INVERTEBRATE PHOTORECEPTORS 669 



shrimplike forms have been studied extensively, largely in terms of hor- 

 monal interaction. Gammarus and the isopods Ligia and Idotea have 

 been investigated also. A broader treatment among crustaceans is that 

 of Bennitt (1924), though his work was done prior to any appreciation of 

 the hormonal mechanisms involved. Changes in insect eye color with 

 light have been described and investigated, but although use of pigment 

 migration as a tool for investigation of other visual physiology has been 

 suggested, little has been analyzed except the mechanism itself. 



Studies of visual acuity (resolving power) of the arthropod compound 

 eye have been made from the standpoint of the dioptric apparatus, but 

 more reliably from investigations of the activities and responses of intact 

 animals ; good agreement between these methods is characteristic of most 

 papers. Kalmus (1937) tested the response of newly emerged nymphs 

 of the walking-stick insect Dixippiis to vertical stripes alternating light 

 and dark. Hecht and Wald (1933, 1934) established a relation between 

 illumination intensity and acuity in Drosophila and found at best a reso- 

 lution Ho 00 as good as that of the human eye and }-{q that of the honey- 

 bee. A number of insects were investigated for minimal angle for recog- 

 nition of flickering light and for moving stripes. The close relation 

 between acuity, intensity of illumination, and contrast discrimination 

 enters into these papers. Evaluation of intensity discrimination sepa- 

 rately was made for Mya by Hecht (1924a,b) and drawn together into a 

 basic theory (Hecht, 1935a, b). Electrical-response studies of intensity 

 discrimination in the xiphosuran single ommatidium were presented by 

 MacNichol and Hartline (1948), and behavior measurements in the 

 honeybee in this regard include the work of de Haan ( 1928a, b). Wolf 

 ( 1933a, b), and Hoermann (1934). Hundertmark obtained values for the 

 walking-stick insect Dixippus (1937b) and the nun moth Lymantria 

 (1937a,c). 



Practical values for these measurements of acuity and intensity dis- 

 crimination are to be found in terms of form vision and pattern recog- 

 nition. Von Buddenbrock (1935d) claimed the first description of 

 compensatory eyed-tentacle movements to rotating visual fields for any 

 gastropod (in Helix). Peckham and Peckham (1887) reported that there 

 was definite indentification by sight of the egg sac in the spider Theridion 

 and that a male Astia (jumping spider) followed visually after a female 

 even 10 or 12 in. away. Plateau (1887b) considered 1 cm as a maxi- 

 mum distance for form recognition in jumping spiders and wolf spiders 

 (lycosids), with scarcely better in scorpions and far worse in all other 

 arachnids. The Peckhams contested this interpretation (1894), citing 

 many instances of prey and mate recognition at greater distances. Rain- 

 bow (1898) cited further examples of fair visual performance in arachnids. 

 Baltzer (1924) believed that the appearance of prey must be meaningless 

 to the web-spinning Epeira, but Peters (1932) demonstrated that running 



