RELATION BETWEEN TREY AND PREDATOR — ROEDER 297 



in this nerve and the muscle response (muscle potential) were de- 

 tected by electrodes inserted into the muscle through holes in the 

 proximal end of the femur. The contraction of the extensor tibiae 

 was registered by an electromechanical transducer attached to the 

 tibia. 



The sequence of events recorded in this way is shown in plate 5. 

 The small deflection on the solid line indicates the moment of stimu- 

 lation of nerve 3B near the metathoracic ganglion. The small diphasic 

 deflection which follows by about 1.5 msec, indicates the arrival in 

 the muscle of the efferent impulse. This is followed by the large 

 monophasic muscle potential lasting for 3.5 msec. The muscle po- 

 tential is the sign of membrane depolarization and the spread of exci- 

 tation over the muscle fibers. As the muscle potential decays, con- 

 traction (shown by line broken at i.o msec, intervals) begins and 

 reaches a maximum in an additional 4.0 msec. 



The duration of these events may be summarized in the following 

 form : 



msec. 



1. Excitation time of sensilla — unknown, probably 0.5 



2. Conduction time in cereal nerve i .5 



3. Synaptic delay in last abdominal ganglion 1.1-1.5 



4. Conduction time in giant fibers 2.8 



5. Synaptic delay in metathoracic ganglion unknown 



6. Conduction time in fast motor fiber of 3B 1.5 



7. Neuromuscular delay and muscle potential 4.0 



8. Development of contraction 4.0 



Total time less event 5 1 5-8 



The total duration of events i through 8 is considerably less than 

 the average startle time of 54 msec, as determined in the intact insect. 

 However, this total does not include event 5. The synaptic delay in 

 the metathoracic ganglion is difficult to assess for two reasons. First, 

 it is necessary to place the insect under considerable restraint if elec- 

 trodes are to be placed under its nerves. It seems probable that this 

 condition produces a state of frequent or continuous "startle" in the 

 insect and causes failure of the rapidly adapting synaptic system in 

 the metathoracic ganglion. Second, in the few cases where volleys 

 in the ascending giant fibers elicited motor discharges from the meta- 

 thoracic ganglion it was evident that several successive volleys were 

 necessary, that is, that the metathoracic synapses operate through 

 temporal summation (Roeder, 1948). If this is so it becomes impos- 

 sible to define the synaptic delay, which then depends upon the state 

 of adaptation of the synapse, that is, upon the number of successive 



