Insects survive in many different environmental conditions, across the world. But, when winter hits temperatures can be extreme in places, reaching -60℃, and colder! So how do insects survive this extreme fluctuation in temperature? Some insects migrate to avoid these temperatures, but some species stay put, and have physiological adaptations to survive the winter months. Thousands of species spanning several orders, including Lepidoptera, Coleoptera and Mecoptera, use two techniques to survive: freeze tolerance and freeze avoidance, which have evolved independently for many species (Dennis, et al, 2015; Duman, et al, 2004; Li, 2016).
1) Freeze Tolerance:
As temperatures start falling in autumn, insects begin to synthesise 3 components key to their winter survival, these are: antifreeze proteins (AFPs), polyols and ice-nucleating agents (INA proteins).
Freeze tolerant species survive by encouraging ice formation in extracellular spaces, using INA proteins. Through osmosis, water is drawn through the cell membrane creating an equilibrium, through these two methods ice is prevented from forming within the insect’s’ cells, which can lead to severe damage and could become lethal (Bale, 2002).
However, the insect is still susceptible to injury from the ice, this is where the polyols come in. These are used to prevent mechanical damage to the insect and have various uses to do this, such as reducing the fluctuation of water across the cell membrane (Bale, 2002).
The insect has one final hurdle to overcome to ensure its survival over winter. As the winter months draw to an end the temperature begins to rise, and water may attach to the ice crystals, within the extracellular spaces, and cause secondary recrystallisation. This is where it gets complicated. Using AFPs, insects can prevent the growth of ice crystals as they preferentially grow from surfaces with a small radius. AFPs prevent this by adsorbing onto these low radius surfaces of the ice crystal meaning that that they do not grow, unless the temperature reaches the colligative melting point – the Kelvin effect. Essentially the ice crystal will not grow unless the temperature reaches the hysteretic freezing point. Due to the AFPs the water becomes supercooled, and the freezing point is much lower than usual, termed the hysteretic freezing point (Duman, et al, 2004; Zachariassen and Kristiansen, 2000).
2) Freeze Avoidance:
Freeze avoidance is a completely different strategy, using the same materials. Freeze avoidance works by keeping the insects bodily fluids liquid, throughout the entire winter, as opposed to letting the extracellular spaces freeze (Dennis, et al, 2015).
First things first, the insect has its last meal and finds a nice spot to overwinter. Then it begins the process of removing any ice nucleating substances from its body: it’s water content becomes reduced whilst its fat content increases and the digestive system is emptied (Bale, 2002). The insect then synthesises AFPs and polyols which results in the insect having a very low supercooling capacity and thus preventing any bodily fluids from being able to freeze, as long as the temperature remains above their supercooling point (Overgaard and MacMillan, 2017).
To summarise some insects have complex systems allowing them to survive the extreme cold, and it’s pretty cool!
By Linzi Thompson (Email: email@example.com, Twitter: @Apis_linzi )
Harper Adams MSc Entomology Twitter: @EntoMasters
Bale, JS. 2002. Insects and Low Temperatures: from Molecular Biology to Distributions and Abundance. Philosophical Transactions of the Royal Society B: Biological Sciences. 357, pp.849-862.
Dennis, AB, Dunning, LT, Sinclair, BJ, and Buckley, TR. 2015. Parallel molecular routes to cold adaptation in eight genera of New Zealand stick insects. Scientific Reports. Nature. 5
Duman, JG, Bennett, V, Sformo, T, Hochstrasser, R, and Barnes, BM. 2004. Antifreeze Proteins in Alaskan Insects and Spiders. Journal of Insect Physiology. 50, pp.259-266.
Li, NG. 2016. Strong Tolerance to Freezing is a Major Survival Strategy in Insects Inhabiting Central Yakutia (Sakha Republic, Russia), the Coldest Region on Earth. Cryobiology. 73, pp.221-225.
Overgaard, J, and MacMillan, HA. 2017. The Integrative Physiology of Insect Chill Tolerance. Annual Review of Physiology. 79, pp.187-208.
Zachariassen KE, and Kristiansen, E. 2000. Ice nucleation and Antinucleation in Nature. Cryobiology. 41, pp.257-279.