[Role of the principal and accessory eyes in the photomenotactic orientation of the funnel spider Agelena labyrinthica (CL.)].

The photomenotactic orientation of funnel spiders (Agelena labyrinthica [Cl.]) returning to their retreat was studied by measuring the direction of their course on the horizontal webs. 1. If a lamp was shifted 90 degrees just before the spider caught its prey the mean return course deviated less than 90 degrees from the direction of the retreat. This deviation was not only a function of the azimuth of the light and of the amount it had been moved, but also varied with the course taken when approaching the prey, the duration of the interval without light stimulation while the lamps were changed, the light stimulation experienced while at the retreat, the extent to which vision was experimentally reduced, and the light intensity. 2. Elimination of the principal or of the accessory eyes had little or no significant effect on the photomenotactic orientation. Spiders able to use only the median eye or one principal eye could no longer use the lamp to orient by. 3. Animals with full vision showed an increased (decreased) deviation from the direction of the retreat if the light intensity was abruptly increased (decreased), as did animals without principal eyes or without accessory eyes. The mean return directions were, however, not dependent upon light intensity changes before reaching the prey or after an interruption of web contact. The threshold for photomenotactic orientation using the accessory eyes is 0.366-10(-4) sb; the corresponding value for the principal eyes is 100 times larger. 4. If the lamp was not visible on the outward run the spiders were unable to perform correct return runs by light orientation without optical information from previous runs. If they could see a lamp while at the retreat their return course was shifted in the direction away from the light. Animals without principal eyes did not show this effect. 5. Spiders that had not performed an outward run did not keep to a straight course. Animals with full vision spontaneously showed negative phototactic orientation, those without principal eyes were usually positively phototactic. 6. Animals, whose web contact had been interrupted or animals without principal eyes showed a similar deviation from the correct return direction as did unmanipulated animals.