210 THE PHYSIOLOGY OF INSECT SENSES 



Goldsmith and Ruck (1958) believe that there are two types of receptor 

 in this organ. 



The exact nature of the contribution of dorsal ocelli to behaviour is 

 still elusive. Alone they are insufficient for phototaxis (Homann, 1924 ; 

 Bozler, 1926; Muller, 1931; Wellington, 1953; Cornwall, 1955; but 

 compare Gotze, 1927 and Wellington, 1953). They are not, however, 

 totally without effect in light-directed behaviour. Elimination of ocelli 

 may cause phototaxis to be temporarily reversed (Muller, 1931), 

 reduce the speed of response to Hght (Bozler, 1926; Muller, 1931), or 

 alter the direction of response to two light sources (Muller, 1931). In 

 Periplaneta the characteristic persistent daily rhythm depends upon 

 stimulation of the oceUi (Marker, 1956). As a consequence of these 

 actions the ocelli are generally considered to be * stimulatory' organs 

 (Bozler, 1926; Wolsky, 1930, 1931, 1933). 



STEMMATA 



With the possible exception of dermal receptors sensitive to Hght, the 

 lateral oceUi or stemmata are the sole visual organs possessed by many 

 larval insects. Structurally they vary from forms similar to the dorsal 

 ocelli of adults (e.g., larval ocelli of Tenthredinidae) to mere pigment 

 spots equipped with refractive bodies. In blowfly larvae the photo- 

 receptors are small groups of vacuolated cells pocketed among the 

 hypodermal cells of the cephalic region (Bolwig, 1946). 



The most thoroughly studied stemmata are those of lepidopterous 

 larvae (Grenacher, 1897; Hesse, 1901; Dethier, 1942, 1943). Each 

 resembles an individual ommatidium. It possesses two lenses, one 

 corneal, the other analogous to the crystalline cone of compound eyes. 



Table 9 



(From Dethier, 1953 c) 



Optical constants of the six ocelli of Isia Isabella 



