Open Access

Philip Steinhoff

• The central nervous system (CNS) is the integration center for the coordination and regulation of all body activities of animals and the source of behavioral patterns, behavioral plasticity and personality. Understanding the anatomy and the potential for plastic changes of the CNS not only widens the knowledge on the biology of the respective species, but also enables a more fundamental understanding of behavioral and ecological patterns. The CNS of species with different sensory ecologies for example, will show specific differences in the wiring of their CNS, related to their lifestyle. Spiders are a group of mesopredators that include stationary hunting species that build webs for prey capture, and cursorial hunting species that do not build capture webs. These distinct lifestyles are associated with major differences in their sensory equipment, such as size of the different eyes. In this thesis, I aimed to answer if a cursorial mesopredator would change its behavior due to different levels of perceived predation risk, and if this behavior would be influenced by individual differences (chapter 1); how the visual pathways in the brain of the cursorial hunting jumping spider Marpissa muscosa differs from that of the nocturnal cursorial hunting wandering spider Cupiennius salei (chapter 2); to what degree the visual systems of stationary and cursorial hunting spiders differ and whether CNS areas that process vibratory information show similar differences (chapter 3); and finally if the CNS in stationary and cursorial hunting spiders shows different patterns of neuroplasticity in response to sensory input and deprivation during development (chapter 4). In chapter 1, I found that jumping spiders adjust their foraging behavior to the perceived level of risk. By favoring a dark over a light substrate, they displayed a background-matching strategy. Short pulses of acute risk, produced by simulated bird overflights, had only small effects on the behavior. Instead, a large degree of variation in behavior was due to among-individual differences in foraging intensity. These covaried with consistent among-individual differences in activity, forming a behavioral syndrome. Our findings highlight the importance of consistent amongindividual differences in the behavior of animals that forage under risk. Future studies should address the mechanisms underlying these stable differences, as well as potential fitness consequences that may influence food-web dynamics. In chapter 2, I found that the visual pathways in the brain of the jumping spider M. muscosa differ from that in the wandering spider C. salei. While the pathway of the principal eyes, which are responsible for object discrimination, is the same in both species, considerable differences occur in the pathway of the secondary eyes, which detect movement. Notably, M. muscosa possesses an additional second-order visual neuropil, which is integrating information from two different secondary eyes, and may enable faster movement decisions. I also showed that the tiny posterior median eye is connected to a first-order visual neuropil which in turn connects to the arcuate body (a higher-order neuropil), and is thus not vestigial as suggested before. Subsequent studies should focus on exploring the function of the posterior median eyes in different jumping spider species, Foraging behavior, neuroanatomy, and neuroplasticity in cursorial and stationary hunting spiders as they show considerable inter-specific size differences that may be correlated with a differing connectivity in the brain. In chapter 3, I described all neuropils and major tracts in the CNS of two stationary (Argiope bruennichi and Parasteatoda tepidariorum) and two cursorial hunting spiders (Pardosa amentata and M. muscosa). I found major differences in the visual systems of the secondary eyes between cursorial and stationary hunting spiders, but also within the groups. A. bruennichi has specialized retinula cells in two of the secondary eyes, which connect to different higher-order neuropils. P. tepidariorum has only a single visual neuropil connected to all secondary eyes, and lacks recognizable mushroom bodies. The neuroanatomy of CNS areas that process mechanosensory information on the other hand, is remarkably similar between cursorial and stationary hunting species. This suggests that the same major circuits are used for the processing of mechanosensory information in both cursorial and stationary hunting spiders. Future studies on functional aspects of sensory processing in spiders can build on the findings of our study. In chapter 4, I found that developmental neuroplasticity in response to sensory input differs between a cursorial (M. muscosa) and a stationary hunting spider (P. tepidariorum). While deprivation of sensory input leads to a volume increase in several visual and mechanosensory neuropils M. muscosa, neither sensory deprivation nor sensory enrichment had an effect on the volume of neuropils in P. tepidariorum. However, exposure to mechanical cues during development had an effect on the allometric scaling slope of the leg neuropils in both M. muscosa and P. tepidariorum. Future studies should focus on the genetic and cellular basis of developmental neuroplasticity in response to sensory input in order to explain the observed patterns.