Arthropods have small but sophisticated brains which have enabled them to adapt their behavior to a diverse range of environments. The enormous evolutionary success of arthropods in terms of species richness and diversity depends on the sophistication of their brains. Advances in neurobiology have clarified some of the sensory and motor mechanisms of the arthropod brain, but the basic rules of computation underlying the central functions of the arthropod brain remain unknown. Consequently, it is not known how the basic design of the arthropod brain differs from, or is analogous to, that of other animals, especially mammals. In this report, we argue that characteristic features of the arthropod “microbrain” can be ascribed not only to the limited number of its constituting neurons but also to the optimization to life with a small body.