Energy: Forms, Sources and their Utilization in Insects
Authors: Ramya, R.S1., Ranjith, M², Manimaran, B³.
¹ICAR- National Bureau of Agricultural Insect Resources, Bengaluru, India
²Directorate of Plant Protection, Quarantine and Storage, Faridabad, India
³ICAR- Indian Agricultural Research Institute, New Delhi, India

The word Energy is derived from the ancient Greek word “ Energeia” meaning activity or operation". It is often understood as the ability of a physical system to do work on other physical systems. Energy cannot be created or destroyed. Any form of energy may be transformed into another form. In the context of physical sciences, several forms of energy have been defined. These include thermal energy, chemical energy, electric energy, radiant energy, nuclear energy, magnetic energy, elastic energy, sound energy, mechanical energy and luminous energy. These forms of energy may be divided into two main groups; kinetic energy and potential energy. Kinetic energy is defined as the energy possessed due to motion or the work needed to accelerate a body of a given mass from rest to its stated velocity while potential energy is the energy possessed due to position. Energy transformation or conversion is the transformation of one form of energy to another. For example, in photosynthesis, radiant energy is converted to chemical energy; chemical energy is converted to mechanical energy in insect flight and sensory transduction in insect nervous system happens by the conversion of mechanical or chemical energy to electrical energy. Because energy is defined via work, the SI unit for energy is the same as the unit of work – the joule (J). The calorie is a pre-SI metric unit of energy. 1-gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C at standard atmospheric pressure. This is 4.2 joules.

Sources of energy in insects

The main sources of energy in insects are carbohydrates, lipids and proteins. Carbohydrates release 4 kcal of energy per gram and stored as glycogen and trehalose in insects. The first metabolite used is trehalose. Lipids release 9 kcal of energy per gram, stored as triacylglycerides and transported as diacylglycerides in insects. Fatty acids stored as triglyceride is used for energy production through β-oxidation. Proteins release 4 kcal of energy per gram. An important protein used by insect as source of energy is proline.

Utilization of energy in flight

The fuels providing energy for flight vary in different insects. Hymenoptera and Diptera commonly use carbohydrates; locusts, aphids and migratory Lepidoptera depend mainly on fats, but use carbohydrates during short flights and the early stages of sustained flight. Some Diptera and possibly Coleoptera metabolize amino acids, especially proline, at the flight muscles, although the energy is ultimately derived from lipid reserves. Fat is more suitable than carbohydrate as a reserve for insects that make long flights because it produces twice as much energy per unit weight.

Carbohydrates in the flight muscles are usually the immediate source of energy at the start of flight. Insects engaging in long-range migration, such as locusts, some butterflies and planthoppers switch from using carbohydrates as the main source of fuel to using lipids. This switch occurs some 15-30 minutes after takeoff and, in general, the lipids are obtained from the fat body where they are stored as triacylglycerides. In a few species, proline provides the major substrate oxidized by the flight muscles although lipids in the fat body are the ultimate source of fuel. This occurs in the tsetse fly, Glossina, and some beetles, such as Leptinotarsa. Proline has high solubility, so accumulate in high levels and does not require any carrier molecules.

Utilization of energy in metamorphosis

Energy for metamorphosis is usually derived from carbohydrate reserves.i.e., from glycogen and trehalose. But there are exceptions likeGalleria mellonella which rely mainly on lipids and Lucilia cuprina which utilize lipids and proteins rather than carbohydrates.

Utilization of energy in reproduction and embryonic development

Glucose and trehalose together with amino acids are utilized for sperm maintenance in seminal vesicle and spermatheca. Energy for embryonic development is derived from carbohydrates and lipid yolk.

Utilization of energy in diapause

Diapause in insects crave upon the reserves of triglycerides, glycogen and trehalose and to a lesser extent on proteins. Glycogen and trehalose are most important during early stages as they supply glucose and act as precursors for polyhydric alcohol synthesis. Long term survival relies upon the lipid reserves.

Conclusion

By understanding how insects manage their energy budgets during processes like diapause we can identify one of the critical dimensions defining the limits of insect survival in seasonal environments. Although management of an energy budget is critical for understanding the narrow question of how insects survive adverse conditions and synchronize development, diapause also offers a good model for probing more basic questions related to energy storage and utilization. Because many insects appear to be on a tight energy budget, the potential of capitalizing on this vulnerability by searching for new agents that could selectively prevent fat accumulation, elevate diapause metabolism, or deplete energy reserves prematurely would appear to be a tactic that could be exploited to manipulate pest populations.




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