Nocturnal foraging enhanced by enlarged secondary eyes in a net-casting spider

Animals that possess extreme sensory structures are predicted to have a related extreme behavioral function. This study focuses on one such extreme sensory structure—the posterior median eyes of the net-casting spider Deinopis spinosa. Although past research has implicated the importance of vision in the nocturnal foraging habits of Deinopis, no direct link between vision in the enlarged eyes and nocturnal foraging has yet been made. To directly test the hypothesis that the enlarged posterior median eyes facilitate visually based nocturnal prey capture, we conducted repeated-measures, visual occlusion trials in both natural and laboratory settings. Our results indicate that D. spinosa relies heavily on visual cues detected by the posterior median eyes to capture cursorial prey items. We suggest that the enlarged posterior median eyes benefit D. spinosa not only through increased diet breadth, but also by allowing spiders to remain active solely at night, thus evading predation by diurnal animals

Nocturnal foraging enhanced by enlarged secondary eyes in a net-casting spider

Animals that possess extreme sensory structures are predicted to have a related extreme behavioral function. This study focuses on one such extreme sensory structure—the posterior median eyes of the net-casting spider Deinopis spinosa. Although past research has implicated the importance of vision in the nocturnal foraging habits of Deinopis, no direct link between vision in the enlarged eyes and nocturnal foraging has yet been made. To directly test the hypothesis that the enlarged posterior median eyes facilitate visually based nocturnal prey capture, we conducted repeated-measures, visual occlusion trials in both natural and laboratory settings. Our results indicate that D. spinosa relies heavily on visual cues detected by the posterior median eyes to capture cursorial prey items. We suggest that the enlarged posterior median eyes benefit D. spinosa not only through increased diet breadth, but also by allowing spiders to remain active solely at night, thus evading predation by diurnal animals

Ogre-Faced, Net-Casting Spiders Use Auditory Cues to Detect Airborne Prey

Prey-capture behavior among spiders varies greatly from passive entrapment in webs to running down prey items on foot. Somewhere in the middle are the ogre-faced, net-casting spiders (Deinopidae: Deinopis) that actively capture prey while being suspended within a frame web. Using a net held between their front four legs, these spiders lunge downward to ensnare prey from off the ground beneath them. This “forward strike” is sensorially mediated by a massive pair of hypersensitive, night-vision eyes. Deinopids can also intercept flying insects with a “backward strike,” a ballistically rapid, overhead back-twist, that seems not to rely on visual cues. Past reports have hypothesized a role of acoustic detection in backward strike behavior. Here, we report that the net-casting spider, Deinopis spinosa, can detect auditory stimuli from at least 2 m from the sound source, at or above 60 dB SPL, and that this acoustic sensitivity is sufficient to trigger backward strike behavior. We present neurophysiological recordings in response to acoustic stimulation, both from sound-sensitive areas in the brain and isolated forelegs, which demonstrate a broad range of auditory sensitivity (100–10,000 Hz). Moreover, we conducted behavioral assays of acoustic stimulation that confirm acoustic triggering of backward net-casting by frequencies in harmony with flight tones of known prey. However, acoustic stimulation using higher frequency sounds did not elicit predatory responses in D. spinosa. We hypothesize higher frequencies are emitted by avian predators and that detecting these auditory cues may aid in antipredator behavior

Multimodal sensory reliance in the nocturnal homing of the amblypygid \u3ci\u3ePhrynus pseudoparvulus\u3c/i\u3e (Class Arachnida, Order Amblypygi)?

Like many other nocturnal arthropods, the amblypygid Phrynus pseudoparvulus is capable of homing. The environment through which these predators navigate is a dense and heterogeneous tropical forest understory and the mechanism(s) underlying their putatively complex navigational abilities are presently unknown. This study explores the sensory inputs that might facilitate nocturnal navigation in the amblypygid P. pseudoparvulus. Specifically, we use sensory system manipulations in conjunction with field displacements to examine the potential involvement of multimodal—olfactory and visual—stimuli in P. pseudoparvulus’ homing behavior. In a first experiment, we deprived individuals of their olfactory capacity and displaced them to the opposite side of their home trees (\u3c5 m). We found that olfaction-intact individuals were more likely to be re-sighted in their home refuges than olfaction-deprived individuals. In a second experiment, we independently manipulated both olfactory and visual sensory capacities in conjunction with longer-distance displacements (8 m) from home trees. We found that sensory-intact individuals tended to be re-sighted on their home tree more often than sensory-deprived individuals, with a stronger effect of olfactory deprivation than visual deprivation. Comparing across sensory modality manipulations, olfaction-manipulated individuals took longer to return to their home trees than vision-manipulated individuals. Together, our results indicate that olfaction is important in the nocturnal navigation of P. pseudoparvulus and suggest that vision may also play a more minor role

Female mate choice for multimodal courtship and the importance of the signaling background for selection on male ornamentation

Conspicuous visual ornaments are frequently incorporated into complex courtship displays that integrate signal components from multiple sensory modalities. Mature male Schizocosa crassipes (Walckenaer, 1837) wolf spiders wave, arch, and tap their ornamented forelegs in a visual courtship display that simultaneously incorporates seismic components. To determine the importance of modality-specific signal components in female mate choice, we used a signal ablation design and compared the mating frequency of female-male pairs across signaling environments with manipulated modality-specific transmission properties. We found that the successful transmission of isolated visual or seismic signaling was sufficient for mating success; neither signaling modality was necessary. Additionally, the environment enabling the successful transmission of composite, multimodal displays yielded the highest mating frequencies. Our results indicate the presence of selection from S. crassipes females for multimodal courtship and suggest that multimodal signaling may facilitate mating across variable signaling environments. We next explored the influence of ornamentation per se on female choice by phenotypically manipulating males into two groups: (i) intact (brushes present) and (ii) shaved (brushes absent). We compared the mating frequencies of intact versus shaved males in the presence versus absence of seismic signaling. Males with brushes intact had higher mating frequencies than shaved males, but only under specific signaling conditions – in the presence of seismic signaling. Female choice for male brushes then appears dependent on the signaling background, making brushes themselves an unlikely target of direct selection. Our results emphasize the complex nature of female choice, highlighting the potential for both trait interactions and environment-dependent selection

Sensory Ecology of the Net-Casting Spider, Deinopis spinosa

Sensory systems are both highly beneficial and highly costly. The balance between these benefits and costs is what shapes the sensory systems of animals. A better understanding of how sensory systems evolve can illustrate how the environment might shape animal behavior, morphology, and ecology. This dissertation examines the sensory ecology of a nocturnal, visual specialist predator, the net-casting spider, Deinopis spinosa. An integrative approach, I provide insight into the benefits of specific sensory systems in foraging and mating contexts, as well as potential constraints for sensory system development in D. spinosa. First, I used field and laboratory visual occlusion trials to decipher potential benefits of possessing the enlarged posterior median eyes (PMEs) of D. spinosa. I found that spiders without PME vision seemed unable to capture cursorial prey items walking beneath their web. These prey items tended to be larger than prey captured from the air. Thus, PME vision benefits D. spinosa specifically in the ability to capture cursorial prey items that might be of greater nutritional quality. Second, I investigated potential plasticity of peripheral (e.g. eyes) and central ( e.g. brain) structures related to vision. I compared absolute and relative eye diameters of principal and secondary eyes in D. spinosa and found that mature males had smaller secondary eyes, but larger principal eyes when compared to penultimate males. Relative investment in central brain structures mirrored peripheral investment. Such adaptations likely enhance mate-searching efforts, as well as longevity. Lastly, I investigated the role of olfaction in male mate-searching behavior. I found that mature males were sensitive to airborne cues given by mature females, and preferred to associate with female odor over conspecific penultimate males’ odor, and thus likely use airborne cues in mate-searching. Taken together, I utilized field and laboratory based behavioral assays in conjunction with quantifications of external and internal sensory morphology to gain a better understanding of sensory system benefits and constraints within an enigmatic, sensory specialist, the net-casting spider. Future studies within this family of spiders have great potential for further elucidating how sensory systems are shaped through time

Sensory system plasticity in a visually specialized, nocturnal spider

The interplay between an animal’s environmental niche and its behavior can influence the evolutionary form and function of its sensory systems. While intraspecific variation in sensory systems has been documented across distant taxa, fewer studies have investigated how changes in behavior might relate to plasticity in sensory systems across developmental time. To investigate the relationships among behavior, peripheral sensory structures, and central processing regions in the brain, we take advantage of a dramatic within-species shift of behavior in a nocturnal, net-casting spider (Deinopis spinosa), where males cease visually-mediated foraging upon maturation. We compared eye diameters and brain region volumes across sex and life stage, the latter through micro-computed X-ray tomography. We show that mature males possess altered peripheral visual morphology when compared to their juvenile counterparts, as well as juvenile and mature females. Matching peripheral sensory structure modifications, we uncovered differences in relative investment in both lower-order and higher-order processing regions in the brain responsible for visual processing. Our study provides evidence for sensory system plasticity when individuals dramatically change behavior across life stages, uncovering new avenues of inquiry focusing on altered reliance of specific sensory information when entering a new behavioral niche

Enigmatic Ornamentation Eases Male Reliance on Courtship Performance for Mating Success

Female preferences are frequently invoked to explain the widespread occurrence of elaborate male ornaments, yet empirical data demonstrating such preferences are sometimes equivocal or even contradictory. In the wolf spider Schizocosa stridulans, despite evidence of strong female choice, prior research has been unable to link the conspicuous sexually dimorphic foreleg ornamentation of males to their mating success. We conducted three experiments aimed at determining the function of this previously enigmatic ornamentation. Our first two experiments used males with phenotypically modified foreleg phenotypes in simple and complex mating environments in order to examine the relationship between the presence/absence of ornamentation and male mating success. In both experiments, we found no relationship: courtship rate was the sole predictor of mating success. In a third experiment, we used males with naturally varying foreleg ornamentation in mating trials. Ornamentation was subsequently quantified and we again examined the factors influencing male mating success. As in our first two experiments, we found courtship rate to be a good predictor of mating success. Additionally, we discovered that foreleg ornamentation and courtship rate interact to influence male mating success. At low courtship rates, males with more foreleg ornamentation have a mating advantage, whereas at high courtship rates, males with less foreleg ornamentation have a mating advantage. We discuss several potential explanations for these results. In summary, we provide the first evidence of a benefit of foreleg ornamentation in male S. stridulans and suggest that this benefit is realized by the interaction between ornamentation and courtship rate

Ogre-Faced, Net-Casting Spiders Use Auditory Cues to Detect Airborne Prey (video)

7-minute video presentation of research. Prey-capture behavior among spiders varies greatly from passive entrapment in webs to running down prey items on foot. Somewhere in the middle are the ogre-faced, net-casting spiders [1] (Deinopidae: Deinopis) that actively capture prey while being suspended within a frame web [2–5]. Using a net held between their front four legs, these spiders lunge downward to ensnare prey from off the ground beneath them. This “forward strike” is sensorially mediated by a massive pair of hypersensitive, night-vision eyes [5–7]. Deinopids can also intercept flying insects with a “backward strike,” a ballistically rapid, overhead back-twist, that seems not to rely on visual cues [4, 5, 8]. Past reports have hypothesized a role of acoustic detection in backward strike behavior [4, 5, 8]. Here, we report that the net-casting spider, Deinopis spinosa, can detect auditory stimuli from at least 2 m from the sound source, at or above 60 dB SPL, and that this acoustic sensitivity is sufficient to trigger backward strike behavior. We present neurophysiological recordings in response to acoustic stimulation, both from sound-sensitive areas in the brain and isolated forelegs, which demonstrate a broad range of auditory sensitivity (100–10,000 Hz). Moreover, we conducted behavioral assays of acoustic stimulation that confirm acoustic triggering of backward net-casting by frequencies in harmony with flight tones of known prey. However, acoustic stimulation using higher frequency sounds did not elicit predatory responses in D. spinosa.We hypothesize higher frequencies are emitted by avian predators and that detecting these auditory cues may aid in antipredator behavior