Internal clocks, or circadian rhythms, are nearly ubiquitous across taxa (e.g., animals, plants, fungi, and cyanobacteria), and it is widely believed that a biological clock benefits organisms by enabling them to schedule behavioral and physiological changes in anticipation of predictable changes in environmental conditions. Theory and evidence suggest it is important that the internal clock resonate closely with the 24-h daily cycle. Recently, however, Cyclosa turbinata (Walckenaer, 1841) (Araneidae) was revealed to have a circadian clock with a period of about 19 h, which was presumed to be anomalous. Here, we report on the behavioral rhythms of a nocturnal orbweaver, Metazygia wittfeldae (McCook, 1894), from the same family. Under laboratory conditions of a 12:12 h light:dark cycle, we found that locomotor activity initiates shortly after dark, reaching a peak early in the dark phase, continuing at a lower level throughout the remaining dark phase, and then diminishing shortly after lights-on. Locomotor activity continued to cycle under constant dark conditions with a mean free-running period of 22.7 h. We also found a second component in the free-running activity (mean 11.5 h) which correlated very tightly with the free-running period. Thus, M. wittfeldae has what can be considered a typical circadian clock resonating with the 24-h day. Notably, however, there were two outliers close to the 19-h period observed in C.Turbinata, suggesting that there may be sufficient variation in clock period among araneid spiders upon which selection could act leading to the short-period clocks in C.Turbinata.

This study examines locomotor activity in samples of Frontinella pyramitela collected over its active season (April-October) to investigate whether seasonality and/or age may help explain the exceptional variability typically found in spider clock systems. Despite its noteworthy variability (%CV= 7.7), we have found that Frontinella has a mean free-running period of 28.4±2.18 hours that does not significantly vary over time. There is no correlation between day length and free-running period, indicating that varying length of FRP is not a function of photoperiod length. In LD 12:12 h, the window of activity is significantly smaller in April, gradually widening as the season continues, which may be evident of a shift in foraging strategy. Frontinella’s clock appears to mature before its entrainment mechanisms are fully developed, and towards the end of its season, there is strong evidence of circadian misalignment which may be a product of physiological age.

A major advantage of having behavior controlled by a circadian clock is that the organism may be able to anticipate, rather than respond to, important daily events in its environment. Here, we describe the behavioral rhythms of locomotor activity and web building in the orb-weaving spider Cyclosa turbinata (Walckenaer, 1841). Web building occurs late in the scotophase, in absolute darkness, and is initiated and completed before lights-on under light:dark cycles in the laboratory. This scheduling presumably enables web-building to occur under the cover of darkness, thereby avoiding visual predators. Locomotor activity occurs predominantly in the dark with a sharp peak within one hour after lights-off and a broader peak occurring before lights-on. The locomotor activity rhythm free runs under constant dark and constant temperature conditions, thus indicating endogenous circadian control. Evidence from the free running rhythm suggests that the first peak under light:dark cycles is a result of masking but that the second peak is attributable to the endogenous circadian oscillator. The period of the free run is exceptionally short, about 19 hours. In comparison with locomotor activity, web building is quite sporadic under constant dark conditions, making detection of periodicities difficult and, therefore, whether web-building is under endogenous circadian control or is driven by exogenous factors remains unresolved.

Despite recent interest, there still is relatively little known about the ecology and physiology of diel and circadian rhythms in spiders. However, previous work on spiders suggests that there is a striking amount of variation in circadian period both among, and within, species, when compared to model organisms. Whereas previous studies of behavioral rhythms in spiders focused on females, here we describe the diel and circadian patterns of locomotor activity in male Parasteatoda tepidariorum (C. L. Koch, 1841) (Theridiidae). We found that the males showed mostly nocturnal activity under a light:dark cycle, with activity peaking very early after lights off and steadily declining to near zero just prior to lights on. Under constant darkness most individuals showed significant circadian rhythmicity with a mean free-running period of about 21.2 h. Though not the shortest average free-running period described for spiders, being so out of resonance with the 24 h solar day strains conventional circadian rhythm theory. Our data also suggest that the phase angle of entrainment for locomotor activity is in the mid-to-late photophase, but that activity may be masked by light. Of particular note is that both the diel and circadian activity patterns reported here for male P. tepidariorum are similar to those reported elsewhere for females of the species. This study deepens our understanding of the nature and variation in circadian rhythm in spiders and builds a case for further developing spiders as a model system for research integrating the fields of chronobiology and ecology.