Small brains: Body shape constrains tissue allocation to the central nervous system in ant-mimicking spiders

<p>In Batesian mimicry, mimetic traits are not always as convincing as predicted by theory &ndash; in fact, inaccurate mimicry with only a superficial model resemblance is common and taxonomically widespread. The &lsquo;selection trade-offs hypothesis&rsquo; proposes a life-history trade-off between accurate mimetic traits and one or more vital biological functions. Here, using an accurate myrmecomorphic (ant-mimicking) jumping spider species, <em>Myrmarachne smaragdina</em>, we investigate how myrmecomorphic modifications to the body shape impact the internal anatomy in a way that could be functionally limiting. Specifically, via X-ray micro-computed tomography (microCT), we quantify how the spider&rsquo;s constricted prosoma, which emulates the head and thorax of ants, impacts the size of the central nervous system (CNS) and the venom glands. We found that, relative to their whole-body mass, the CNS&nbsp;of the ant-mimicking jumping spider was smaller when compared with a relatively closely related non-mimic jumping spider, indicating that some trade-off between mimic accuracy and size of neural anatomy, as articulated by the &lsquo;selection trade-offs hypothesis&rsquo;, is a possibility. Our explorative evidence enables and encourages broader investigation of how variable mimic accuracy impacts the neuroanatomy in ant mimics as a direct test of the &lsquo;selection trade-offs hypothesis&rsquo;.&nbsp;</p>