The sensory fields of four Wenlock trilobites Proetus (Proetus) concinnus, Warburgella (Warburgella) stokesii, Warburgella (Warburgella) scutterdinensis, and Harpidella (Harpidella) maura are described and found to be species specific. Certain features, such as the small cell polygon size, and prismatic layer comprising about one tenth of the cuticle thickness are characteristic of Order Proetida. Intraspecific variation is related to ontogenetic development, most markedly in the field of the free cheek of P. (P.) concinnus. The proetid occipital pustule may have functioned as a light-sensitive organ. The tropidium W. (W.) stokesii is a ridge with a central canal or groove, rather than a tubular structure, and probably contained a sensory receptor rather than a circulatory system. Despite microstructural distribution being species specific, it is of limited practical value in taxonomy as the fine details are easily lost during diagenesis.
