216 



THE EYE IN EVOLUTION 



Jerrymander 



JEBRYMANDERS (soLiFUG^) — active, pugiiacious, non-venomous, 

 nocturnal creatures found in warm countries — possess a pair of median 

 (principal) eyes situated on a small tubercle and one or two pairs of 

 lateral eyes usually rudimentary, difficult to see and probably function- 

 less. Both types are simple cupulate ocelli with direct (verted) retinse 

 (Scheuring, 1913; Demoll, 1917). 



MITES and TICKS (acarina). mites are minute Arachnids of which 

 over 20,000 species are known, found almost universally in the earth or 

 in water, salt and fresh, often of parasitic habit on or within animals 



(including man) and plants whether alive 

 or decaying after death : well-known 

 human j^arasites are SarcojJtes scabiei (the 

 itch-mite) causing scabies, and Dernodex 

 foUiculorum found in the hair follicles ; 

 the harvest-mite (chigger) is a virulent 

 pest to both man and animals (particularly 

 rodents), while others infest insects (Isle 

 of Wight bee disease) and others plants 

 (gall mites, red spiders, etc.). Many, such 

 as SarcojJtes are without eyes (Fig. 219) ; 

 others, such as the Prostigmata and the 

 Hydracarina (fresh-water mites) are pro- 

 vided with 2, 4 or 6 ocelli on the front and 

 lateral aspects of the head depending on 

 the species, the individual organs being 

 sometimes fused (Fig. 220 and 221) (Lang, 

 1905). Each possesses a convex lens often 

 difficult to distinguish from the surround- 

 ing skin. 

 TICKS (ixoDiDEs) are larger than mites and are frequently of 

 biological importance as causing disease (tick-fevers) in man ^ and 

 animals. 2 Most types are without eyes, but such species may have 

 thin transparent areas on the dorsal surface which perhaps respond to 

 differences in the intensity of light. When visual organs are present 

 they are extremely rudimentary, being minute ocelli mounted curiously 

 on the animal's shoulder (Fig. 222). 



Figs. 220 and 221. — The Eyes of Fresh-water Mites (Hydracarina). 



Fig. 219. 



-The Mite, Sarcoptes 



SCABIEI. 



(Female) ( X 125) (Sutton and 

 Sutton, Hh. o/ Dis. of the Skin, 

 Mosby). 



Fig. 



220. — The 4 separate ocelli of 

 Limnesia. 



Fig. 221. — Hijgrobates, showing fusion 

 of the anterior and posterior ocelli 

 (after P. Lang). 



^ Texas fever. Rocky Mountain spotted fever, etc. 

 2 Red -water fever in cattle, heart -water in sheep, etc. 



