298 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



volleys which must sum in order to produce a motor discharge. If it 

 is assumed that the minimum requirement for excitation is two volleys 

 in sequence separated by an interval of 2.0 msec, and that there is 

 a ganglionic delay of 2.0 msec, then 4.0 msec must be added to the 

 total time as determined above. This total time is approximately 

 21 msec, an interval not much less than the minimum startle response 

 time of 28 msec, determined in the intact insect. 



The variability of the startle response time (28 to 90 msec.) and 

 the rapid decline of the startle response in free roaches exposed to 

 repeated puffs of air can also be ascribed to temporal summation and 

 rapid adaptation in this metathoracic synaptic system. Short response 

 times probably occur when the metathoracic synapses are disadapted 

 and the stimulus is strong, when only a small number of serial volleys 

 are needed to excite. After recent stimulation a longer sequence 

 would be needed to discharge the partially adapted synapses. Other 

 factors, such as the presence of the head ganglia, play a part in trans- 

 mission at this synaptic system (Roeder, 1948). 



Earlier in this discussion it was suggested that conduction velocity 

 in nerve fibers plays a significant part in the effectiveness of predator 

 evasion. In the analysis given above, events 2, 4, and 6 include only 

 simple conduction along axons. The sum of the times occupied by 

 these events is 5.8 msec. — about 10 percent of the average startle 

 response time of 54 msec. If the shortest values of the startle re- 

 sponse time are considered, then impulse conduction must occupy 

 more than 20 percent of the total time. It is fruitless to speculate on 

 the actual survival value to an organism of an improvement of i or 2 

 percent in its chances of being able to evade a predator (considered 

 highly significant by Ford, 1957), but it is worthwhile to point out 

 that in the cockroach system this end would be accomplished merely 

 by a 1 0-percent increase in condition velocity in the axons concerned 

 in the response without any improvement in other nerve functions. As 

 pointed out earlier, an improvement of this kind seems possible in 

 invertebrates only through an increase in fiber diameter, although in 

 the vertebrates the appearance of a thick myelin sheath has been asso- 

 ciated with an increase of several hundred percent in conduction 

 velocity. 



Measurements of the startle response time in other animal groups 

 could not be found in the literature, although they probably exist. It 

 is interesting to note that the shortest startle response time in man, 

 that of the eye blink to the sound of a pistol shot, is 20 to 54 msec. 

 (Landis and Hunt, 1938). Other facial responses follow 52 to 140 

 msec after the stimulus. These figures fall into the same range as 



