524 



Comparative Animal Physiology 



traction (except at low frequencies in certain instances: fusion at 10/sec. 

 in haltere muscle), and the fact that there is no synchrony between oscilla- 

 tion of the muscle and incidence of motor nerve impulses or electrical stimuli, 

 seem to indicate that these are not concerned directly with contraction of 

 these insect musclesJ^ 



The natural stimulus for initiating haltere oscillation seems to be the dis- 

 tortion of the thorax caused by the indirect flight muscles. This can be 

 simulated bv pressing on the sternum of a slightly anesthetized fly with 

 a needle. Contraction of the haltere muscle causes dorsiflexion of the 

 haltere; relaxation of the muscle is accompanied by ventral movement due 



Fig. 193. Ventral (V) and dorsal (D) views of the left haltere of Lucilia sericata. c, 

 condyle of secondary articulation; h, main hinge line; m, point of muscle attachment; 

 P, strong point in the articulation; hp, basal plate; dsp, dorsal scapal plate; dHp, dorsal 

 Hicks papillae; Icho, large chordotonal organ; up, undifferentiated papillae; vsp, ventral 

 scapal plate; vHp, ventral Hicks papillae, (X 100). (Pringle.'^) 



to the elasticity of the hinge. The ventral movement again acts as a 

 stretch stimulus, and the cycle is repeated, presumably as long as the ar- 

 rival of nerve impulses to the haltere muscle maintains a heightened ex- 

 citability. 



The haltere, its joint, and the muscle form a mechanical system which 

 oscillates at its resonance frequency. Reducing the mass of the haltere 

 increases its frequency of oscillation. 



Removal of the halteres seriously interferes with the ability to fly (Fig. 

 194), particularly affecting the ability to maintain orientation in the hori- 

 zontal plane. The flight abnormalities are not always the same but they 

 always exist, unless some powerful orienting stimulus, as a source of light, 

 is present. 



How the halteres regulate orientation during flight is shown in Figure 195. 

 The center of gravity of the haltere lies to the rear of the haltere axis. When 



