A general description, with illustrations, of the larva of Tarsocheylidae, based on material of Tarsocheylus Berlese and Hoplocheylus Atyeo and Baker, is presented for the first time. In light of observations on this instar, the homology of structures and application of standard systems of notation of GRANDJEAN, especially that for the leg tarsi, are determined more precisely than was formerly possible. A previous interpretation, that the third opisthosornal dorsal plate EF in postlarval instars of tarsocheylids is compounded from the tergites of two segments, is confirmed by the component plates E and F remaining separate in the larva. Observations on the ontogeny of various structures, particularly on the changes between larva, protonymph, and deutonymph, treat the opisthosomal dorsum and caudal bend, the proral and tectal setae of tarsi I to IV, and the denudation of setae on leg IV of the protonymph. The significance of these ontogenetic phenomena for cladistic analysis and classification of the Tarsocheyloidea and the Heterostigmata is discussed. Diagnoses are provided for the family Tarsocheylidae and its genera, based on the larva and nymphal instars, in comparison with the subcohort Raphignathae, the Heterocheylidae, and the Tarsonemina.