Data from: Viability selection by invertebrate predators in the polyphenic scavenger fly Sepsis thoracica

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.