Compound slit sense organs on the spider leg: Mechanoreceptors involved in kinesthetic orientation

Summary

• 1. The hunting spiderCupiennius salei Keys is able to direct its locomotion by making use of information about its own previous movement sequences (kinesthetic orientation). After a blinded spider is chased ca. 25 cm away from a prey-fly, it returns to the original capture site despite the preclusion of other possible orientation clues. The mean starting direction of such returns differs from the ideal return direction by only 2 ° (Fig. 4a, 5). Of all runs 95% are “successful” in that the animals approach the capture site as close as 5 cm (mean value) (Fig. 3).

• 2. Mechanical destruction of compound slit sense (“lyriform”) organs on femur and tibia of all legs results in disorientation of the spiders: more than 2/3 of their returns pass the capture site at a distance of more than 10 cm (Fig. 3, 4b). In addition, the mean angular deviation of starting directions increases significantly. The difference between the mean starting angles of the treated groups and the mean of intact animals, however, is significant only in some cases.

• 3. A special effort was made to evaluate not only thestarting directions and the “success” of a return path, but theentire return route, which is comprised of several path segments based upon each stopping and/or turning point. To this end a “walking error” e_n was determined for each segment (Fig. 8). For intact animals the error increases abruptly at the point nearest to the capture site. We therefore conclude that the spiders control also their walking distance kinesthetically. In the case of operated animals the mean “walking error” calculated from those segments lying before the “nearest point” increases by a factor of 4 to 5, as compared with intact spiders, whereas it remains about the sameat the “nearest point” itself (Fig. 9).

• 4. Small holes pierced into the leg cuticle near intact lyriform organs of otherwise intact “control animals” do not influence the success, starting angle, and walking errors of returns.