High trait variability among insects reflects a combination of intra- and inter-phenotype variations. Our aim was to assess if the trait distribution of body measurements can be more significantly influenced by sex (intraspecific variance) or by species (interspecific variance). To achieve this, we collected in Namibia tettigoniids belonging to two congeneric species of armoured ground crickets: Acanthoplus discoidalis (a significant pest in African croplands) and the long-legged Acanthoplus longipes. We measured in the field the total body length, the maximal pronotal width and length, and the femur and tibia lengths of the hind legs in 106 adults. We also derived the body mass from length and width values of the sampled specimens. No significant differences emerged in the two species by sex. A discriminant analysis clearly shows that at species level the locomotory traits as captured by tibia and femur lengths and the size traits as captured by body and pronotal lengths account for 99 % of the total variance and clearly separate this pest from its congeneric species. In essence, it is not primarily the body size that differentiates the two species, but rather the pronotum and hind leg larger sizes of A. longipes. Different eco-ethological requirements, like the peculiarity of the calling song and the movements within the vegetation (and the consequently needed energy), independently force these functional traits.