Setiform Sensilla and Prey Detection in the Bird-Spider Sericopelma Rubronitens Ausserer (Araneae, Theraphosidae)

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Abstract
The setiform varieties on legs and pedipalps of Sericopelma rubronitens Ausserer (Araneae, Theraphosidae) were examined for their external structure and their func- tion with scanning electron microscopy and electrophysiological methods. The legs and palps carry a basic pelage of dark-brown hairs (covering hairs) and-on the ventral surface of the tarsi and metatarsi-grey hairs (pile-hairs). These two hair types have no receptor function; interspersed with them are several other types of hairs. Four types of mechanosensitive sensilla were found: 1) multi-innervated slightly curved tactile hairs; 2) single-innervated straight tactile spines; 3) trichobothria, probably multi-innervated, and responding to the slightest disturbance of the air; 4) club-shaped sensilla with unknown innervation, which are very sensitive to vibrations of the soil. Trichobothria and club-shaped sensilla play an important part in detecting and localizing a prey. In all probability, the tactile hairs become important only after the prey has been caught. The ventral spines on the hind legs play an important part in the defensive behaviour of the spider. We also identified thermoreceptors which only respond to an increase in temper- ature. It is likely that these receptors take part in the regulation of the spider's rhythm of activity, as they must be able to perceive an increase in temperature of the air and soil. Presumably, their sensitivity is too low to detect a remote warm- blooded animal. Although behavioural studies indicated the presence of olfactory receptors on the tarsi, we were not able to localize these sensilla. Moreover, we did not find evidence that smell takes part in the decision of the spider to catch a prey. On the coxae of the pedipalps a brush of curved hairs was found which probably serves to filter the food. On these coxae bulb-shaped structures are also present, carrying a characteristic fingerprint-like pattern of ridges; the function of these structures is still unknown.

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