1981] 
Edwards — Phidippus mystaceus 
209 
sensitive setae in araneids were innervated and structured in essen- 
tially the same manner as pheromone receptors of insects. Hill 
(1977a, b) noted that the whorled setae on the tarsi and palpal 
cymbia of several species of Phidippus also resembled insect 
pheromone receptors; male P. mystaceus have the same type of 
setae on their palpal cymbia (Fig. 1). 
The behavior in P. mystaceus of engaging the palpi against the 
substrate is probably derived from similar behavior among its 
relatives. Males of other species of Phidippus move their palpi up 
and down or back and forth during courtship. This behavior 
appears to pre-adapt them for engaging the substrate, since only a 
slight change in the amplitude and / or attitude of these movements 
would bring the palpi into contact with the substrate. The same 
movement occurs more intensely and rapidly when a male encoun- 
ters silk made by a female, in association with presumed chemotac- 
tile exploration; it is likely that this is the evolutionary pathway of 
the development of the use of sound in P. mystaceus. 
The shape and arrangement of the macrosetae at the tip of the 
cymbium are such that a downward, forward pressure would engage 
them with the substrate. By dragging the palpus backward, enough 
leverage apparently is produced to move the fan-shaped file across 
the relatively stationary plectrum; however, the backward move- 
ment and bend of the palpus also may be a prerequisite to 
positioning the macrosetae onto the substrate. Once the palpus is 
anchored onto the substrate, the cymbium is rotated laterally 
outward, then the palpi are returned to their starting position. The 
macrosetae are arranged in a circle around the tip of the cymbia in 
P. mystaceus, with more macrosetae on the ectal edge than on the 
ental edge, which enables the palpus to remain engaged with the 
substrate as it rotates outward. The structure involved in sound 
production by rotating is the linear file; the cymbium must be 
rotated sideways due to the lateral position of the file. 
By simulating the direction of palpal movement with a model, it is 
apparent that the backward movement would cause the fan-shaped 
file to be drawn across the plectrum, while the rotary movement 
would bring the linear file into contact with the plectrum. The fan 
shape of the proximal file would accommodate the arc-shaped 
movement as the palpus is bent on the backward stroke; however, 
sound does not seem to be produced by the fan-shaped file. Only 
