Title: Where the rubber meets the road: How do phytophagous beetles hold onto host plants?
Abstract: Famed entomologist and Harvard alum Thomas Eisner (AB’51, OEB Ph.D. ’56) once said, “Insects are the most versatile of evolutionary innovators. Pick an insect at random, and chances are there is something about the way it feeds or defends itself, or reproduces, that is unique”1. This diversity is especially obvious within the largest group of insects: the beetles, which inhabit a wide range of ecological niches and utilize multiple feeding strategies, including carnivory, saprophagy and mycophagy. However, plant-feeding remains one of the most successful strategies, with phytophagous beetles making up over half of all beetle diversity.
For phytophagous beetles, life on plants poses three fundamental challenges: nutrition, exposure and attachment. Because their hosts range across flowering plants, including many monocots and dicots with differing and complex surfaces, leaf beetles present an opportunity to study and compare adaptive functional morphologies utilized for attachment as they ambulate across these diverse microtopographies. For this, I focused on the beetle tarsus as it constantly comes into contact with various surface asperities. Tarsal structures that are suited for smoother plant surfaces may not be ideal for surfaces with significant asperities. In addition to considering how these interact with specific surfaces, attachment may be especially challenging when forces are introduced by beetle movements, or when environmental factors are acting on the plant.
In Chapter 1, I present a large dataset that describes morphological features present in the tarsal attachment systems of Chrysomelidae beetles. This includes descriptions and measurements of the adhesive setae and terminal tarsal claw. These descriptions highlight the diversity of tarsal modifications utilized for attachment and locomotion, as well as the synergistic nature of these functionally different features. Chapter 2 introduces the topic of host plant surfaces, specifically focusing on microroughness and also examines the ecological and environmental pressures relevant to Chapter 1. Because each beetle species in Chapter 1 corresponds to a particular host plant, I was able to quantify the surface roughness of the plant leaves using a novel gel-based profilometry technique. I also provide a literature review describing some of the additional physical features displayed by these host plants, and life history descriptions for each corresponding beetle. Because each beetle species utilizes its plant surface in different ways (e.g. feeding vs. migration), these additional descriptions were important in highlighting how tarsal morphologies are enhanced by a combination of host plant roughness and locomotive strategy.
Finally, in Chapter 3, I designed a biomechanical study that observed how tarsal structures actively interact with various surfaces as beetles land after flight. Using a plant-inhabiting, predaceous beetle species (Hippodamia convergens), I separated beetles into three groups, removing an element of their attachment system (tarsal claw or adhesive setae) in two groups while leaving the final group unmodified as a control. Inducing flight and eventual landing observations allowed detection of whether these features were activated in ways similar to regular ambulation where setae are more effective on smoother surfaces, while claws are adapted for rougher asperities. Initial results describe unique body positioning as the beetles landed on surfaces of different orientations (horizontal vs. vertical). Additionally, while there was some correlation between surface roughness and the presence of each tarsal feature, there was an overall lack of success in the claw removal groups, which highlights the importance of this feature in ensuring attachment during this less controlled form of locomotion.
1. Eisner, T. (2005). For love of insects. The Belknap Press of Harvard University Press.
Committee: Brian Farrell (Advisor), Stacey Combes (UC Davis), Missy Holbrook, Naomi Pierce (Chair)