Reading the rather gruesome recent Guardian article on aircraft stowaways who freeze yet survive (less than a quarter of them) reminded me of aphids. Aphids (greenfly, blackfly) are those squishy things that infest bean and rose shoots, preventing them from developing normally. They are soft-bodied and stay out in the open all year, yet aphids, in high latitudes like Scotland and Canada, can survive frosts.
Playing with aphids and temperature seemed to be a favourite zoology project in my undergrad uni, at Newcastle upon Tyne, a famously cold place in winter. One of my friends worked on this and has since had a stellar career in entomology in the UK. In temperate countries aphids are important – they are major pests, as they are vectors of plant viruses (e.g. in wheat) and also can develop huge numbers very quickly, each one feeding on the host plant juices. As Shelley might have said:
Hail to thee blithe aphid
Pest thou ever wert
While other insects pollinate
All you can do is hurt
(Yeah, okay, apologies not just to Shelley but anyone reading this – I made it up to put in my third-year exam almost 40 years ago; there’s a few more verses…)
How they breed quickly is another little magic thing about aphids – they are facultatively parthenogenic. Big words for a simple process – it means that in the right conditions the females keep producing babies without having all the fuss and nonsense of sex. That makes them easy to keep in the lab as well, so they are often ‘lab rats’ for experimental work.
Anyhow, going back to the cold, adult aphids are not tolerant of severe cold. Depending on the species, they can survive two weeks at -2°C (continuously), but do not survive temperatures much lower than that even for short periods. Cold-climate aphids get through winter in thick-walled eggs. But other insects, such as flies, may be freeze tolerant, surviving temperatures down to -40°C. How do they do this?
Survival of these insects is based on having a mixture of chemicals in the blood system – some that can allow partial freezing in the blood but outside cells, and some, such as glycol, sugars and various proteins, that are antifreeze agents.
We don’t have these chemicals in our bodies, so have much more difficulty surviving cold if we allow our body heat to drop much. Of course, insects are cold-blooded, having no internal heat-generating system.
If those insects are not amazing enough, there’s a non-biting midge called Polypedilum (family Chironomidae) living in central Africa with larvae that can survive severe dessication (water content down to 3%) for 17 years and then be ‘woken up‘ with water to develop normally.
Such dessicated larvae can be heated to 100°C, frozen to -270°C, given high doses of radiation, or immersed in 100% ethanol, yet survive to develop normally.
Who needs science fiction? I suspect these things will be around long after we are gone.
Have a question specifically for Chris? Email editor@2508mag.com.au
