Activation of the Fibrillar Muscles in the Bumblebee during Warm-up, Stabilization of Thoracic Temperature and Flight

The fibrillar muscles of bumblebees may be used to drive the wings during flight and during fanning for nest-temperature regulation (Hasselrot, i960), to produce buzzing sounds, to vibrate the substrate during pollen collecting (Michener, 1962), and to generate heat prior to flight or during the incubation of brood (Heinrich, 1972 b). During fanning, buzzing and the production of other vibrations, the generation of heat is incidental to the performance of mechanical work, but during warm-up and brood incubation the primary task of the fibrillar muscles is heat production. During warm-up the indirect muscles are usually excited by synchronous impulses (Mulloney, 1970; Kammer & Heinrich, 1972), but the motor patterns during the stabilization of thoracic temperature in stationary bumblebees, and during flight at different ambient temperatures, have not been examined previously. Little is known about the relationship between muscle activation and (1) the rate of heat production during warm-up at different thoracic and ambient temperatures, and (2) the temperature excess maintained during stabilization of thoracic temperature at various ambient temperatures. Bumblebees fly at low as well as at high ambient temperatures (Heinrich, 1972 a) and require a high thoracic temperature in order to be able to continue flying (Krogh & Zeuthen, 1941). However, during flight the primary function of the fibrillar muscles is to drive the wings, and the production of heat is obligatory and continuous. Since the fibrillar muscles can be used specifically for heat production during warm-up, it is of interest to know whether their activation is varied also during flight in order to control heat production. We here report on the activation of the fibrillar muscles at different muscle temperatures during warm-up, during the stabilization of thoracic temperature at a variety of ambient temperatures, and during fixed flight at different ambient temperatures.