Impact of Relative Humidity on the Biology of Pardosa milvina Hentz, 1844 (Araneae: Lycosidae) A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Ryan D. Bell, B.S. Graduate Program in Entomology The Ohio State University 2009 Thesis Committee: Dr. Glen R. Needham, Advisor Dr. David J. Horn Dr. Richard A. Bradley 1 Copyright by Ryan D. Bell 2009 2 Abstract Pardosa milvina is a small wolf spider commonly associated with agricultural ecosystems. P. milvina produces dragline silk that is attached to the substrate over which it moves, but is not used in capturing prey. The effect relative humidity on P. milvina behavior and biology was examined through a series of experiments. The water balance constraints of P. milvina were studied to determine its body water content and its water loss rate at 0% RH. The calculated water loss rate is comparable to that of other terrestrial arthropods, and body water content was similar to other Pardosa spp. To examine the degree to which prey items are utilized as a water source, a study was conducted to determine if dehydrated spiders were more likely to take prey than hydrated spiders of comparable satiation levels. The individuals tested did not show an increase in prey taking when under water stress, as no spiders in either treatment took prey. Although they did not take prey, the dehydrated spiders regained a significantly greater mass when presented with water, indicating that free-standing water sources are preferred over prey if the spider is not hungry. The effect of relative humidity on silk deposition was examined, which necessitated the development of a technique for visualizing the silk. A difference in silk production between spiders maintained at different relative humidity levels was not found. Although there was no difference between relative humidity treatments, an ii analysis of a subset of individuals by mating status did reveal a difference in silk deposition between mated and virgin females. Virgin females deposited significantly more silk than mated spiders. iii Dedication To Chrissy Joy Bell, for everything she does iv Acknowledgements I would like to thank my advisor, Glen Needham, as well as my committee members Dave Horn and Rich Bradley for their guidance throughout this study. I would like to thank George Keeney and the staff of the OSU Insectary for their assistance in setting up prey cultures. I also thank Josh Benoit for his assistance and input and Justin Whitaker for collecting help. v Vita July 14, 1983………………………………Born – Towanda, Pennsylvania 2005……………………………………….B.S. Biology, Susquehanna University Selinsgrove, Pennsylvania 2005-present………………………………Graduate Teaching Associate, The Ohio State University, Columbus, Ohio Publications Bell, Ryan, A.L. Rypstra, & M.H. Persons. 2006. The effect of predator hunger on chemically-mediated antipredator responses and survival in the wolf spider Pardosa milvina (Araneae: Lycosidae). Ethology. 112:903-910. Schonewolf, K.W., R. Bell, A.L. Rypstra, & M.H. Persons. 2006. Field evidence of an airborne enemy-avoidance kairomone in wolf spiders. Journal of Chemcial Ecology. 32:1565-1576. Fields of Study Major Field: Entomology vi Table of Contents Abstract…………………………………………………………….................. ii Dedication……………………………………………………………………. iv Acknowledgements……………........................................................................ v Vita………………………………………………………………………......... vi List of Tables…………………………………………………………………. viii List of Figures………………………………………………………………… ix Chapter 1: Characteristics of Water Loss and Gain in Pardosa milvina…….. 1 Chapter 2: The Effects of Relative Humidity on Silk Deposition……………. 25 References……………………………………………………………………. 42 vii List of Tables Table 1.1. Water balance characteristics of P. milvina ………………………. 23 Table 1.2. Percent water changes……………………………………………... 24 viii List of Figures Figure 1.1. Water loss regressions for P. milvina …………………………… 22 Figure 2.1. Mean percent body mass (mg) lost ……………………………… 40 Figure 2.2. Mated vs. virgin silk indices…………………………………….. 41 ix Chapter 1: Characteristics of Water Loss and Gain in Pardosa milvina ABSTRACT. Pardosa milvina is a small wolf spider commonly associated with agricultural ecosystems. The water balance constraints of P. milvina were studied to determine its body water content and its water loss rate at 0% RH. The calculated water loss rate is comparable to that of other terrestrial arthropods, and body water content was similar to other Pardosa spp. To examine the degree to which prey items are utilized as a water source, a study was conducted to determine if dehydrated spiders were more likely to take prey than hydrated spiders of comparable satiation levels. The individuals tested did not show an increase in prey taking when under water stress, as no spiders in either treatment took prey. Although they did not take prey, the dehydrated spiders regained a significantly greater mass when presented with water, indicating that free-standing water sources are preferred over prey if the spider is not hungry. INTRODUCTION The ability to regulate internal water balance is crucial for the survival of all organisms. Terrestrial arthropods must contend with the challenges of small size, exposure to varying temperature and humidity extremes while allowing for gas cxchange across exoskeleton. Many studies have looked at the effects of relative humidity on survival and the behavioral and physiological ways in which organisms mitigate these stresses (Wharton 1985, Hadley 1994). One of the key factors influencing terrestrial 1 arthropod water balance is their diminutive size, which results in a greater surface area to volume ratio compared to most vertebrates. A large surface area and small water volume places the arthropod in jeopardy due to evaporative water loss unless certain fundamental behavioral and physiological attributes are in play (Hadley 1994). Being small has its positives as well. For example, this may allow for more effective use of more optimal microhabitats of temperature and relative humidity, which can mitigate evaporative water loss (Hadley 1994). The epicuticle of the exoskeleton serves as the main barrier to passive movement of water in and out of the arthropod body. Studying the cuticle water permeability properties can provide important clues about the desiccation sensitivity of specific spider species. To maintain water balance, the individual must match water loss with water intake or risk dehydration. Learning how much water can be lost before they become immobilized or moribund is crucial information to characterizing a spider’s water balance physiology. Body water activity (aw) is a term that is used to quantify the mole ratio of water to solutes in the arthropod. The aw will normally be around 0.99aw, corresponding to 99% water molecules (Edney 1977), while the outside environment will generally be much lower. The transpiration of water through the cuticle represents a constant loss of water that is dependent upon cuticle permeability and temperature (Hadley 1994). Even with a loss of half the body water the aw will remain near 0.99. This constant rate, which characterizes the water-loss properties of an individual, can be gravimetrically 2 determined by taking periodic measurements of that individual kept near 0 av or 0% humidity (RH). Vapor activity (av) is determined by dividing RH by 100. When kept in this condition at constant temperature there is no passive vapor sorption to confound the gravimetric determination. Passive movement of water vapor into an arthropod is proportional to the ambient av. In addition to transpiration through the cuticle, there is also water lost during respiratory gas exchange. Water can also be lost via excretions or secretions. Water loss has been associated with leg grooming and glandular secretions used in short term evaporative cooling (Pulz 1987). Spider silk deposition is an additional avenue of water loss not common among other arthropods. Water gain can come in the form of drinking, eating, metabolic water production, and active vapor uptake. Food contains some moisture, which can be an important water source for some species. Metabolic water refers to water molecules that are produced from chemical reactions, especially respiration, and active water uptake is the ability of an organism to extract water vapor from the air (Machin 1976; Gaede & Knülle 1997). As a group, spiders have received little attention from a water balance standpoint. They have a humidity receptor called the tarsal organ located on the tarsus of each leg, which contributes information that likely influences microhabitat selection (Barth 2002; Ehn and Tichy 1994; Foelix 1996). Many species, including those of the Lycosidae, utilize both tubular tracheae and book lungs, which allow gas exchange while limiting 3 water loss (Foelix 1996). Spiders have been known to uptake water from the soil if moisture is sufficiently available (Humphreys 1975), and would gain water from prey during feeding and drinking. As with other terrestrial arthropods, spiders gain water passively from the surrounding air via passive sorption that is proportional to the ambient relative humidity (Wharton 1985). Other arachnids, most notably ticks, extract water vapor from unsaturated air by an active process (Needham and Teel 1986, 1991; Gaede and Knülle 1997). They splay their palps while extracting water from the air (Sigal et al 1999), probably to