The organization of the spider nervous system as revealed by dopamine immunolabeling and confocal microscopy

Spiders (Arthropoda: Araneae) display remarkably diverse and sophisticated behaviors despite possessing fewer than 100,000 neurons. This makes spiders excellent model systems for understanding the evolution of behavioral complexity and the neural mechanisms that underlie it; however, little research to date has been done on arachnid neurobiology. To address this gap, we utilized immunocytochemical techniques to characterize the distribution of dopamine (DA) in the central nervous system (CNS) of the Floridian wolf spider Hogna lenta (Lycosidae).  DA is a universally important modulator of many complex behaviors across animal taxa, and our study is the first to examine its distribution in any spider species.  We discovered that DA is globally expressed throughout the CNS of H. lenta. In the supraesophageal mass, we found that DA was particularly concentrated in the first and second optic neuropils (ON1 and ON2) of all eight eyes, indicating that DA plays a critical role in modulating visual processing. DA was found throughout multiple layers of the arcuate body and the bridge of the mushroom body as well, both sites of higher order sensorimotor integration. We also observed large dopaminergic neurons in the abdominal neuromeres, which appear to project anteriorly to the supraesophageal mass via large plurisegmental tracts. In addition to these findings, our immunolabeling revealed distinct divisions in the leg and abdominal neuromeres, which may reflect ancestral segmentation of the nervous system. Taken together, these findings reveal that DA is widespread in the spider CNS and underscore the important modulatory functions of DA in the Araneae.