Discuss mechanisms of thermoregulation among insects

Insects can be categorised as either endothermic, predominately winged species, or ectothermic,

largely wingless species. When an organism is endothermic it is known to thermoregulate– which is

the ability to maintain its body temperature within specific parameters in order to protect and fund

various metabolic processes (Heinrich 1974). This essay will provide detail on airborne insects which

have an extremely simplified thermoregulatory system dependent on physiological adaptations of

muscular activity involved in flight. Additionally, it will focus on behavioural, structural and social

mechanisms of temperature regulation such as: the use of ambient heat, external coatings and

dependency of the metabolic output from others as well as discussing methods preventing

overheating both in a physiological and behavioural sense.

The most basal physiological appendage of insects, in regards to thermoregulation, is the muscles

responsible for flight, located on the thorax. This is the predominant source of heat generation for

most insects, which is only 20% efficient, where the remaining energy fuels the production of

thermal waste energy (Heinrich 1974). This process of heat production can be associated with the

evolutionary development of wings – proto-wings – which were initially used as a means of

thermoregulation, not flight (Lewin 1985). Some select organisms, however, due to their size

convectively loss this heat to their surroundings; namely small insects. Hence they have acquired

structural adaptations that halve the rate of internal heat degradation such as the thick exterior

covering of pile on moths (Heinrich 1974). Insects also display physiological and structural

adaptations which are used for reducing internal temperature. This includes the circulation of

hemolymph to disperse heat throughout the entirety of the insect’s body – as the thorax is highest

in temperature, where its wings are adjoined - allowing for greater rates of heat dispersion as there

is more exposed surface area (Chown 2004). Moreover, convective cooling is utilised where an

organism bodily fluids undergo convective currents to disperse heat into its ambient surroundings

(Chown 2004). However, not all insects have gained such complex internal mechanisms for

thermoregulation and rely on the apparatus that is their behaviour.

Organisms dependant on behaviour, namely butterfly species, for thermoregulation also exploit

their wings, for heat absorption (Lewin 1985). They do this by appressing their wings perpendicular

to the sun and wallow in the ambient temperature allowing it to heat their bodies. A study by Kemp

and Krokenberger (2004) showed that Pieris rapae butterflies in Northern Queensland bask in

sunlight from 07.00 to 17.00 through the duration of winter. Similarly, to cool down the butterflies

simply fold their wings upright and move to shaded areas. These behavioural patterns have led to

the favourable evolution of insect morphology such as wing pigmentation – to increase absorption

of heat. Thus behavioural thermoregulation operates with three main features: morphology (wings),

behaviour (wing posture) and ambient temperature (Kemp and Krokenberger 2004). Other

alternative behaviour can be seen in bee species, particularly Hylaeus cornutus, who coat themselves

in water from a plant’s xylem. This allows for evaporative cooling where the coating of water is

heated from the insect’s own internal temperature and is then evaporated away from the body

(Chown 2004). Insects that use flight to generate high internal temperatures undergo cyclical

behavior to regulate internal temperature, this includes i) pre-flight warm-up or “shivering” where

ATP is catabolically used to warm muscles, ii) the process of flight and iii) rest, which is a process that

allows the organism to decrease their internal temperature (Heinrich 1974). Other organisms glide

through the air which reduces heat production during flight and hence the cycle of flying followed by

rest can be avoided (Heinrich 1974). Some insects, however, do not simply rely on individual

mechanisms for thermoregulation but rather operate as an entire colony generating a temperature

gradient or climate.

By studying the Hylaeus cornutus bee, it can be observed that the machinery of thermoregulation

can operate on a social scale as well as individually (Stabentheiner and Brodschneider 2010). Bee

colonies operate as collectives where different organisms have different roles and functions in the

insect community; these include cleaning, nursing, food processing, guarding and foraging

(Stabentheiner and Brodschneider 2010). Some of these organisms in the colony are restricted to the

hive and use their wing muscles infrequently; as a result they are dependent on other organisms for

heat otherwise their internal temperature would be lower than their ambient surroundings (May

1979). This process is vital to the young of the colony such as larvae and pupae who need to be kept

within specific temperature factors or else they can face detrimental deformations during

development (Stabentheiner and Brodschneider 2010). The maturity of the colony is hence decisive

of the overall climate or temperature gradient for a population – that is a colony constituted mainly

of young larvae and pupae will have a lower overall temperature in comparison to a colony that has a

greater amount of working adults (Stabentheiner and Brodschneider 2010). It should be noted that

this process of social thermoregulation can be generally seen in any insect colony that function as an

aggregate with levels of organisation (Stabentheiner and Brodschneider 2010).

Thermoregulation is a vital process of homeostasis that allows an organism to control its internal

temperature within particular boundaries regardless of the external surroundings. Endothermic

insects contain capabilities which function physiologically, behaviourally and socially to maintain

constant heat generation. In doing so the evolution of insect wings has underpinned the

fundamental application of heat regulation in these species which have been followed by a number

of evolutionary structural adaptations to maintain generated temperature levels. For others the use

of ambient temperatures and exploiting the heat output of others organisms have also allowed

thermoregulation in some species. Unsurprisingly these insects have greater evolutionary ‘fitness’,

than ectotherms, as they have greater mobility and are not subject to environmental variability

(Lewin 1985).

Reference List

Chown SL, Nicolson S (2004) Insect Pysiological Ecology: Mechanisms and Patterns. Oxford

Scholarship 18, 234-277.

Heinrich B (1974) Thermoregulation in Endothermic Insects. Science 185, 747-756.

Kemp DJ, Krokenberger AK (2004) Behavioural thermoregulation in butterflies: the interacting effects

of body size and basking posture. Australia Journal of Zoology 52, 229-236.

Lewin R (1985) On the origin of insect wings. Science 230, 248.

May ML (1979) Insect Thermoregulation. Annual Reviews 24, 313-319.

Stabentheiner A, Kovac H, Brodschneider R (2010) Honeybee Colony Thermoregulation – Regulatory

Mechanisms and Contribution of Individuals in Dependence on Age, Location and Thermal Stress.

Plus One 5, 297-308.