Presumed Glomerular Neuropils in the Central Nervous Systems of Spiders (Araneae)

While spiders are known for their tactile and vibratory senses, their sense of smell has not been well researched at all. Only relatively few behavioral studies have been published showing that spiders can perceive pheromones and may be able to smell prey or predators. Less is known about the nature of their olfactory neurons and sense organs (sensilla), and nothing is known about how and where olfactory information is processed in the spider's central nervous system. This comparative anatomical study is the first to describe presumed chemosensory glomeruli in the central nervous system of spiders. Such glomeruli are the anatomical hallmark of olfactory processing in other taxa. This study finds presumed olfactory glomeruli residing ventrally in each neuromere of the legs and pedipalps (feelers) of all spider families examined. Unlike olfactory glomeruli in most insects or vertebrates, in spiders, these presumed chemosensory glomeruli are often elongated rather than spherical. Anterograde tracing of neurons originating from chemosensory sensilla on the legs shows that these presumed chemosensory glomeruli are supplied by primary chemosensory afferents, at least some of which may supply more than one glomerulus. In addition, a previously described central structure, referred to as the Blumenthal neuropil and presumed to receive hygro- and thermo-sensory input, is here shown to also comprise glomerular components. One or more of these central Blumenthal glomeruli are particularly large in male spiders. In analogy to insect macroglomeruli, this suggests that these glomeruli might be involved in processing pheromone information. These findings are discussed with reference to insect and vertebrate olfactory information processing.