Distribution of histamine in the CNS of different spiders.

Immunohistochemistry is used to demonstrate histamine-immunoreactivity in the CNS of spiders. We found histamine-immunoreactivity in the photoreceptors of different spiders. Therefore, we suggest that histamine is a neurotransmitter of photoreceptors in all arthropods, since it is also known to occur in the photoreceptors of the other main arthropod taxa (Merostomata, Crustacea, and Insecta). We also describe a system of only six omnisegmental histamine-immunoreactive neurons within the central nervous system. These histamine-immunoreactive neurons can be divided into two subgroups: a dorsal system with two cells per hemisphere and a ventral system with only one cell per hemisphere. All six cells have extended arborizations in both the motor and the sensory areas of all neuromeres in the suboesophageal ganglionic mass. In contrast to araneomorph spiders, two additional sets of histamine-immunoreactive neurons were detected in mygalomorph spiders. The first set consists of seventeen cells with their cell bodies located in the cheliceral ganglion and projecting to central areas of the protocerebrum. The second set contains many if not all sensory projections from the tarsal organs on all eight legs and the pedipalps to the Blumenthal neuropil.