Thermoregulation in Erythemis Simpiliciollis

Temperature regulation in many species is imperative to survival; however, in dragonflies it is particularly important. For this group of species temperature regulation not only assists in maintaining a constant body temperature, it is also vital for flight. However, how or if dragonflies regulate body temperature is not widely known. This study investigates the relationship between air temperature and body temperature of dragonflies and examines the influence sunlight has on dragonfly body temperature; this study also scrutinizes the differences between dragonfly body temperatures in and out of sunlight. Furthermore because use of flight muscles is a known conductor of heat energy this study looks at the affect of muscle activity on body temperature.

This experiment was conducted by collecting dragonflies of the species Erythemis simpiliciollis. Their body temperature was measured and their activities at time of capture were recorded. The data revealed that dragonflies participate in thermoregulation by changing microclimates when needed. This conclusion explains dragonfly's constant behavior of basking and retreating to shade.

Introduction

There are three hundred-seven known species of dragonflies found in North America (Evans, 2007). A majority of these species including the Eastern Pondhawk scientific name Erythemis simpiliciollis are found near bodies of water including but not limited to lakes, rivers, and streams.

E. simpiliciollis as well as all other dragonfly species are members of the order Odonata and the suborder Anisoptera. Specifically, E. simpiliciollis are members of the family Libellulidae, which also includes the Bar-winged Skimmer and the Twelve Spotted Skimmer dragonflies (Mitchell, Lasswell, 2005). Members of this family as well as all of those in the order Odonata reproduce sexually and have two primary life stages: nymph and adult. The nymph phase is the larval stage of the dragonfly life cycle; during this phase, dragonflies reside exclusively in water or damp environments (Borror, White, 1970). After a stent of time in the water, the nymphs emerge and through incomplete metamorphosis transform into adult dragonflies (Berger 2004). As adults E. simpiliciollis can range in size from one to one and a half inches (Borror, White, 1970).

Adults of some dragonfly species have different characteristics traits in males and females. For example the male E. simpiliciollis has a green face while the majority of their bodies are blue in color (Mitchell, Lasswell, 2005). Likewise, the female E. simpiliciollis is generally green in color (see figure 4 in results).

Due to their close proximity to aquatic habitats dragonflies' primary food sources are generally also found near water. A main component of their diet consists of other insects including but not limited to midges, mosquitoes, and smaller dragonflies (Evans, 2007). Nymphs though they are merely infants are also impressive aquatic predators; they often feed on tadpoles and occasionally small fish (Berger, 2004). Due to their carnivorous and occasionally cannibalistic nature dragonflies are often considered to be among the top predators of the insect world. In fact, their order name Odonata translates from Greek to English as "toothed one's" which denotes dragonfly's characteristic jagged lower jaw that is used for preying on other insects (Berger, 2004).

Dragonflies like many other insects are endotherms and consequently it is extremely important that they maintain a stable body temperature (May, 1982). Moreover, maintaining an acceptable body temperature is essential for survival because in order to participate in flight their flight muscles must be warm (Berger 2004). To control body temperature dragonflies often bask in the sun or alter their body position, orienting it with the location of the sun in a pose known as obelisk position (Berger, 2004). To keep warm some dragonflies also bring their wings in close to their body (Berger, 2004). Others flutter their wings, which warms their muscles and by default their bodies (Evans, 2007). Cold is not the only problem for dragonflies, extreme heat can also be detrimental. To resolve this issue some dragonflies regulate body temperature by quickly plunging themselves underwater, or they simply retreat to shaded areas (Berger 2004).

In this study the goal is to determine if air temperature differs from the body temperature of dragonflies. Moreover, this study will also look at whether or not air temperature influences body temperature and will investigate the affect of sunlight exposure on body temperature. In addition to this, this study will explore whether or not muscle activity affects body temperature.

Methods

This study was conducted on the causeway outside of the University of Pittsburgh's Pymatuning laboratory of Ecology in Linesville, Pennsylvania. To complete this study, large insect nets were obtained as was a temperature probe attached to a thermocouple. The dragonflies used in this study were Erythemis simpiliciollis which were chosen due to their large population in the area surrounding Pymatuning Lake.

E. simpiliciollis were caught in the insect nets and their activity at time of capture was recorded as either: basking in the sun, sitting in the shade, flying in the sun, or flying in the shade. If the dragonfly had to be pursued it was also recorded, as was the organism's sex as determined by their predominant color either blue for male or green for female. The temperature probe attached to the thermocouple was then used to first determine air temperature at time of capture. This was done by simply holding the probe at arm's length and waiting for the temperature reading to stabilize. Once air temperature was determined, the temperature probe was then inserted gently into the thorax of the specimen, specifically in the area between the wings, to determine body temperature. When probed in this specific area E. simpiliciollis is not harmed. The temperature recorded in the data is the maximum temperature as determined by the probe. After body temperature was collected and recorded E. simpiliciollis was then released, unharmed into its natural environment.

After sampling E. simpiliciolli the results were compiled using the statistical computer program PRISM. Using this program linear regressions and Man Whitney-U tests were performed using an alpha value of 0.05.

Results

The compiled data between air temperature and body temperature when subjected to the Man Whitney-U test revealed a p- value of 0.0001. This value is less than the alpha value of 0.05; therefore, the means are not the same. A linear regression was also constructed between the two groups (see figure 2). The regression resulted in a slope of 0.5163 and a p-value of 0.0001 which is less than the alpha value of 0.05, thus it is concluded that there is a relationship between the two groups: air temperature does influence body temperature. Moreover, the standard deviation of body temperature was 3.14 while the standard deviation of air temperature was 1.99 (see figure 1).

A Man Whitney-U test done between prior activity and no prior activity resulted in a p-value of 0.2087. This p-value is greater than the alpha value of 0.05 and thus the means are the same. Additionally, a Man Whitney-U test performed between E. simpiliciollis found in the sun and those found in the shade. This analysis resulted in a p-value of 0.5987, which is greater than the alpha value of 0.05, and the means are the same.

Discussion

During the sampling period a collection of forty-seven dragonflies were located and their body temperature measured. Of these forty-seven, all were females. This anomaly may be due to the fact that males primarily inhabit the areas surrounding their aquatic breeding sites (May, 1977). Furthermore, though the area where sampling took place was fairly close to an area typical for dragonfly breeding, it was also fairly terrestrial and thus the number of males in the area was noticeably different from the number of females.

The data from this study reveals that the means between body temperatures of Erythemis simpiliciollis and air temperature are different. Likewise, the data does conclude that air temperature does influence body temperature (see figure 2). This conclusion implies that dragonflies are not explicitly dependent on the temperature of the air to control bodily functions. This result agrees with the claim that dragonflies are endotherms and are affected by their external environment (May, 1982). Furthermore, this entails that E. simpiliciollis is doing something to alter and maintain a comfortable internal body temperature.

Continuing on, the results illustrated that the two groups: one in the shade and one is the sun, have the same body temperature in E. simpiliciollis. Muscle activity between the groups participating in flight and not participating in flight was also found to be similar. This is in contrast to the idea that during flight, heat convection is taking place and causes significant heat gain (May, 1979). This data is most likely due to the fact that the dragonflies are participating in thermoregulation. Thermoregulation is best defined as "the maintenance, by active behavioral or physiological responses of an organism in its natural environment, of body temperature relatively independent of environmental temperature" (May, 1979). Therefore, based on the definition of thermoregulation the dragonflies would have low body temperature if they were in fact attempting to warm up through participating in flight. This conclusion would also explain why dragonflies in the sun and in the shade have similar body temperatures. The dragonflies are controlling internal temperature through activity and changing microclimates, thus temperature would be stable.

In sum, dragonflies partake in thermoregulation to control their internal environment. From the results, it can also be concluded that among other methods dragonflies can perform thermoregulation by basking in the sun and retreating to shaded areas, they also control body temperature by participating in flight or by resting.

Literature Cited

Berger, Cynthia (2004). Dragonflies. Mechanicsburg, PA: Stackpole Books.

Borror, Donald J. & White, Richard E. (1970). Insects. New York, NY: Houghton Mifflin Company.

Evans, Arthur V. (2007). Field Guide to Insects and Spiders of North America. New York, NY: Sterling Publishing Co.

May, Michael L. 1977. Thermoregulation and Reproductive Activity in Tropical Dragonflies of the Genus Micrathyria. Ecology 58:787-798.

May, Michael L. (1979). Insect Thermoregulation. Annual Review of Entomology. 24, 313-349.

May, Michael L. 1982. Heat Exchange and Endothermy in Protodonata. Evolution 36:1051- 1058.

Mitchell, Forrest L. & Lasswell, James L. (2005). A Dazzle of Dragonflies. New York, New York: Nevraumont Publishing Company.

Smith, Ray F. & Pritchard, A. Earl (1973). Aquatic Insects of California. Berkeley, CA: University of California Press.