Beetles can be found almost anywhere, including almost every lake. Also beetles can often be found in very large number. One of the main reasons for being so successful is because beetles make very hard exoskeletons from chitin. It works as an armour and protects them from predators and hostile environments as it is not affected greatly by force, natural chemicals and temperatures.
This is also the reason why parts of beetles can survive very well as fossils. Beetle remains of up to 25 million years old have been found and their chitin has not changed a lot. The stable isotopes in fossil chitin are still pretty much the same as they were at moment when the chitin was made. This makes it possible to measure stable isotopes in fossil beetles and estimate past temperatures from it.
Some groups of beetles (including the water beetles used for this study) can also be used to estimate past temperatures in a different way. This other method is already well developed and it works well. Therefore it is possible to compare the temperature estimates that were found in this study with temperature estimates from the other method.