Date Presented

Spring 5-2026

Document Type

Thesis

First Advisor

Dr. John Styrsky

Second Advisor

Dr. Allison Jablonski

Third Advisor

Ms. Kimberly Geier

Abstract

Defecation is the elimination of digestive waste by animals. In insects, these wastes are composed of both undigested food and nitrogenous wastes in the form of uric acid, and can be either liquid or solid form as they exit the anus (Weiss, 2006). Food is ingested, ground, and some of it is stored in the foregut of the insect while the midgut digests food chemically and absorbs nutrients. What remains after the foregut and midgut is then moved into the hindgut, where they accumulate with uric acid coming from the malpighian tubules (Weiss, 2006). According to Weiss (2006), rectal pads in the hindgut of the insect reabsorb ions and water, and the waste products that remain after reabsorption are eliminated by the insect as a combination of feces and uric acid. While defecation is a vital component of the digestive process, feces are often regarded simply as waste, despite the fact that several organisms utilize fecal matter for a variety of purposes. There are several possible uses of feces in ecological contexts, such as prevention of desiccation, defense against natural enemies, shelter, camouflage, and fertilizer (Weiss, 2006). Additionally, some species use waste for defensive purposes in ways that benefit others as well as themselves. For example, certain hemipteran taxa excrete honeydew as a waste product that attracts ant species, which act as defensive mutualists by protecting the hemipterans from predators and parasitoids in order to feed on the honeydew (Weiss, 2006). The use of feces in defense against natural enemies has been documented in various beetle species, commonly in protective strategies during the early stages of life. In the case of the leaf-beetle species Neolema sexpunctata and Lema trilinea, the larvae use their own feces to construct a shield-like structure on their backs. Morton and Vencl (1998) investigated the effectiveness of fecal shields in larvae of N. sexpunctata and L. trilinea against the predatory ant Formica subsericea. In both beetle species, larvae with intact fecal shields exhibited nearly 100% survival, whereas those whose shields were removed experienced nearly 100% mortality. Morton and Vencl (1998) determined that this protection was primarily due to the chemical defense provided by the fecal shields rather than the shields acting solely as a physical barrier. Several beetle species utilize feces for defense, some for the physical barrier the fecal matter creates, and some for the chemical protection the excrement provides. Although less well documented, some beetle species may also utilize feces to protect their eggs. The water scavenger beetle, Coleostoma stultum (Hydrophilidae), for example, uses fecal matter to defend its eggs in a double-layering of substrates. When the female water scavenger beetle lays eggs, they wrap the eggs in silk cocoons before also coating the central area of the cocoon lid with fecal matter (Matsushima and Yokoi, 2023). To study the effects of the fecal coating on hatchability, Matsushima and Yokoi (2023) conducted an experiment in which they observed female water scavenger beetles laying eggs, stopping some of the females from depositing feces on the egg cocoon as their experimental group, while allowing others to deposit their feces on the cocoon as a control group. Cocoons of both treatment groups were observed regularly and then collected after three days for incubation and, later, examination in a laboratory. Matsushima and Yokoi (2023) also performed an experiment to investigate the defensive mechanisms of the fecal coating. For this experiment, individual egg cocoons with and without the fecal coating were exposed to their dominant natural predator, the pill bug, Armadillidium vulgare. It was found that egg cocoons with a fecal coating experienced significantly less predation than those without a fecal coating, resulting in much higher hatching success (Matsushima and Yokoi, 2023). Using feces for egg protection is better known in leaf beetles (Chrysomelidae). Damman and Cappuccino (1991) investigated egg defense in the leaf-beetle Microrhopala vittata, which lays its eggs in clumps and covers them with fecal matter. By comparing eggs with fecal coverings to those without, they found that egg mortality was reduced by two key aspects of female oviposition behavior: (1) covering eggs with feces and (2) depositing eggs in masses (Damman and Cappuccino, 1991). Many species within the genus Neochlamisus construct a case made of fecal matter for each individual egg they lay. In those species, the individual larvae live in their fecal egg case, expanding it with their own feces as they develop. The functionality of the fecal egg case in this example ends after it acts as a pupal case for the individual (Brown and Funk, 2005). Based on these various accounts, it is clear that instances in which feces have been used in protection have been previously documented. In one other species of leaf-beetle, the dogbane beetle (Chrysochus auratus), the females build a case made of feces to surround their eggs, but the reason for this has never been investigated (Figure 1). Approximately half of the fecal egg case is built before the eggs are laid inside, followed by additional defecation to close the fecal egg case. These fecal egg cases are produced primarily on the underside of leaves of the beetle's host plant, spreading dogbane (Apocynum androsaemifolium). Little research has been done on the dogbane beetle, specifically regarding the purpose of the fecal egg case in its laying behavior. Possible explanations for the use of fecal egg cases in dogbane beetle laying behavior include prevention of desiccation and protection from natural enemies. In this study, I investigated oviposition behavior and the use of feces in egg protection by dogbane beetles. I set up two experiments to test if where the beetles oviposit their eggs and if the fecal covering that dogbane beetles apply to their eggs reduces egg mortality. In one of these experiments, I clipped leaves with fecal egg cases present on the abaxial leaf surface (underside) into an inverted position to expose them to direct sunlight to test the hypothesis that the fecal cases prevent desiccation of the eggs. I predicted that eggs in fecal cases on inverted leaves would experience greater mortality. In the other experiment, I tested the hypothesis that the fecal egg cases protect the eggs by removing and carefully resecuring the fecal cases in an inverted position, thereby exposing the eggs to predators and desiccation. I predicted that egg survival and hatching success would be reduced in inverted egg cases in which eggs were exposed compared to naturally oriented fecal cases in which eggs were protected.

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