Material detectable with antisera to the pigment-dispersing hormone (PDH) is regarded as a component of the circadian clock residing in some insects in the optic lobe. This paper demonstrates that the position of the PDH-positive neurones and the course of their processes are similar in all representatives of the insect cohort Polyneoptera. A basic morphological pattern, which includes the proximal frontoventral (Pfv), distal posteriodorsal (Dpd) and posterioventral (Dpv) clusters of PDH-positive neurones, was found in the examined species of locusts, crickets, walking sticks, cockroaches, earwigs and termites. The Pfv cluster is located close to the accessory medulla and usually consists of a set of smaller and a set of larger perikarya. The Dpd and Dpv clusters occupy a dorsal and a ventral position, respectively, at the distal edge of the medulla. These clusters are lacking in stonefly and praying mantid species. The fan-like arrangement of PDH-positive fibres within the frontal medulla face (the locusts and the praying mantid have an additional, smaller fan on the posterior medulla face) is another characteristic feature of Polyneoptera. One (two in the locusts and the praying mantid) nerve bundle runs from the optic lobe to the lateral protocerebrum where it ramifies. One branch gives rise to a fibre network frontally encircling brain neuropile in the area of mushroom bodies. One thin fibre in the crickets and the earwig, and several thicker and anastomosing fibres in the other insects, connect the brain hemispheres. The arrangement of other PDH-positive structures specifies taxa within Polyneoptera. Specific features comprise the presence of PDH-positive perikarya in protocerebrum (walking stick and termite), deutocerebrum (cricket, walking stick, and one cockroach species), tritocerebrum (another cockroach species), and the suboesophageal ganglion (cricket, walking stick and termite). In the walking stick and the termite, PDH-positive fibres pass from the cephalic to the frontal ganglion and from there via the recurrent nerve to the corpora cardiaca where they make varicosities indicative of peptide release into the haemolymph.