Humans and most other mammals are called "warm-blooded," meaning that our body temperature arises from heat generated within our bodies. All our cells are actively engaged in chemical reactions that generate heat. They are like the engine of a car that runs all the time. Even at idol the engine consumes gasoline and produces carbon dioxide and heat. Like the automobile engine we consume fuel (carbohydrates, fats), burn it and use the resulting energy in several ways: 1)to grow, repair and replace tissues, 2) store energy (as carbohydrate and fat), 3) keep our body temperature constant (98.6 F°), and 4) move about our environment. The cells of a "cold-blooded" animal do the same things, but they don't produce enough energy to raise body temperature above that of its surroundings. The only way a cold-blooded animal can raise its temperature is by finding warmer surroundings like basking in the sun. In fact, an insect sitting in the sunlight may have a higher body temperature than a human. Because of this it is better to use the terms ectothermic and endothermic in place of cold-blooded and warm-blooded. (Ectothermic means heat is obtained from external sources; endothermic meand heat is generated from internal chemical reactions.)
Bumblebees are partially endothermic. They can generate enough heat to fly if the temperature of their environment is not too cold. They can accomplish this feat because of three features: 1) the way their body fluid circulates, 2) their powerful flight muscles and 3) the way their abdomen is attached to their thorax.
Let's begin on a cold morning. The bee contracts all its flight muscles simultaneously. The muscles that move the wings up are straining against the muscles that move the wings down, so the wings don't flap, they just shudder a little. It's an isometric exercise – muscle tension without work. It looks like they might be shivering. The contraction of these opposing muscles generates heat and the thorax begins to warms up. The hairs that densely cover the bumblebee's thorax act as insulation, so some of this heat is retained and warms the body fluid that bathes the organs in the thorax.
Insects have an "open" circulatory system, meaning that they lack circulatory vessels like arteries and veins and their tissues are simply bathed in a fluid (called hemolymph, the insect's blood) that is slowly circulated. The circulation of hemolymph is accomplished by a heart, which is a simple muscular tube that runs from the abdomen through the thorax to the head. The heart beats with a wave of contractions that push the hemolymph inside the tube forward toward the head end. (This is like our esophagus works when we swallow food or water.) The heart keeps pushing hemolymph forward so the hemolymph in the head end is forced to move back toward the abdomen. So bee blood slowly flows from the head back through the thorax into the abdomen and then gets picked up by the heart and pumped forward to the head again. That's how the blood gets circulated.
The blood vessel in the thorax runs between the flight muscles and is warmed by the heat produced when they contract.
Now the way the bee body is built becomes important. The thorax and abdomen are only narrowly connected, like a wasp waist. (You have to have a bee in hand to see this – the thorax and abdomen look like they are broadly connected, but they really aren't.) The heart must go through this narrow connection and all the hemolymph in the head and thorax must flow through the same connection as it moves from the head back to the abdomen. The blood coming from the head and throax has been warmed by its contact with the contracting flight muscles. The hemolymph in the heart that is coming from the abdomen is cooler and it is warmed by the warmer hemolymph moving through the connection between thorax and abdomen. This heating of incoming hemolymph preserves the higher temperature in the thorax, and, as the flight muscles continue to contract, the thoracic temperature rises until the flight temperature is reached. This mechanism is called a counter current heat exchange and the principle is used in many mechanical devices, e.g., air conditioners, to improve their performance.
If a bee starts early in the morning when the temperature is in the low 70s it won't be able to fly. It must reach a minimum of 86 degrees F before its flight muscles can operate with the necessary speed of contraction. Once that temperature is reached it can fly away. (The temperature in the thorax can reach as high as 104 degrees F or more – the equivalent of a delirious fever for a human. And bees are supposed to be cold blooded!