When a darner does a splash-dunk, it is sometimes unable to get back out of the water. It becomes stuck, and will die (unless it's close enough for me to save it), either by drowning or being eaten by a fish.
The plot below shows data for the frequency for becoming stuck – the sticking frequency – as a function of the date of observation. The first observation of a darner getting stuck (a sticking event) was about the middle of the season (9/19/11) and we continued to see splash-dunks and sticking events until the end of October. The plot also shows the temperature versus date of observation in ˚F, using the same vertical axis as for the sticking frequency.
No sticking events were seen when the temperature was above 65 ˚F. In fact, we saw 91 dunks before observing the first sticking event. As the season continued, and the temperature dropped, and the sticking frequency increased.
Notice the nice correlation between temperature and sticking frequency. By the end of the flight season the temperature was in the 40s and the sticking frequency was about 25%. At temperatures this low, the flight mechanism is inefficient, and the dragonfly is often just not strong enough to break free of the water. Thus, a dragonfly splash-dunking at the end of the season has a one in four chance of not making it back out again. Quite a high mortality rate. Clearly the benefit of splash-dunking (possibly to clean their bodies and wings) is fairly significant for the population to continue the behavior in the face of a 25% mortality.
The next plot shows the sticking frequency as a function of temperature in ˚F. The data indicate a smooth, systematic drop off in sticking frequency with increasing temperature.
The drop-off in sticking frequency is approximately exponential, as indicated by the smooth curve that gives a good fit to the data. The exponential curve predicts a small probability for sticking events at 65 ˚F and higher, but none were observed. The probability of these events is so low that it will take more observations to get good results in this temperature range.