BLACK WALL ON SPIDERS. 



259 



which they occur, and to them the adhesive property of the snare is chiefly to be 

 ascribed. In attempting to determine by experiment the cause of adhesion in the 

 blue bands, I ascertained that bodies with highly-polished surfaces, such as the 

 bulbs of thermometers and burnished metallic rods, if carefully applied to them, 

 may be withdrawn without deranging their structure, though the viscid globules in 

 the nests of geometric spiders adhere to the same bodies as soon as they are brought 

 into contact with them. From this circumstance I was led to infer, that the blue 

 bands are fibrous, although their structure is so exceedingly fine that I cannot 

 detect it even with assistance of the microscope ; and that the imperceptible fila- 

 ments of which they are composed adhere to objects, not in consequence of being 

 glutinous, but solely by attaching themselves to inequalities on their surface." — 

 p. 473. 



Several naturalists had remarked, that geometric spiders employ one 

 sort of silk for the frame-work, or outside lines of their nets, and another 

 sort for the part which is efficient as a snare ; but by more minute 

 attention to this, Mr. Blackwall thinks he has detected no fewer than 

 three sorts. 



" Not only," he says, " the garden spider, but every geometric species with which 

 I am acquainted, employs three distinct kinds of silk, if a liquid gum can with pro- 

 priety be termed silk, in the construction of its net. The boundary lines, radii, 

 and first formed spiral line being unadhesive, and possessing only a moderate share 

 of elasticity, are evidently composed of a different material from the last formed 

 spiral line, which is exceedingly viscid, and elastic in a remarkable degree. Now 

 the viscidity of the elastic spiral line may be shown to depend entirely upon the glo- 

 bules with which it is studded ; for if they be removed by careful applications of 

 the finger, a fine glossy line remains, which is highly elastic, but perfectly unadhe- 

 sive. As the globules, therefore, and the line on which they are disposed, differ so 

 essentially from each other, and from the rest of the snare, it is reasonable to infer 

 that the physical constitution of these several portions of the net must be dis- 

 similar. 



" When exposed to the desiccating influence of the sun, and of air briskly agi- 

 tated, the nets of geometric spiders speedily lose their adhesive property ; but when 

 formed in situations from which light is excluded, and where the atmosphere is not 

 liable to be perceptibly disturbed, I have known them retain their viscidity for a 

 long period. In a net of Epeira diadema, constructed in a glass jar, which was 

 placed in a dark closet, where the temperature was not subject to great and sudden 

 fluctuations, the globules preserved their adhesive property almost unimpaired, and 

 the last-formed spiral line its elasticity for more than seven months." — p. 479. 



Mr. Blackwall gives an interesting and original account of the man- 

 ner in which Clubiona atrox makes what he terms the flocculus, a very 

 different thing, as our readers may perceive by comparison, from the 

 vague and unsatisfactory details given of similar processes from the 

 time of Homberg to our own days. 



" When the spider," he says, " purposes to form a flocculus, it presses its 



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