INVERTEBRATE PHOTORECEPTORS 653 



important in the capture of prey. Distance estimation is evident in 

 both naiad and adult stages of odonate insects (Demoll, 1913; Baldus, 

 1924, 1926), robber flies (Melin, 1923), and tiger beetles. Originally it 

 was believed that instinctive reactions led to the capture of prey when 

 ommatidia whose axes intersected at the correct distance were stimulated 

 simultaneousl}^ and symmetrically. But revision of this view is neces- 

 sary since partially blinded odonates and tiger beetles will adapt their 

 behavior to approach prey monocularly, pivot, and seize at the appro- 

 priate instant (Baldus, 1924, 1926; Abbott, 1949). 



Under normal circumstances the various areas of a compound eye 

 apparently serve definite reaction patterns. Painting over one eye or 

 symmetrical parts of both often leads to postural modifications if the 

 animal stands still or to circus movements if it progresses. Descriptions 

 of these modifications are included in a great many texts as evidence for 

 the " muscle-tonus " theory of animal behavior. Unfortunately the 

 observers did not continue their experiments long enough to discover 

 what remarkable modifications can be made b}^ the arthropod nervous 

 system, gradually eliminating the postural abnormalities and circus move- 

 ments and achieving remarkably normal responses. Hence ommatidia 

 that normally serve certain reflexes can take over for others that are 

 blinded, if the central nervous system is given time to make the adjust- 

 ments (Rabaud, 1921, 1925). 



Among crustaceans, trilobites, xiphosurans, and the ametabolous and 

 hemimetabolous insects, compound eyes grow at each molt. The process 

 has been followed in one instar after another of the praying mantis 

 Sphodromantis (Sztern, 1914; Przibram, 1930; Yamanouti, 1933), the 

 cockroach Blatta and walking-stick insect Dixippus (Przibram, 1930), 

 the back swimmer Notonecta (Bernard, 1934), and various other insects 

 and crustaceans (Bernard, 1937). New ommatidia are added along one 

 or more margins, and previously formed ommatidia enlarge. Thus in 

 Sphodromantis the number of ommatidia rises from 3144 at hatching 

 through 10 molts to 8107. Dixippus, by contrast, adds no ommatidia, 

 and the total increase in dimensions is 126 per cent, with a doubling of 

 eye area from birth to maturity. 



The development of the compound eye appears to depend upon nor- 

 malcy of the supra-esophageal ganglion. Damage to this ganglion usu- 

 ally results in failure of the eye to differentiate. In Drosophila the genetic 

 degeneration of the eye is through factors acting on the eye itself, not 

 indirectly through the ganglionic background (Richards and Furrow, 

 1925). Degeneration of the compound eyes in cavernicolous arthropods 

 and deep-sea crustaceans is common and apparently follows a genetic 

 course influencing the eye itself. The literature on these degenerate eyes 

 is quite large (Hanstroem, 1929). Some abyssal forms show hypertrophy 

 of the compound eyes rather than degeneration, and this is believed to be 



