Abstract.  Most fliers and some large perching species of odonata exhibit “wing whirling” activities to elevate body temperature.  Smaller perching species that do not exhibit “wing whirling”, orientate themselves to maximize surface area to collect radiant heat. As body temperature rises for flight, it is in direct correlation with the individual’s body weight and ambient temperature.  Some continuous flying species regulate body temperature by alternating powered flight with periods of gliding as well as controlling circulation within the abdomen and thorax. Because of high rates of heat loss smaller species are incapable of continuous flight.  Large, well-thermoregulated species extend their flight activities earlier and later than smaller less thermoregulated species.  The smaller species’ inability to become active early and stay active late reduces competition for perches and reduces interference in mating for the larger species.

INTRODUCTION

   The temperature of an insect is of great importance in determining its activities (DIGBY, 1955) and life history.  Odonata adults have incorporated a variety of thermoregulatory techniques to control body temperature. A minimum and maximum temperature range in which they can function properly is known to exist.  As poikilotherms, insects must acquire a suitable temperature to operate, yet overheating can be dangerous (HEATH, 1971; EDNEY, 1971; SHEELY, 1982).  Through posture, orientation to radiant heat, circulatory modification, to name a few, Odonate regulate themselves to optimize periods of activity.  In this paper I will review how body temperature is maintained through the incorporation of one or more thermoregulating controls by behavior.

PROPERTIES OF THERMOREGULATION

   Corbet (1963) classified adults into two categories:  “fliers” and “perchers”.  During periods of activity, fliers are predominantly on the wing.  Perchers roost during active periods only to take flight on occasion to catch prey or to interact with other odonata.

Warming Strategies During Low Ambient Air Temperatures in Endotherms

  When ambient air temperatures are low, dragonflies and damselflies exhibit warming up strategies.  For most fliers, and some larger perching species, early activity at a low ambient temperature is produced endothermically.  Thus metabolic heat produced during wing shivering\whirling raises body temperature. Church (1960b) noted that superficial air sacs surround the thoracic musculature in dragonflies, and that these sacs reduce heat loss from the thorax.  Body temperature increases dramatically towards maximum warm up temperatures shortly after wing whirling is established.  Control of circulation of haemolymph from the thorax to the abdomen is presumed in some species of endotherms (MAY 1976).  Thus further control of heat loss can be attained by limiting heat passing from the thorax to the abdomen.

Warming Strategies During Low Ambient Air Temperatures in Heliotherms

   May (1976) stated that the stationary habits of perchers enable them to be well suited heliotherms, due to their ability to select areas that favor thermal conditions.  Perchers obtain most of their body temperature through body position in relation to the sun.   Because of most perchers small size, heat is lost quickly, thus their need to replenish adequate heat loss by basking behaviors (HILFERT & RUPPEL, 1998).
 
   Tracy et al. (1979) established there are two major postures that perchers exhibit at low air temperatures to increase body temperature.  The first occurs when the dragonflies and damselflies rest their bodies on a substrate.  Their wings are pitched down at a forty-five degree angle towards the substrate.  The downward angle of the wings is thought to trap warm air close to the thorax, reducing convective cooling.  They also state that this posture is used to elevate body temperature above that of ambient air, in transition to other postures and activity.

   The second posture maximizes solar heat input.  The individual perches horizontally to the direction of the sun’s rays, its body is off the substrate and its wings held straight out, parallel to the substrate (TRACY et al., 1979).  This posture maximizes surface area in relation to the sun’s energy.  Individuals demonstrate this posture throughout most of their active period.

   Hilfert and Ruppel (1998) illustrated an example of these postures.  They observed that in the zygopteran, Ishnura elegans spent the night resting inactively on grass close to its mating site with its body axis resting on vegetation, posture 1.  Heat transfer from the grass to I. elegans was noted.  The temperature of the substrate was significantly higher than that of the ambient air temperature.  As the individual warmed, it moved to the sunlit side of the plant to take on the normal zygopteran posture, posture 2, where only the legs touched the substrate.  The body was orientated towards the sun to except radiant heat.  As the air temperature increased nearing the minimum flight temperature, the activity period began.

Strategies of Thermoregulation During High Air Temperatures in Endotherms

  In flying species, as ambient air temperatures increases the amount of powered flight decreases (MAY, 1976). The switching between powered flight and gliding reduces metabolic heat produced in the thoracic musculature.  Most species incorporate periods of powered flight, followed by periods of gliding at high air temperatures (MAY, 1976).  I assume that individuals are trying to avoid high body temperatures at high air temperatures.   Corbet (1963) suggests that the anal lobe on certain species wings is enlarged specifically for gliding.

   May (1976) established that circulation between the thorax and the abdomen can reduce increasing body temperatures.  Circulation of heat from the well-insulated thorax to the poorly insulated abdomen results in great heat loss through convection during flight.  By varying air speed, May concludes that some species have the ability to control heat loss.  This is possible since heat loss at four meters per second is almost twice that of still air.  Typical speeds for Odonates is from zero to nine meters a second (CORBET, 1963).   It is suggested that further heat loss may occur in some species that depress their abdomens during flight in response to high temperatures.  It is thought that these species are possibly trying to shield their abdomens from solar radiation.  In doing so, increased heat loss may occur from the increased surface area of the abdomen in relation to wind resistance furthering convective heat losses.

Strategies of Thermoregulation During High Air Temperatures in Heliotherms

      As perchers approach their maximum temperature tolerance they adopt two different behavioral postures.  These heliotherms adopt a position in which their abdomen is pointing directly up or towards the sun and their wings well above the tergal plane (MAY, 1976).  This was defined by Corbet (1963) as an obelisk.   The second posture is where the organism faces directly into the sun with its abdomen and thorax depressed (TRACY et al., 1979).  Both postures optimize the avoidance of direct solar radiation.  Tracy et al. (1979) suggest that by perching facing the sun convection occurs quicker and this pushes the body temperature closer to ambient air temperatures.

   May (1977) also concluded that behaviorally, odonates control body temperature in relation to perching sites such as partially shaded vs. sunny patches.  Their perching height can also be a factor.  Ambient temperatures close to the ground differ from those a meter or higher (MAY, 1976).

CONCLUSION

  In order for some odonates to endothermicly produce heat they must expend enormous amounts of energy.  Energy expenditure can be predicted by the following: thermal conductance decreases as body mass increases, thus body temperature increases with body mass (MAY, 1976).  This explains why larger species’ exhibit this type of behavioral warming and that smaller species have to behavioral thermoregulate posturally.

   Advantages to endothermic heat production are obvious.  Body temperature is independent of ambient air temperature (MAY, 1976). The ability to become active independently from the ambient air temperature reduces competition early in the active period and later in the active period from species unable to thermoregulate in this manner (HILFERT & RUPPEL, 1998).

   Even though smaller bodied dragonflies and damselflies are unable to regulate adequate heat endothermicly, they are good heliotherms.  Through postures that either absorb radiant heat or postures that reduce the amount of radiant heat, they can monitor body temperature effectively.  Choosing perching sites is also important in their behavioral temperature regulation.  As shown by May (1976) they can pick temperature specific locations according to body temperature.  A major disadvantage to thermal heating by the sun is that on cool cloudy days activity may be severely limited to those who can produce heat endothermicly.

   For both large and small species of dragonflies and damselflies, keeping within the required temperature zone is a must.  If temperatures rises too high, loss of motor control can be expected (May, 1976).  Since dragonflies and damselflies have to be ready to catch prey, escape predation, and interact with other Odonates, it is important to stay within optimal parameters, otherwise injury or death may occur (MAY, 1976).
 

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