UC Berkeley

Animals that engage in active predation need to acquire sufficient sensory information to locate, capture, and overtake their prey. Different types of foraging and prey capture behaviors can require specific types of sensory input. The exact sensory requirements for prey capture depend on hunting strategy, prey defensive behavior and morphology, and habitat characteristics. Within spiders there have been several independent evolutionary enlargements and modifications of eyes, allowing for changes in the visual capabilities of the spider. In these spiders, different pairs of eyes can be responsible for different visual tasks. Vision in spiders – and in all animals – is dependent not only on the properties of the viewer but on the environment as well, particularly on the quality of the ambient light. Spider vision exists alongside other senses, such as vibratory and trichobothria senses, that can be used in prey capture. In this dissertation I studied how spiders use different senses in combination to guide prey capture. To do so, I performed lab prey-capture experiments on wild-caught spiders in which I restricted their access to sensory information, through manipulations to the environment or the spider.

In the first chapter, I found that Habronattus formosus jumping spiders never catch a prey insect in complete darkness, and that they are slower to capture in dim light conditions. I then tested how H. formosus captures prey under a variety of manipulations. By varying light conditions and substrate, I restricted visual and/or vibratory information and measured its effects on hunting behavior of spiders with two types of insect prey. By doing so I characterized how senses are used together or in isolation in prey capture, and how this sensory usage changes with context, such as different prey. I found that H. formosus are more likely to capture flies than crickets, and more likely to capture prey under bright light than dim light. I also found that these spiders caught prey more quickly on flexible substrates than on stiff substrates. This was the first study of actual prey capture performance of Habronattus in different sensory environments, and the first to explicitly examine multimodality of prey capture in these spiders.

In the following chapter I studied sensory usage in prey capture across lighting contexts in H. formosus, but with a focus on vision. Through selective blindfolding of the enlarged forward-facing principal eyes of these spiders, I tested the role that the specific vision of this pair of eyes plays in prey capture. Using similar methods to those in the preceding chapter, I ran fully crossed behavioral trials measuring prey capture success-rates and speeds of blindfolded and sham-painted spiders in either bright or dim lighting conditions. I found that blindfolding of the principal eyes of H. formosus diminishes prey capture rates, as do dim-light conditions. These spiders also caught prey more slowly under dim or blindfolded manipulations. Furthermore, I found that spiders with occluded principal eyes never caught prey in dim light, suggesting that these eyes are necessary for low-light prey capture. This study confirmed and expanded upon previous research documenting the importance of salticid principal eyes in prey capture.

In the final chapter I studied the role of vision from another eye-pair in prey capture, but in flattie spiders (Selenopis). Within this family there has been an independent evolutionary eye enlargement of a different pair of eyes, the backwards-facing posterior lateral eyes. Flattie spiders capture prey differently from jumping spiders, ambushing their prey and using a rapid spinning motion to capture prey that is behind or beside them. Using a factorial design, I studied how Selenopid striking behavior changes with different access to visual information. I found no effect of blindfolding of the backwards-facing posterior lateral eyes on the strike dynamics of a flattie spiders. Strike characteristics such as angular and linear speeds of strikes did not change with the occlusion of these large eyes. Unlike H. formosus, these spiders did catch prey under dim light when their largest pair of eyes were covered. This was the first behavioral study of vision in Selenopidae, and the first step towards understanding the currently unknown function of their enlarged eyes.

Spiders display an incredible array of sensory abilities. This fact, combined with the diversity of prey capture strategies in spiders and the concomitant diversity of their sensory systems, make this order of arachnids an excellent system for behavioral studies of sensory ecology. Additionally, the way in which spiders subfunctionalize their vision across their pairs of eyes make them particularly well suited for the behavioral study of vision. Despite their small eyes and small brains, spiders can acquire and process the sensory information to perform tasks that can be quite complicated. These stark limitations also make them great subjects of study to move towards an understanding of the sensory ecology of animals overall. Though it’s most obvious in the tiny and fascinating world of spiders, we animals are all forced to meet our sensory needs only within what is allowed by the constraints of physics, evolution, development, and ecology.