PICTURE: A modern, male, non-biting mosquito (Chironomidae: Tanytarsini) on a blooming apple tree in Munich, Germany. Chironomids occur from the tropics to the Arctic and Antarctic and have existed for a long time More
Photo credit: Viktor Baranov
MUNICH – Scientists use many proxy to reconstruct the ancient climate of the earth. For example, pollen, diatoms, geochemical isotopes, and fossils all help piece together puzzles of past climates. The ubiquity and wide geographical range of insects – such as the non-biting mosquito (Order Diptera, Family Chironomidae), a type of fly – have made them a useful tool for reconstructing paleoclimate around the world in the recent geological past.
Previous research used chironomids to reconstruct the climate during the Holocene (roughly the last 11,000 years) and to compare the fossil insects with modern mosquitoes. “These [technique] is pretty good, but has one major disadvantage: the further back you go in time, the less accurate what we know about reporting on modern animals becomes, “says Viktor Baranov, paleontologist at the Lüdwig-Maximilians-Universität in Munich Die Today’s small dragonflies may not have had a range similar to that of the meter-long Cretaceous dragonflies around 100 million years ago, he explains.
Now Baranov and his team have developed a technique that goes back in time, using the size of fossil mosquito bodies and wings to reconstruct temperatures up to the beginning of the Cretaceous period about 145 million years ago. Baranov said they wanted to add a proxy that could be useful for fossils found in geological environments where other proxies may be unusable. Their results, presented at the General Assembly of the European Geoscientific Union (EGU) in 2021, show that the ratio of insect body size to latitude appears to be a promising new indicator of paleotemperature.
The ability to use insects for paleoclimate work deep in the geological past was tempting, and Baranov said he had started sifting through literature to see if this could be feasible. “I have found that some workers have written down anecdotal evidence in very old newspapers – it appears that the Afrotropic representative of this Congo-based genus appears to be considerably smaller than close relatives from France,” says Baranov.
Given Bergmann’s rule that animals like polar bears or Amur tigers living in the Arctic are large-body animals while their tropical cousins are small, Baranov thought that “there is a similarly reliable, quantitative relationship between body size could give and temperature “fossilized mosquitoes. To test his theory, he had to start measuring insects.
Lots of mosquitoes measure
The ubiquity of insects makes them a good candidate for uncovering global patterns, says Baranov. The Munich collection alone contains around 2 million specimens of chironomids, from the Antarctic to the Arctic, from Australia to England.
The team examined 6,300 specimens that lived sometime between the Triassic (245 million years ago) and today, with good geographic location information. They measured body and wing lengths and found that in chironomids in the northern hemisphere, the further north their range, the larger both became. This means that, like polar bears, the mosquitoes got bigger with colder temperatures.
“The average mosquito becomes one millimeter longer for every five degrees of latitude,” says Baranov. “That was very exciting for me to learn. Because it means that at least in this group there seems to be a very strong signal.”
The results show the potential of using invertebrate fossils for quantitative analysis to reconstruct paleotemperature over millions of years, says Baranov.
In addition, this temperature-size relationship could be useful in predicting what might happen to modern insect populations exposed to climate change. “I’m also working on the decline in insects and the drivers of modern insect extinction,” says Baranov. “It seems that rising temperatures affect insect size, and that affects their ability to fly around.”
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