206 THE PHYSIOLOGY OF INSECT SENSES 



Studies of the electrical response (ERG) of the eye to flickering light 

 agree beautifully with recent behavioural studies. As described 

 earlier in this chapter, there are oscillations in the ERG which are 

 synchronized with the stimulus flashes. In *fast* eyes (e.g., Calliphora, 

 Apis) the synchrony persists at even higher frequencies than those 

 judged by optomotor responses to be fusing. In 'slow' eyes (e.g., 

 Dixippus, Tachycines, Periplanetd) the responses, both behaviourally 

 and electrophysiologically, are slower. These findings and the con- 

 clusions of others (Zerrahn, 1933 ; Wolf and Zerrahn-Wolf, 1937) that 

 the decisive feature of pattern recognition is the transitory stimulation 

 ofommatidia suggested to Antrum (1948 a, 1948 b, 1949, 1952, 1955 b) 

 the idea of temporal resolution. 



The concept is based upon the assumption that the ommatidium is 

 the optical unit of the eye. In rapidly flying insects the angle subtended 

 by ommatidia in the horizontal direction is approximately twice that 

 in the vertical (del Portillo, 1936). During horizontal movement a 

 point being observed remains in the visual field of a single ommatidium 

 longer; consequently, the summation time for this stimulus is pro- 

 longed. Points of a pattern that succeed each other rapidly fuse less 

 readily. Accordingly, the capacity for discrimination is improved 

 through movement (Antrum, 1949). Studies of ERGs show that two 

 points of light succeeding each other within a critical time fuse if they 

 pass over the ommatidial grating vertically, but act as individual 

 stimuli if they pass horizontally. The electrophysiological responses of 

 *fast' eyes reveal features which tend to favour reception of repetitive 

 stimuli of short duration. The ERG shows that the on-effect for stimuli 

 of 1-To^ second depends only upon intensity. It is independent of 

 duration. The off"-effect depends on the product of intensity and 

 duration. Furthermore, in the 'fast' eyes, according to Antrum, a 

 positive off'-effect originating in the lamina ganglionaris prevents sus- 

 tained depolarization as occurs in 'slow' eyes, and thus 'prepares' the 

 eye for the next stimulus. 



Insects also appear to be responsive to stroboscopic (apparent) 

 motion. In contrast to the negative findings of Gaffron (1934), 

 Antrum and Stocker (1952) demonstrated that this phenomenon 

 exists provided that certain critical conditions are met. For strobos- 

 copic motion to be perceived the time sequences must be short for those 

 insects ('fast') whose eyes possess high temporal resolution and long 

 for those ('slow') species whose eyes possess low temporal resolution. 

 A close relationship exists between the times that are important in 

 motion perception, whether real or apparent, and the times for which 



