On their poorly insulated parts, they have countercurrent heat exchangers, in which heat of warm, outflowing blood is transferred to cold, inflowing blood. They can swim faster than other fish. Their dark lateral muscles of a tuna are the warm muscles. Tuna blur the boundaries between warm and cold-blooded animals. Metabolic generation of heat by insects and plants Bumblebees are the most well-studied of endothermic insects.
Sphinx moths are also well-studied.
Sphinx moths, like bumblebees, are large for a flying insect and need to be warm to fly. In the morning, they contract their thoracic flight muscles to generate metabolic heat. When the moth is warm enough it can fly. One idea was that moths vary metabolic heat generation. Heinrich proposed that their metabolic rate is fairly constant, and that the moths thermoregulate by varying the rate of heat loss to the environment.