Which arthropod has cerci on its abdomen that differ between males and females?

The correct answer is earwigs, which are characterized by the presence of cerci on their abdomen that show sexual dimorphism. In earwigs, males typically have larger and curved cerci, whereas females have smaller, straighter cerci. This physical differentiation is crucial for mating behaviors, as it can be involved in grasping during reproduction.

Ants lack cerci altogether, having evolved a cast system and other morphological adaptations that make their reproductive roles more about the colony structure than individual traits like cerci. Millipedes, on the other hand, also do not have cerci; they possess a different anatomical structure meant for locomotion and defense. Beetles have cerci too, but they are generally less distinct between the sexes compared to earwigs, where the differences are notable and play a significant role in their reproductive process. Thus, earwigs exemplify the unique trait of having differentiated cerci based on sex.