P. (P.) botosaneanui n.sp. has been collected in the interstitial habitat of the Caribbean shores on the eastern coast of Cuba. This note is divided into 6 parts: 1) the description by L. Botusaneanu of the stations where this species has been collected and data on the possibility for the specimens of this taxa to swim and to creep between the grains of sand; 2) the description and the affinities of the n. sp., which is closely related to bonetianus Silv. from Mexico; 3) the study of the male genitalia made possible the definition of 4 instars (male 1 to male 4) which seem common to all the Parajapyx; 4) the study of the 9 genitalia, which possess always the same number of phanera, whatever the size may be, does not permit the identification of instars; 5) the study of the armature of the internal margin of the cerci shows for the first time among the Parajapygidae a striking dimorphismus both between the sexless and sexed instars and between male and female, these latter retaining, when “adult”, an ornamentation identical to that of juvenil males: 6) the study of evolution and progressive complication of the chetotaxy from the sexless instars to the elder ones.