HEAT TRANSFER IN DRAGONFLIES: 'FLIERS' AND 'PERCHERS' · PDF file Heat transfer dragonflies in...

Click here to load reader

  • date post

    20-Aug-2020
  • Category

    Documents

  • view

    0
  • download

    0

Embed Size (px)

Transcript of HEAT TRANSFER IN DRAGONFLIES: 'FLIERS' AND 'PERCHERS' · PDF file Heat transfer dragonflies in...

  • J. exp. Bio/. (1978), 74. *!-& 17 With 17 figures

    Printed in Great Britain

    HEAT TRANSFER IN DRAGONFLIES: 'FLIERS' AND 'PERCHERS'

    BY BERND HEINRICH AND TIMOTHY M. CASEY

    Department of Entomological Sciences, University of California, Berkeley, CA 94720 ami Department of Environmental Physiology, Rutgers University,

    New Brunswick, NJ 08903

    {Received 19 July 1977)

    SUMMARY

    1. Both 'perchers' (Libellula saturata) and 'fliers' (Anax junius and Aeshna multicolor) remained active in the field in sunshine at air tempera- tures from at least 24 °C to 36 °C.

    2. The percher basked at low air temperatures and regulated exogenous heat input by postural adjustments. It markedly reduced flight activity at high air temperatures but flew nearly continuously at intermediate tempera- tures.

    3. In direct sunlight, the abdomen of L. saturata heated faster than the thorax, but this percher exhibited little or no capacity to transfer heat between abdomen and thorax.

    4. In contrast, the fliers gave no evidence of behavioural thermoregula- tion, but both showed impressive capacities for heat transfer from thorax to abdomen.

    5. When heated exogenously on the thorax the temperature of the entire abdomen of both fliers increased uniformly, but with endogenous heat production during pre-flight warm-up there was only a slight temperature increase near the anterior portion of the abdomen.

    6. Removal of abdominal air sacs or immobilizing the abdomen with wax to prevent all abdominal pumping did not significantly alter the capacity to transfer heat from thorax to abdomen.

    7. Ligation of the heart anywhere along the length of the abdomen abolished heat transfer. Given sufficient exogenous heat input, fliers that can regulate their thoracic temperature by transferring the excess heat to the abdomen died in about 2 min due to overheating when the heart was occluded. Under our experimental conditions the fliers appeared to thermo- regulate exclusively via a control of blood circulation.

    INTRODUCTION

    It has been assumed for a long time that dragonflies thermoregulate. Corbet (1962, pp. 125-133) discussed the possible role of various postures and behaviour patterns in thermoregulation and recognized two types of Odonata: those which, when active, remain continuously on the wing which he called 'fliers', and those which spend most of the active period on a perch from which they make short flights, which he called

    erchers'.

  • 18 BERND HEINRICH AND TIMOTHY M. CASEY

    May (1976) presented the first data on dragonfly body temperatures, giving direcU evidence that both perchers and fliers regulate their thoracic temperature. He examined primarily the role of postural adjustments of perchers to incoming solar radiation, but he also showed that the flier Anaxjunius could transfer heat from thorax to abdomen. The fliers heat up the thorax during flight due to endogenous heat production, but the perchers heat up the abdomen while basking. We here examine physiological mechanisms of heat exchange between thorax and abdomen in a large percher and two large fliers.

    MATERIALS AND METHODS

    All of the dragonflies used in this study were captured by hand net along the creek at Del Puerto Canyon, Stanislaus County, near Patterson, California. Immediately after capture the animals were confined in paper triangles, in 100% R.H. in a dark box kept in the shade. They were used in the laboratory within 48 h.

    We worked primarily with three species: Anax junius (Drury), Aeshna multicolor Hagen (both Aeshnidae), and Libellula saturata Uhler (Libellulidae).

    Field measurements included the timing of flight and perching durations. Durations of activity of animals encountered by walking along the stream-bed of the Canyon were timed to the nearest second with a stopwatch. Since individuals were not identified it is probable that not all measurements were of different individuals. Body temperatures of tethered individuals were measured in ' still' (inside a cardboard box within 20 cm high sides) and moving air (unbaffled and under field conditions when animals were active; air velocity was undetermined) with 36-gauge copper-constantan thermocouples using a portable Omega potentiometer. Animals were held in place between pins on a styrofoam pad, and the thermocouples were implanted about 1-2 mm into the thorax and between the tergites of the abdomen. As examined and discussed previously (Heinrich & Pantle, 1975) the conduction of heat down the thin thermocouple wires has negligible effect on body temperature in relatively large insects such as dragonflies. Changes in body temperature were read at 10-30 s intervals after the animals had been exposed to direct solar radiation.

    Temperature changes of thorax and abdomen during heating and cooling live and dead individuals in the laboratory were measured and recorded continuously on a Honeywell multichannel potentiometric recorder. As in the field, the animals were held in place with pins on a styrofoam pad. The tethered animals had one 36-gauge copper-constantan thermocouple implanted in the thorax and 2-4 in the abdomen. Abdominal thermocouples were placed between overlapping tergites without piercing the intersegmental membranes. They were glued in place with a mixture of molten beeswax-rosin. Heat was applied to either thorax or abdomen with a narrow beam of light from an incandescent microscope lamp. Aluminium foil was placed between thorax and abdomen to minimize passive heating of the non-illuminated body part. Immediately following an experiment, the animal was killed by the injection of a drop of ethyl acetate and the experiment was repeated with all leads left in place. Unless otherwise indicated, the heat lamp was in the same position to within 1 mm.

    Passive rates of cooling were measured using recently killed animals and from these measurements we calculated Newtonian cooling constants.

    Frequency and relative amplitudes of the heart and the abdominal pumping

  • Heat transfer in dragonflies

    Aeshna Libelluia

    A

    Aeshna

    Pleuron Tergite

    Sternite

    Fig. i. Anatomy of the abdomen of Aeshna and Libelluia. Note difference in length and width in the two species. Libelluia abdomen is shown while expanded and flattened (left) and while contracted (right). Cross section of abdomen of Aeshna schematically illustrates major internal body parts surrounded by air sacs. Dashed lines show where wax was applied (see Methods) to prevent abdominal pumping.

    movements were measured with paired electrodes connected to Model 2991 Biocom Impedance converters and recorded with a Beckman R411 dynograph recorder (for details see Heinrich, 1976).

    In the dragonflies there is no telescoping of the abdomen during abdominal pumping; all abdominal pumping consists of lateral abdominal movements. In some experiments, as indicated, we prevented abdominal pumping by glueing the ventral lateral edges of the tergites with beeswax-rosin (see Fig. 1). Where indicated, the heart was exposed by removing a dorsal section of an abdominal tergite and underlying dorsal air sacs (Fig. 1). The heart, which we raised out of the abdomen with a bent insect pin, was ligated with a human hair.

    Blood volumes were measured by removing a portion of the dorsal abdominal

  • 2 0 BERND HEINRICH AND TIMOTHY M. CASEY

    40 r-

    0 07.00 08.00 09.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00

    Time of day (h)

    Fig. 2. Increase in air temperature in shade on a sunny day in late June at the Del Puerto study site, and the abundance of Libdlula saturata (O) and Aeshnids ( x) {Anaxjumus and Aeskna multicolor) throughout the day.

    tergites and withdrawing as much blood as possible with z fi\ capillary tubes. These volumes thus represent only fluid that was not bound to the tissues. We made our measurements only on living animals either immediately after capturing them, or within a day after capture while they had been maintained continuously at ioo% R.H.

    RESULTS

    Ecology and behaviour. In the area where we observed and captured dragonflies, air temperatures were typically high and solar radiation was unimpeded by clouds. In late June, air temperatures in shade increased from 19-4 °C at 07.00 h to 36-8 °C at 14.20 h (Fig. 2). Counts of animals seen along a 100 m section of the creek showed that both groups of dragonflies peaked in abundance at intermediate temperatures at 10.00-11.00 h and declined during the hottest part of the day. The Aeshnids were absent from the study area between 14.00 and 15.30 h, but we observed an occasional L. saturata even during the hottest part of the day. At other times 2-6 animals could usually be sighted at any one point in the study area.

    The behaviour of the dragonflies changed markedly at different temperatures. The Aeshnids either flew 'continuously' (not seen to land) or they perched, invariably in the shade. The perchers that remained visible at the study area throughout the day displayed a wide range of behaviour possibly related to thermoregulation. We did not observe L. saturata until about an hour after the sun was on the study area. At the low air temperatures (24-1-25-5 CC), in the early morning, they spent long durations perching with the dorsum of the thorax and abdomen generally oriented at nearly right angles to the incoming solar radiation (Fig. 3). At intermediate temperatures (27-5- 31-0 °C), however, most animals encountered along the creek perched only for shor|

  • Heat transfer in dragonflies 2 1

    400r

    300

    e % 200 00 c

    100

    4- 24 26 28 30 32 34 36 38

    Air temperature (°