When animals engage in contests over resources, for the majority of species ‘fights’ are usually settled by some form of communication rather than injurious combat. Nevertheless, restrained fighting has favoured the evolution of weaponry that can be used to overpower or grapple with opponents during trials of strength. In addition, animals may also produce agonistic signals to advertise their fighting ability, the production of which are limited by physical constraints or energetic costs, thus honestly reflecting the physical attributes or stamina of the sender. Here we investigate the relative allometry of weaponry in two species of Madagascan hissing cockroaches (the wide-horned cockroach, Gromphadorhina oblongonota and the flat-horned cockroach, Aeluropoda insignis), both of which engage in territorial pushing contests involving the use of pronotal horns. We also investigate the production of the acoustic ‘hissing’ signal made by the cockroaches forcing air through specialised spiracles during aggressive displays. By combining contest observations with performance capacity measures of strength and resistance to exertion, and by using X-ray computed tomography to compare the structure of visible weaponry and the internal signalling systems used in aggressive behaviour, we reveal which factors are associated with resource holding potential in these two species.