BLACKWALL ON SPIDERS. 



of the elastic spiral line ; this product multiplied by 24, the mean number of cir- 

 cumvolutions formed by the elastic spiral line, gives 3360, the mean number of 

 globules contained between two radii, and produces 17,360, the total number of viscid 

 globules in a finished net of average dimensions. A large net, fourteen or sixteen 

 inches in diameter, I have found, by a similar calculation, to contain upwards of 

 120,000 viscid globules, and yet Epeira apoclisa will complete its snare in about 

 forty minutes on an average, if it meet with no interruption. Astonishingly great as 

 this number of globules is, each is separated from those adjacent to it by a sensible 

 space ; indeed the material of which they are composed is so fluid, that they run 

 together the moment they are brought into contact. The globules and the intervals 

 between them may be distinctly seen with the assistance of a magnifier of the 

 power of ten." p. 478. 



It appears that these globules had been observed by Dr. Hooke, 

 who says in his Micrographia : 



" I observed further that the radiating chords of the web were much bigger and 

 smoother than those that were woven round, which seemed smaller and all over 

 knotted or pearled with small transparent globules, not unlike small crystal beads 

 or seed pearls, thin strung on a clew of silk; which, whether they were so spun 

 by the spider, or by the adventitious moisture of a fog (which I have observed to 

 cover all these filaments with such crystalline beads) I will not now dispute." 

 p. 202. 



Connected with their mode of spinning, the claws of spiders are 

 important, and have been taken notice of by most naturalists who have 

 written on the subject ; but Mr. Blackwall, with his usual originality, 

 has extended these observations to several important minutiae : 



" Naturalists," he says, " appear to concur in the opinion that the tarsi of 

 spiders are armed at their extremities with three claws, which occupy the upper 

 and anterior portion of the foot. That this is the case with some species cannot 

 be denied ; other species, however, belonging to various genera, Mygale avicularia, 

 Drassus melanog aster, and Salticus scenicus, for example, have only two claws on 

 each foot ; and if the tarsi of the larger geometric spiders indigenous to Great 

 Britain, such as Epeira cicatrosa, Epeira diadema, and Epeira apoclisa, be examined 

 under the microscope with a high magnifying power, it will be distinctly perceived 

 that the inferior part of their feet is provided with several claws, which have a con- 

 siderable degree of curvature, are finely pointed, and are furnished with tooth-like 

 processes on the under side ; and should the investigation be extended to other 

 retinary spiders, the feet of many species which construct complicated snares 

 will likewise be found to exhibit a similar organisation. As the best means of 

 guarding against errors, to which the inspection of limbs defective in structure 

 might conduce, it is advisable to select the legs of vigorous individuals which have 

 recently moulted, whenever, such can be procured. 



" The supernumerary claws were first observed by me, in examining the feet of 

 Epeira apoclisa ; and on every instance I counted as many as five, which, with the 

 three upper ones previously known, give a total of eight claws on the same foot, 

 distinguishable at a glance from the coarse setaceous bristles in their vicinity. 



