GRAPHIC .MICROSCOPY. 



By E. T D. 



No. XIII. —The Red Water-Mite (Eylais Extendens (?)). 



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N reference to the se- 

 ries of twelve Plates 

 published during 

 the past year, the 

 writer is desirous 

 of thanking nu- 

 merous correspon- 

 dents for valuable 

 information, 

 chiefly relating to 

 minute details of 

 st ru c ture, and 

 n omenclature ; 

 and to reply to 

 suggest ions, 

 worthy of consid- 

 eration, which, if 

 accepted (in some 

 instances), would 

 advance the selec- 

 tion of subjects 

 beyond the scope of " popularity " into the 

 region of biological research. With low powers, 

 and special illumination seeking picturesque effects, 

 many deep and important peculiarities, possibly 

 bearing upon classification, necessarily cannot be 

 detailed or even exhibited ; and to enter into intri- 

 cacies of structure, with higher objectives (as has 

 been proposed), would far outstrip the motive of 

 the work ; it may therefore be desirable to repeat that 

 the raison d'etre of " Graphic Microscopy " is to en- 

 courage the observations of our younger subscribers, 

 in presenting them with an attractive and accurate 

 picture of a distinct subject : simple, popular, easily 

 procurable, and susceptible of artistic treatment. 



The present example, drawn from life, under 



moderate amplification, necessarily screening specific 



character (the same individual furnishing the dorsal 



and ventral aspect), fairly represents in form and 



No. 241. — January 1885. 



general appearance, a British fresh-water spider-mite, 

 of the Order Acarina, and Family Hydrachnea, of 

 which there are six Genera ; four, Arrenurus, Atax, 

 Eylais, Hydrachna, the most attractive in beauty j 

 Diplodontus and Limnochares, parasitic and some- 

 what obscure. 



The specimen figured is an elegant and lively 

 individual of a form commonly met with : Eylais, so 

 designated from collation with published description j 

 the writer, however, was fortunate in the proof of the 

 plate having been seen by Mr. C. F. George of 

 Kirton in Lindsey, whose articles in this journal, on 

 t'hese particular organisms, commencing Sept., 1882, 

 continued at intervals until August 1883, and illus- 

 trated with woodcuts "of scientific importance and 

 accuracy, establish a basis of valuable authority on 

 the subject ; the number of the eyes in these minute 

 creatures appears to be a matter of important obser- 

 vation, and, as Mr. George remarks, their singular 

 peculiarity and arrangement cannot be overlooked. 

 In the drawing, their position is only indicated ; 

 detail would have demanded an amplification so 

 expansive as to have taken the subject beyond the 

 scope of a general view of the entire creature ; under 

 deeper examination, four eyes are discovered in pairs, 

 upon the dorsal surface of the cephalothorax, ap- 

 proximating so closely that each pair is seemingly 

 combined in one ; even in such minute Acarina these 

 organs possess an ocular power superior to those of 

 insects, and often the appearance of a solitary eye • 

 under closer examination is found to be what has 

 been happily termed " helved," the eye projecting on 

 two rounded semi-circles, reniform, a fusion of two 

 stemmata ; a divided eye ball. Such a minute condi- 

 tion could not have been shown in a field involving 

 the general aspect of the upper and under sides of the 

 same individual, but it is a peculiarity of significant 

 importance in the decision of species, and upon this 

 point (apart from the portraiture of the specimen), Mr. 



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