The number, complexity, and arrangement of spider eyes vary across spider families and often correlate with behavior. In addition, the size and organization of the visual processing regions of the protocerebrum also vary (Long 2019). The structural and functional unit of the nervous system of spiders, like other animals, is a neuron. Additionally, the spider central nervous system is composed of discrete synaptic regions called neuropils and the inside of the neuropil is comprised of nerve fibers and glial components. The central nervous system of spiders is composed of two major regions called the supra-esophageal and sub-esophageal regions (Barth 2002; Strausfeld 2012). The supra-esophageal region is considered to be the brain of the spider and it consists of the protocerebrum and the deutocerebrum. The sub-esophageal region is comprised of tritocerebrum and leg ganglia (Strausfeld 2012). In addition, the sub-esophageal region contains afferents from all sensory systems except the eyes (Barth, 2002; Strausfeld, 2012). Moreover, this is the largest region of the spider central nervous system. For example, in Cupiennius salei, the sub-esophageal region makes up about 85% of the total central nervous system (Barth, 2002). The protocerebrum contains the optical lobes that receive information from the spider’s eyes. Among them, the most important are structures called the central body, proto-cerebral bridge and paired mushroom bodies. All of them receive information from various sensory and motor cells. The input of information into the protocerebrum from the rest of the central nervous system supports the idea that it serves as an integration center (Babu and Barth, 1984).