Most spiders can be characterized as animals that fatigue rapidly, especially when compared to insects. The maximum duration and rates of a spider's burst activities (e.g., behaviors involved In attacks or escape) are limited by biochemical factors. Rapid depletion of stores of high-energy phosphate compounds (principally arginine phosphate and adenosine triphosphate) early in maximal activities quickly results in a marked slowing of the spider's movements. As the spider continues to struggle, the build-up of anaerobic by-products (principally D-lactic acid) and associated effects result In complete fatigue within ca. two minutes. Defects in the hydraulic leg extension mechanism of spiders resulting in insufficient hemolymph to extend the legs (due to loss of hemolymph from the prosoma and legs to the opisthosoma) do not appear to cause fatigue directly. Instead, the high prosomal hemolymph pressures needed for vigorous activity prevent the heart from pumping freshly oxygenated hemolymph to the prosoma and Its active muscles. The result is that over a two minute bout of struggling probably less than 10^ of a spider's energy is derived from aerobic sources^ with the remainder coming from anaerobic glycolysis and high-energy phosphate stores. This contrasts with ca. 99% aerobic dependence in non-burst activity such as web-building. The exact relative dependence of a given species on each of these three energy sources during peak activity appears to be correlated with respiratory surface area: the smaller this area, the more dependency on anaerobic metabolism. Correlations probably also exist with the circulatory system. During recovery, lactate diffuses from the prosoma to the opisthosoma where it is metabolized, probably to complex carbohydrates. The reverse process occurs during activity. Recovery takes 30 minutes or longer in completely exhausted spiders, the exact time being correlated inversely with the respiratory surface area. Transport of lactate from the prosoma is analogous to the process of blood transport of lactate from muscles to the liver in vertebrates and is an adaptation to allow quick recovery of a spider's running ability. Temperature has less effect on spider locomotion than would be expected based on the temperature dependence of a spider's aerobic process. However, recovery is slowed in spiders that are at temperatures away from that to which they are acclimated. The metabolic abilities of spiders, both aerobic and anaerobic, are low to moderate when compared to other groups of predatory animals (vertebrates. Insects). However, a spider's use of silk and poisons probably have been major factors in lowering the need for highly developed metabolic capacities.