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Topic 10. Thermoregulation.

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(Name)

I. MATERIALS.

Obtain an adult hawkmoth of the family, Sphingidae, a white-lined sphinx moth or the tobacco hornworm, Manduca sexta. However, in a pinch, any large noctuid could be used, or even a bumble bee or honey bee. The advantage of using Lepidoptera is that if they get loose, the worse they can do is fly away or land in your soup instead of delivering a possible sting.

A thermocouple or thermister thermometer (to be inserted into the thorax).

A thermocouple reference junction interface.

A recorder to monitor temperature. A pen recorder is preferred.

A lamp with a tungsten bulb that will generate heat, or some other heat source.

A magnifying glass, such as one used for reading fine print to focus heat.

Clamps and rods to hold the lamp and glass to focus light on the moth.

Dissecting tools.

Tackiwax.

A small soldering iron, the trigger-operated instant-on type.

A Variac, or other voltage regulator.

II. LEARNING OBJECTIVES.

Understand the heating and cooling of the body of the hawkmoth, Manduca sexta.

How:

Measure thoracic temperature by adult moths during external heating.

Observe lack of thermoregulation after disrupting the circulatory system.

Therefore, appreciate the importance of circulation in thermoregulation.

III. INTRODUCTION.

Small insects such as ants and termites can regulate their environmental temperature by a judicious choice of nest site, depth in the soil, judicious use of insulating materials, or orientation to sun. Size of the insect is important in retaining heat. If the insect is too small, the surface to volume ratio is so unfavorable, heat is lost faster than it can be generated. However, above a certain size, the surface to volume ratio along with the presence of insulation, usually by body hairs or scales, allows heat to be built up and retained in the body with acceptable efficiency.

The selection of site is vital to insects that live in alpine conditions. Grylloblattids and certain stenopelmatids occupy niches around glaciers on mountains. These insects can survive in conditions of outside air temperatures below minus 60°C because the temperature under a few feet of snow or ice is usually closer to 0°C. Indeed the grylloblattids are quickly killed outside the temperature range of from minus 5.8°C to plus 10°C.

Although the best known thermoregulators are honey bees, who are able to maintain a hive temperature within narrow limits through even harsh winters, the process of thermoregulation in insects is as common as the porch light on a summer's evening. Noctuid moths are attracted to porch lights starting in late spring and early summer, depending on the latitude. Once an adult moth lands and folds its wings, the process of generating heat ceases because the flight muscles are the main source of heat build-up. If the thoracic temperature drops too low, the adult moth will usually activate the antagonistic sets of direct flight muscles synchronously in isometic contraction so that heat is generated, but the wings vibrate, but flight is not initiated. This is seen as a characteristic "shivering" of the adult moth. Muscle functions are optimized in these moths at elevated temperatures. The hawkmoth thermoregulates the thorax to 44°C, which can be some twenty degrees Centigrade above ambient temperatures when they are active at night, which is remarkable.

Temperature maintenance by bees and moths is accomplished by the dorsal vessel conducting hemolymph from the abdomen through the thoracic flight muscles much like radiator water through an internal combustion engine. However, instead of fluid channels through the motor and attachment to a water pump and a radiator, the insects circulate hemolymph through loops of the dorsal vessel in the middle of the thoracic muscles and use the abdomen for a radiator.

When the thorax is too hot, the dorsal vessel and ventral diaphragm both vigorously pump hemolymph through the thorax to draw heat away eventually to the abdomen where it is dissipated. When the thorax cools below a set point, the amplitude and frequency of contractions of the dorsal vessel and ventral diaphragms decrease which in turn decreases the flow of heat away from the thoracic muscles.

This system with its precise set temperature implies several things. First, a thermostat mechanism in one of the thoracic ganglia must be operating. Second, the thoracic ganglia must contain neurons that participate in regulation of the ventral diaphragm and dorsal vessel. Third, the regulation must be capable of controlling both amplitude and frequency of contractions. The activity of the ventral diaphragm and dorsal vessel must be correlated.

If the nervous pathway connecting the thoracic ganglia and the pumps and diaphragms in the abdomen is disrupted, or if the delivery of hemolymph via the abdominal dorsal vessel to the thorax is prevented, the ability to thermoregulate would be lost, in exactly the same manner that a water cooled motor would overheat if the radiator function were lost.

An experiment first published by Bernd Heinrich in 1971 (described in Heinrich, 1993) will be duplicated here to demonstrate thermoregulation. Heinrich found that the temperature control of the thorax of the adult hawkmoth could be demonstrated in a resting adult mounted so that the wings do not move by merely heating the thorax. The same thermoregulation mechanism that operated during flight, will function at rest if the thorax is in danger of being overheated.

IV. DIRECTIONS.

Anesthetize, then mount an adult moth in such a manner that the wings are immobilized, the abdomen is free to move, and the legs are on a convenient substrate. Insert a miniature thermocouple into the thorax and secure it in place with Tackiwax. You can warm a small soldering iron by plugging it into an adjustable Variac power source, then melt some Tackiwax on the end of the iron and daub the molten blob of wax onto the cuticle and thermocouple. If the wax is not too hot, it will solidify almost immediately, and this should not harm the moth. Or, secure the thermocouple in place with a drop or two of super-glue.

Check the function of the thermocouple and then connect to monitoring instrumentation. We prefer to convert the temperature signal to a voltage and calibrate a pen recorder to record temperature over time at a very slow recorder speed.

The thoracic temperature should be near ambient to start. Now direct a focused beam of hot light onto the thorax, making sure that the thermocouple is not heated directly. The temperature being monitored should start to raise. Leave the light beam in place and monitor the temperature rise.

Plot the temperature on a graph and provide a proper label:

[time vs. temp graph soon to come]

Supply the proper units for the graph after the experiment is complete. As the thorax is heated, the temperature should rise to a maximum. Draw an asymptotic line to indicate the maximum set temperature that the data implies. Enter the maximum temperature here: ___________. Once you are satisfied that a temperature maximum has been reached, turn off the lamp and continue to monitor the temperature. Plot this as well.

Now there are two options. You may disrupt the circulation of hemolymph by placing a curved probe through the intersegmental membranes of the abdomen and tearing the dorsal vessel, or, in a more difficult operation, you may transect the ventral nerve cord where it passes between the thorax and abdomen. Either of these operations may require that the mounted adult moth is held under a dissecting microscope to see what you are doing.

If you prefer not to attempt these finer operations, then you may achieve the same result by applying a hot needle to the dorsal midline of the abdomen long enough to cauterize the dorsal vessel, or cut the abdomen more severely. As a last resort, you may remove a part of the abdomen or ligate it in a severe enough manner to completely disrupt the flow of hemolymph between the abdomen and thorax.

If any of these manipulations bother you, you may remove the head of the moth adult before beginning or pith the brain, if that seems more humane. Once one of these operations is finished, repeat the heating procedure used before and again plot the results.

[time vs. temp graph soon to come]

If your operation has been successful, there should be a dramatic difference in the shape of the curve obtained.

Draw a picture of the moth with the electrodes in place for a permanent record. A strictly stylized drawing of a side view with circles or ovals for head, thorax and abdomen is fine. Draw arrows from the two labels provided to the appropriate places on the figure.

Drawing of the adult moth used in thermoregulation demonstration.

[Drawing soon to come]

V. SUMMARY ANALYSIS.

Describe what was learned from this exercise.

VI. REFERENCES.

Heinrich, B. 1993. The Hot-Blooded Insects. Harvard University Press, Cambridge, MA. See Chapter one, pp. 17-75.

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(Name)

Topic 11