria mellonella and Manduca sexta (Lepidoptera). Ultrastructural analysis of the epidermis of these intermediates always revealed the presence of only two types of epidermal cell, which produced morphologically perfect cuticles of the previous and future developmental stages. There were no intermediate cuticles at the level of individual cells. It has been determined that a single epidermal cell constitutes the lowest elementary unit in the perception and realization of the developmental messages conveyed by JH to its target tissues. Further investigations revealed that the responses of individual epidermal cells to JH were strictly autonomous and qualitative, i.e. they were executed according to the ‘yes–or–no’ or ‘all–or–none’ rule. The neighbouring epidermal cells could realize independently, side–by–side, the quite dissimilar +JH (somatic growth) or –JH (metamorphosis) developmental programmes, although each of them formed biochemically, functionally, and ontogenetically different structures. The qualitative on– and off– signal given by JH for induction of the stationary (plus;JH) developmental cycle was limited to relatively short, genetically determined, and stage–specific developmental periods of cellular susceptibility to JH. The mosaic mixtures of the heterochronic, larval–pupal or adult epidermal cells, which we found in different proportions on the bodies of the intermediates, revealed two variable, development–related factors: (i) the presence or absence of a minimum effective concentration of JH, and (ii) positive or negative sensitivity of a particular epidermal cell to JH.