THE VISION OF ARTHROPODS 



581 



THE VISION OF INSECTS 



The mastery of a new element and the adventure of the experiences 

 afforded by a third dimension would be expected to give a fillip to the 

 sensory reactions of Insects, while the development of flight with the 

 consequent ease and speed of exploring new environments must stress 

 the importance of efficient distance recejDtors in the gathering of 

 adequate data for effective orientation. These expectations have been 

 realized ; and to Insects much the most important recej^tor-organs are 

 the eyes. Indeed, in their efficiency, 

 theii' capacity to resolve a pattern or 

 to interpret movements, the eyes of 

 Insects excel those of most Verte- 

 brates ; moreover, alone among In- 

 vertebrates many species have a fully 

 developed colour sense, while they 

 have assumed a faculty apparently 

 unique to Arthropods — the power to 

 analyse the plane of polarization of 

 light and orientate themselves there- 

 by. Finally, small though the insect 

 brain may be, and dominated though 

 the creature is by automatic and 

 rigid reflex reactions, it shows an 

 amenability to learning and a power 

 to remember unique in the inverte- 

 brate world ■^^*^" ^'^^' — ■'-^^^ Head of the Moth 



SHOWING THE EyES AND THE EnOR- 



In the behavioural activities of mous Antenn.ts (Richard Cassell). 



Insects other senses are also inijoortant. 



The olfactory sense, indeed, would seem to be more fundamental than vision ; 

 thus it has been shown by Schremmer (1941) that newly emerged specimens of 

 the moth, Plusia gamma, seek flowers by scent only, this faculty being presumably 

 imiate, but that once an association with a particular flower has thus been 

 established, further visits are determined by vision and scent. Moreover, in 

 the recognition of their fellows and as a guide to homing when illumination is 

 ineffective, odour is often a major determinant of conduct ; the male moth, for 

 example, with its extremely sensitive antennte, is said to find a female a mile 

 or more distant by this means alone (Fig. 731) (Bonnett, 1779-83 ; Turner, 

 1907 ; Schneirla, 1929-33 ; Carthy, 1950 ; Vowles, 1955 ; Dethier, 1957). 



The organs of smell are situated on the last 8 segments of the antenna? 

 and consist of minute pits -which are present in large numbers, sometimes ujo to 

 a thousand on a single joint. The taste organs occur not only on the mouth 

 and labial paljDS but also sometimes on the antemije and the feet. The sense of 

 touch is subserved by minute hairs associated with the antennae, the maxillie 

 and the face ; the sette are non-living but each has a sensory cell at its base with 

 nervous connections. Many species are without ears but they are certainly well 

 develojDed in insects cajDable of producing sounds : when they are present each 



