The pectine organs of the scorpion, Vaejovis spinigerus: structure and (glomerular) central projections

The pectines of a new-world scorpion were studied as to their sensilla, nerve supply, and central nervous projections. (i) Pectines and sensilla in Vaejovis are similar to those examined in old-world species previously, although Vaejovis' pectines are larger and equipped with more receptors. The specialized peg sensilla show ultrastructural features characteristic of arthropod chemo- and mechanoreceptors, with the chemosensory exceeding the mechanosensory neuron population about 11-fold in number. (ii) The motoneuron supply of the pectines resembles that of other limbs and apparently conforms to a general arthropod plan. Motoneuron somata occur in three ventral groups, the anterior and posterior ipsilateral, and the contralateral groups. (iii) Pectine afferents terminate mainly in two ventromedial neuropil areas of the fused subesophageal ganglion mass. The larger posterior pectine neuropil shows a distinct glomerular and layered ("lobular") organization, reminiscent of insect antennal lobes and malacostracan olfactory lobes. Afferents enter the neuropil from its periphery, and output neurons leave through a central tract. Most projections show somatotopic organization, and several glomeruli exhibit GABA-like immunoreactivity, indicative of inhibitory synaptic interactions. The glomerular structure of the main pectine neuropil may indicate that such compartmentalisation is advantageous for the initial processing of chemosensory signals. The somatotopic projection of pectin receptors may be related to the use of the pectines in chemosensory orientation to substrate-bound chemicals, and in active sensing.