When the bee contracts its flight muscles isometrically, what temperature can its thorax reach?

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Correct answer: 3 (>40°C)

It should be recalled that a bee is able to leave the hive at cool temperatures (10–12°C), but only if its thorax is at a temperature of around 30°C. This temperature allows enzymes to regulate the chemical process required for the rhythmic contraction (200 beats per second) of the flight muscles.

In humans, when body temperature drops (hypothermia), muscle tone gradually increases, followed by involuntary and rapid contractions of agonist and antagonist muscles, thus without any movement or joint displacement (isometry).

In the bee, the same phenomenon occurs in the thorax, which contains the antagonistic flight muscles, the dorso-ventral and longitudinal muscles. If these muscles contract simultaneously, no wing movement results (isometry), but only heat production, which can locally reach a temperature exceeding 40°C within a few minutes. 

For further reading:  Sense and nonsense of hive thermal insulation