Factors which influence the rate at which Philodromus rufus Walckenaer attacks prey are discussed. Drosophila melanogaster Meign. was used as prey throughout the rearing and testing of P. rufus. Spiders used in these tests were immature at the time of capture and reared to the adult stage in the laboratory. Feeding histories were maintained for all spiders, although the experiments designed to measure the attack response of individual spiders to changing prey density were done with adult males.

In the immature stage there was no difference between the feeding rate of male and female spiders. The adult males, however, were relatively inefficient predators; females killed approximately six times as many flies in their adult lives as males. The irregular feeding pattern of the females, as compared to the fairly uniform feeding pattern of the males, resulted in the males being more amenable to experimental tests. Periods of starvation, molting, and opposition strongly influenced the feeding rate of P. rufus. At high prey densities there was an inverse relationship between hunger and utilization of individual prey. This resulted from uncaptured prey interfering with the feeding spider.

Both feeding time and searching time showed a relationship to prey density. Individual fly activity increased with fly density and this resulted in a functional response curve with an increasing slope. One of the basic requirements for regulation was thereby demonstrated. When prey density was corrected for the increase in fly activity by computing a new density estimate based on fly activity, a typical functional response for invertebrates became evident for hungry spiders but not for well-fed spiders. By using Holling's “disc equation” the difference between starved and satiated spiders was explained as an interaction between prey and predator characteristics. Owing to density-dependent parameters such as prey activity, predator feeding time, and “rate of discovery”, no known mathematical model fitted the data.