The morphological ontogeny of Platynothrus coulsoni sp. nov. is described and illustrated. This species is the most similar to P. quadristriatus Hammer, 1958 as adult, but differs from it mainly by having more longitudinal ridges on the notogaster and fewer genital setae. Most prodorsal and gastronotal setae of the larva of P. coulsoni are of medium size and smooth, except for barbed h₁, and in the nymphs these setae are relatively short. In all instars, one pair of hypostomal setae m is present, and palp femur has one seta. The setation of leg setae of P. coulsoni is generally similar to that of P. punctatus (L. Koch, 1879), but differs from it mainly by having more setae on tarsi I–III of deutonymph and tritonymph, and all tarsi of adult.