Predation is a major factor influencing the fitness and life history of animals. Two key traits affecting prey survival are body size and coloration. Sepsis thoracica males display a sigmoid relationship between these two traits, defining a size threshold above which investment in melanin drastically drops, producing small melanic (black) or large amber morphs. In trying to understand the evolution of this rare dimorphism, we performed laboratory predation experiments to estimate the intensity of adult viability selection exerted by various arthropod predators (bugs, flies, spiders) on male body size and coloration. Selection was performed against two different backgrounds mimicking the natural habitat (dung and grass) in which the camouflage and/or warning effect of the morphs should vary. Body size was mainly under positive selection (larger survived better), which overpowered selection on coloration and varied somewhat among predator species but not backgrounds. No disruptive selection was found, nor did selection change the sigmoid relationship between the two traits. We conclude that, for this fly, predator evasion and escaping skills determined by body size are more effective against invertebrate predators than its conspicuousness determined by coloration, contrasting what has been found for vertebrate predators, where prey coloration is important and negative selection on size dominates. Because arthropod predators have strong effects on insect populations, the positive directional selection imposed by invertebrate predators is likely an important force driving the evolution of body size in S. thoracica and insects in general.