1. Harker has reported (1960) that the locomotor rhythm in the cockroach is maintained only if a secretion from the corpora cardiaca is delivered to the sub-oesophageal ganglion via the pair of small nerves connecting these two organs. Roberts has counter-claimed (1966) that neither this nerve nor the corpora cardiaca themselves are concerned in the control of the rhythm.
2. It has been possible to confirm Roberts's findings that these nerves can be severed, and much (possibly all) of the corpora cardiaca removed, without affecting the periodicity of the rhythm.
3. Brief periods of enforced activity are shown to increase peak activity in the rhythm, whereas stress in the form of minor wounding depresses it.
4. Cockroaches are found to maintain rhythmicity after the almost complete elimination of the medial neurosecretory cells from their brains.
5. It is suggested that the differences between the views of Harker and Roberts concerning the influence of the corpora cardiaca may be explained in terms of the type of actograph used and the possibility that a cardiacum secretion controls the amount of activity but not its periodicity.