INVERTEBRATE PHOTORECEPTORS G43 



neurons which connect with the peripheral spines are sensitive to hght 

 and function as photoreceptors." 



Among the insects a dermal photosensitivity has been reported in the 

 walking stick Aplopus, a true bug Neides, and a cockroach and as the 

 photosensory mechanism of larval flies (dipterans), larval beetles, and 

 ants. Oehring (1934) pointed out some of the difficulties in establishing 

 the functions even of the eyes, since blinding Chironomus larvae by the 

 use of opaque lacquer was unsuccessful unless the whole head capsule 

 was covered; otherwise light passed through the pigmented surface of the 

 epicranium and affected the eyes from within. Oehmig (1939) found the 

 same true of caterpillars. Welsh (1937) indicated that the true photo- 

 receptors in the head region of fly maggots were still unknown, since the 

 sensory papillae previously credited as light-sensitive organs proved gus- 

 tatory. Final identification of the photoreceptors came from extremely 

 careful microdissection technics, in which Bolwig (1946) destroyed limited 

 areas in living housefly larvae and located a clump of rounded cells which, 

 when eliminated, completely blinded the larvae. In early first-instar 

 larvae these cells were not fully developed, and neither was light sensi- 

 tivity. By the second instar the cell group was well organized but not 

 yet enclosed by other than soft tissues. Such larvae orient with amazing 

 precision, apparently evaluating the shadow cast by their own translucent 

 bodies. In the third instar, growth of the main section of the pharyngeal 

 skeleton provides a pocket almost surrounding the sensory cells, and with 

 age this pocket deepens. Early third-instar larvae orient with even 

 greater accuracy than second-instar, but this ability falls off gradually, 

 apparently as a result of increasing opacity of the body. Third-instar 

 larvae, however, are the only ones that will follow a resultant path 

 between two light sources illuminating them simultaneously. 



Arthropod eyes are of two major types: ocelli and compound eyes. 

 The literature on these organs and on the activities of arthropods follow- 

 ing their stimulation is enormous. Degenerate forms are found in many 

 groups, and major puzzles remain in phylogenetic interpretation. Appar- 

 ently even the ocelli have mixed origins. From comparative embryo- 

 logical and neurological studies, Hanstroem (1926) concluded that ocelli 

 included (1) the nauplian eyes of crustaceans, the ocelli of insects, the 

 median eyes of trilobites, the ocelli of xiphosurans, and the eyes of pyc- 

 nogonids — all arising from a dorsal ectodermal mass in the embryo; 

 (2) the lateral ("secondary") ocelli of modern arachnoids and all eyes of 

 diplopods and chilopods — through degeneration from the ommatidia of 

 compound eyes produced by the lateral ectodermal mass of the embryo; 

 and (3) the ventral ocelli of trilobites and xiphosurans and the median 

 ("primary") eyes of eurypterids and arachnoids — arising from a ventral 

 ectodermal mass of the embryo. For crustaceans, Hanstroem provided 

 particularly detailed accounts (1931, 1934a) emphasizing the neural con- 



