INVERTEBRATE PHOTORECEPTORS 645 



In Aranea, at least, form vision was not demonstrated, but the ocelli per- 

 mitted an absolute evaluation of light intensity, used by the spider in 

 determining the nature of its activities. 



The number and arrangement of ocelli in other arachnoids are highly 

 variable. In Solpugida the primary ocelli are usually on a tubercle; two 

 pairs of secondary ocelU may be present elsewhere on the body (Hilton, 

 1932b). In Pseudoscorpionida a pair of primary ocelli are usual; in a 

 few genera a pair of secondary ocelli are found in addition (Hilton, 1931). 

 Among scorpions the primary ocelh are best developed, but from two to 

 five pairs of secondary ocelli may be present laterally on the cephalothorax 

 to the rear of the primary pair. In Pedipalpida ocelli may be lacking 

 altogether, but usually a median pair are present on bead-like elevations, 

 and three pairs of secondary ocelH in more lateral positions (Hilton, 

 1932a). In Phalangida usually one pair of ocelh is found on a turret 

 on the back (Hilton, 1932c).. No correlation has been noted between 

 predatory, scavenging, or parasitic habit and the number or arrangement 

 of ocelli among the Acarina. In Pycnogonida two to four ocelli on a 

 special eminence are usual, but some are eyeless, and others have acces- 

 sory ocelli in addition. 



Among centipedes (Chilopoda) there is a full range from eyelessness to 

 as many as 40 ocelli in a group as an aggregate eye or compound ocellus. 

 Most millipedes (Diplopoda) possess simpler ocelli or none at all. No 

 responses are known to be mediated by the ocelli in either group; blind 

 forms seem to react in the same way as do those with eyes, and dermal 

 photosensitivity is the only obvious explanation (Plateau, 1887a). 



Ocelli are found in many insect orders, the characteristic number being 

 two or at most three (Homann, 1924). Klug (1831) and Imhof (1901a, b) 

 catalogued their presence or absence. Link ( 1908a, b, 1909a, b) described 

 their morphological features in hemimetabolous insects and the adults of 

 holometabolous insects. Some possess a tapetum (Hess, 1920c). Those 

 of adult dragonflies may show strong astigmatism (Tuempel, 1912). In 

 the grouse locust Acridium the ocelli are dimorphic in that those of the 

 female show a double curvature — like a bifocal spectacle lens — producing 

 two images at different distances from the lens (Tuempel, 1914). No 

 explanation is available. 



Immature stages of holometabolous insects are often described as hav- 

 ing ocelU. Hesse (1901a) distinguished between "true" ocelh (without 

 crystalHne cones) in adult insects and hemimetabolous types and "iso- 

 lated ommatidia" with crystalhne cones in larvae of Lepidoptera, Tri- 

 choptera, Neuroptera (s. lat.), some Diptera, and a few Hymenoptera. 

 Since the larval eyes in these holometabolous insects contribute nothing 

 to the adult eyes, their correspondence in structure should be regarded 

 as analogous rather than homologous. Those of dipterous larvae were 

 studied by Constantineanu (1930). 



