HARD WICKE 'S SCIENCE- G OSS IF. 



25 



GRAPHIC MICROSCOPY. 



By E. T. D. 



No. II. — Eyes of Epeira conica. 



^^i^^gr^ HE Arachnida class 

 is one so elevated 

 above the Articu- 

 lata, as to consti- 

 tute the highest 

 condition of the 

 Invertebrata. It 

 has a wide range, 

 extending from the 

 microscopic Tar- 

 digrada (water 

 bears), the para- 

 sitic Demodex, the 

 Acari, and spiders ; 

 reaching the. or- 

 ganised ^lygale 

 and scorpions. 



The Araneidw, 

 or Spiders proper, 

 form one Order 

 of this class, se- 

 parated into two tribes: I. Octonoculina (eight 

 eyes); 2. Senoculina (six eyes), divided into families, 

 genera, and species. 



Epeira conica, the subject of the plate, is a species of 

 the Family Epeiridae of the Tribe Octonoculina, and 

 fairly represents the popular idea of the entire Order, 

 as to them especially belong the wel)s, so well known, 

 formed by precise geometrical rule radiating from a 

 centre, and connected at regular intervals by threads. 

 Interesting as it would be, space does not admit 

 touching on the general anatomy of the spider, but it 

 may be comprehensively stated, that the differences 

 of structure between spiders and insects, consist in 

 the continuation of head and chest in one mass, no 

 articulation of the body into rings or segments, the 

 number of legs, the absence of antennae, the structure 

 of the mouth and palpi, and particularly the presence 

 of none but simple eyes ; these points at once elevate 

 and characterise the spider above the insect. 



The eyes are placed in front of the cephalothorax, 

 or forehead in English species ; they are invariably 

 No. 230.— February 1884. 



six or eight, varying much in size, in the manner of 

 grouping and position, conditions depending entirely 

 on the habits of the species, and especially affected 

 by the necessity of catching prey, under the many 

 diversities of locality in which they are found. 



In spiders, the eyes or ocelli are grouped in the 

 middle of the anterior part of the cephalothorax ; the 

 position of the four median ones, as shown, is gene- 

 ally in a square with two pairs outside ; any variety 

 in the arrangement depends on the habits of the 

 species, and constitutes an important characteristic 

 feature in their classification. Amongst many spe- 

 cialities, those which hide in obscurity have the 

 eyes aggregated in a close group in the middle of 

 the forehead (Clubiona), those which inhabit short 

 tubes terminated by a long web have the eyes 

 separated and spread over the front (Philodromus) ; 

 those which lie in ambush, in and under leaves, and 

 in fissures in walls, have the eyes on a prominence, 

 permitting a slight divergence (Theridion), and in 

 the wandering, hunting, and ground spiders, the 

 eyes are large and grand beyond description, studding 

 in an even row the front margin of the cephalothorax. 

 In a well-known large species with zebra-like mark- 

 ings (Salticus sceniciis), which by sudden leaps, seizes 

 its prey ; .on the edge of the forehead are four 

 " oculars," touching each other in a parallel line, 

 two in the middle, of such dimensions and startling 

 appearance, that when arranged under reflected illu- 

 mination, carefully focussed from the side speculum, 

 they present a sight when the words " ghastly " and 

 "beautiful" may be appropriately associated, for 

 they gleam with weird-like cruelty. These eyes 

 have been minutely investigated, and present a type 

 of structure so far superior to those of insects, as 

 to admit of comparison of similar parts in vertebrate 

 animals ; a cornea, a globular lens, aqueous and 

 vitreous humours, a retina and choroid ; all are found 

 in nearly the same positions, so that the sense of 

 vision in the Araneidce must be, as many must have 

 noticed, extremely quick and keen ; and like the 

 eyes of cats, they are said to gleam in the dark, 



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