LIGHT AND MOVEMENT 



61 



angle to the light, thereby putting itself in the position of the pilot of a 

 ship who can steer otherwise than directly in line with the sun or the 

 pole-star. In the simpler types of orientation, light acts as a stimulus 

 attracting or repelling the animal into a more favourable environment ; 

 in menotaxis light is merely used as a means to an end, guiding the 

 animal to a place where it wishes to go whether favourable or not. 



Four types of response which can be considered as menotactic (the 

 term being used in its widest sense) require particular note — the light- 

 compass reaction, orientation to polarized light, orientation to a visual 

 pattern, and the dorsal (or ventral) light reaction. 



Fig. 37.— 

 Menotaxis. 



The orientation MENOTAXIS wherein the receptor organ is sufficiently 

 Eiysia viridis, evolved to appreciate the direction of a light and is able 

 with respect to ^o inhibit other stimuli so that it can orientate itself 

 onentation ^"angle with reference to it alone, 

 which the longi- There is no doubt that in laboratory conditions 



tudinal axis of the , .,, ,iit ,•[^•^ ri-ij. 



Mollusc makes and With controlled artihcial sources ot light many 

 with the direction Arthropods show a remarkably high degree of accuracy 



of 

 (Fraenkel) 



'^ in maintaining an orientation angle by this means ; 



1 p. 68. 



2 V. Buddenbrock (1937). 



^ The common snail, i^eZf.r — v. Buddenbrock (1919) ; the Mediterranean Gastropod, 

 Eiysia— Fv&er\ke\ (1927). 



« Pardi and Papi (1953). 



= Bartels and Bahzer (1928), Bartels (1929), v. Buddenbrock (1937). 



6 V. Buddenbrock (1931-37), v. Buddenbrock and Schulz (1933). 



' Ruppell and Sehein (1941), Lack (1943), Wilkinson (1949), Matthews (1951-.53). 



8 The caterpillars of the gipsy moth, Lymantria dispar — Ludwig (1934) ; the dung 

 beetle, Oeotrupes sylvaticus — Honjo (1937). 



The LIGHT-COMPASS REACTION, whereby the animal travels at a 

 fixed angle to a light (the orientation angle) either in a straight or a 

 circular direction, was first described by Santschi (1911) in his observa- 

 tions on ants,^ and was so named by von Buddenbrock 

 flight (1917) (Lichtkompassbewegung) (Fig. 37). It is a res- 

 ponse of considerable complexity and of wide distri- 

 bution, occurring in some polychfete worms, ^ in some 

 molluscs,^ in the Amphipod, Talitrus saltator,'^ in Web spider 



spiders returning from a kill in the centre of their web,^ 

 in a large number of insects returning to their nests, ^ 

 and in some birds as a means of navigation.' In general, 

 light -compass reactions may be divided into two types. 

 In the first (tropo-menotaxis, Ludwig, 1934), the 

 reaction is essentially simple and tropic in type, being 

 governed primarily by the intensity of the light, and 

 if two lights appear, their effects are summated and the 

 animal orientates itself balanced at an angle between 

 them ^ ; but the more common reaction is one of telo- 



Oeotrupe 



