Master of Science
Spiders use vibrations to sense their surroundings. It has been suggested that the vibration perception in spiders may be altered by the mechanics of the body. I studied the biomechanics of spiders, at the level of leg joints and the whole body. To study joints, I quantified the allometry of leg joint stiffness in spiders. I found that the stiffness of spider joints increased nearly isometrically with increasing body mass, partly by having shorter and thicker leg segments and also by other unknown means. Using these data, I developed empirically validated biomechanical models which predicted the effects of mechanics on vibrational filtering within the body. Interestingly, both models and empirical data showed that the relatively linear increase in joint stiffness with mass meant that the mechanical filtering of spider bodies may be size independent, indicating that spiders of different masses or ecologies may sense the world in similar ways.
Summary for Lay Audience
Spiders sense the world and communicate using vibrations. Vibrations are caused by forces that make a spider’s web move up and down or side to side periodically. In a spider’s world, vibrations can come from prey caught in the web, a male courting a female, or noise from wind and rain. Spiders sense these vibrations using sensors located near their leg joints. If the vibration makes the joint bend, then the sensor is activated, and the spider can sense that vibration.
I am interested in understanding whether different spider’s bodies move differently when they are vibrated by large prey such as a moth in the web, by small prey such as a fruit fly, or by the wind blowing. Different properties of a spider’s body determine what causes it to vibrate, what causes its leg joints to bend, and how it moves when it vibrates. Two of these are a spider’s mass and the stiffness of its leg joints. Heavier spiders respond to low frequency vibrations, whereas spiders with stiffer joints respond to higher frequencies. But stiffer joints are needed to carry heavier spiders. So, my main questions are, do heavier spiders have stiffer joints? And do these two effects balance each other, or do heavier spiders vibrate differently than lighter spiders?
To answer the first question, I measured joint stiffness in 11 spider species and tested how it changed with spider mass. I incorporated these data into computer simulations of the bodies of three spider species. These simulations predicted how spiders of different sizes with different joint stiffnesses would move when vibrated. I tested simulation results against measurements of real spiders of all three species. I found that heavier spiders had stiffer joints, iv balancing the effect of the increased mass. I found that the models and data from real spiders showed that the net effect of this was that spiders of different sizes vibrated in very similar ways. My thesis therefore suggests that even though spiders may come in various sizes, they may be perceiving the world in very similar ways.
Gartly, Reese L., "Elucidating the Relationships Between Spider Size, Joint Stiffness, and the Mechanical Frequency Response of the Body" (2023). Electronic Thesis and Dissertation Repository. 9835.