Ecological influences on the brain of spiders

<p dir="ltr">This thesis aimed to study the impact of spider ecology on their brain size and structure. Spiders are increasingly becoming popular research models as their diverse ecology and behaviour invites comparative studies. We embarked on developing appropriate methods to image the central nervous system, brain and brain structures by applying X-ray microtomography (microCT). The intent was to consolidate current knowledge and guide researchers. We implemented and evaluated three different preparation and imaging techniques with modifications on a large set of over 200 individuals from four families and 12 species, substantially ranging in prosoma length (1.25 mm - 13.33 mm). The PTA (Phosphotungstic acid) method was the most successful and consistent method we recommend to other researchers. We applied this imaging technique to two distinct ecologies, ant-mimicking and social spiders, with specific predictions on how their lifestyle might impact the size and arrangement of the brain. Ant-mimicry in spiders is very common and can result in considerable morphological modifications, with constrictions on the prosoma that emulates the head and thorax of ants. To understand and quantify this impact, we compared a highly convincing myrmecomorphic (ant-mimicking) jumping spider, <i>Myrmarachne smaragdina</i>, with a non-mimicking, but closely related species using microCT. As expected, the CNS of ant-mimic spiders was significantly smaller than non-mimic spiders, but the higher integration system (arcuate body) was relatively larger. In the next chapter, we investigated how the social environment (living in large family groups or solitary) relates to brain size and structure. We used microCT imaging of two families (Sparassidae and Thomisidae) with representatives of social and solitary species. Overall, we found limited support for the social brain hypothesis, which predicts that social species will have larger brain (or part of the brain) structures, as this would be required to facilitate social behaviour. Only sparassids but not thomisids followed this pattern, possibly because they live in much larger groups. Social thomisids had larger visual neuropils, likely related to other aspects of ecology. While brain size is a key trait of interest for evolutionary and neurobiological researchers, it is crucial to relate brain size to the number of neurons to understand information processing and cognitive abilities. In chapter four, we applied the Isotropic Fractionator (‘the brain soup’) method to estimate neuron numbers. We succeeded in applying the brain soup method in spiders for the very first staining neurons. Our next steps are to quantify neurons and relate neuron numbers to brain volume and, eventually, to behaviour. In the final thesis chapter, I presented information on the social structure of <i>Delena cancerides </i>(social huntsman spiders) and their social behaviour. I found seasonal variation in the complexity of the colony, with multiple adult females, males, and juveniles present at certain times of the year. Furthermore, social huntsman spiderlings regularly share large prey and show little aggression and cannibalism. I discussed this information relative to the classification of their social status in literature. In summary, my thesis research establishes an excellent basis for experimental and comparative work on the neurobiology of spiders.</p>