Evolution, discovery, and interpretations of arthropod mushroom bodies

Mushroom bodies are prominent neuropils found in annelids and in all arthro pod groups except crustaceans, First explicitly identified in 1850, the mus hroom bodies differ in size and complexity between taxa, as well as between different castes of a single species of social insect. These differences l ed some early biologists to suggest that the mushroom bodies endow an arthr opod with intelligence or the ability to execute voluntary actions, as oppo sed to innate behaviors. Recent physiological studies and mutant analyses h ave led to divergent interpretations. One interpretation is that the mushro om bodies conditionally relay to higher protocerebral centers information a bout sensory stimuli and the context in which they occur. Another interpret ation is that they play a central role in learning and memory. Anatomical s tudies suggest that arthropod mushroom bodies are predominately associated with olfactory pathways except in phylogenetically basal insects. The promi nent olfactory input to the mushroom body calyces in more recent insect ord ers is an acquired character. An overview of the history of research on the mushroom bodies, as well as comparative and evolutionary considerations, p rovides a conceptual framework for discussing the roles of these neuropils.