5260 



BLACKWALL ON SPIDERS. 



spinners against one of the glossy lines composing the foundation of its snare, and 

 emitting from them a small quantity of liquid gum, attaches to it several fine 

 threads, drawn out by advancing the abdomen a little, and kept distinct by extend- 

 ing the mammulae laterally. The foot of one of the hind legs is then applied to 

 the superior part of the upper tarsal joint of the other hind-leg, a little above its 

 articulation with the lower joint of the tarsus, and the curious apparatus of spines 

 above described, is brought immediately beneath the spinners at right angles, with 

 the line of the abdomen. By a slight extension of the joints of the hind-legs, the 

 apparatus is forced backwards across the mammulae, the diverging extremities of 

 which it touches in its transit, and is restored to its former position by a correspond- 

 ing degree of contraction in the joints. In proportion to the continuation of this 

 process, (and it is not at all unusual for the spider to pass its spiny apparatus across 

 the points of the mammulae several hundred times in rapid succession,) the inflected 

 lines of the flocculus are found to be produced, the spider making room for them 

 as they accumulate, by elevating and at the same time advancing the abdomen in a 

 small degree, which it effects by slightly extending the joints of the third pair of 

 legs, and contracting those of the two anterior pair. As this operation is generally 

 accomplished in the night, it can seldom be seen to advantage, unless artificial light 

 be employed, some skill in the management of which is required, in order to avoid 

 disturbing the spider. The modus operandi, as nearly as I can ascertain it by the 

 most diligent observation, appears to be this : The points of the lower row of spines 

 are protruded between those of the upper row, and in passing across the extremities 

 of the mammulae comb out the tortuous lines, which run into numerous flexures 

 in consequence of not being kept fully extended. The purpose subserved by the 

 upper row of spines seems to be the extrication of the tortuous lines from the spines 

 of the lower row, by a slight motion outwards, which disengages their points. 

 Now, were the blue bands glutinous, this mode of proceeding would be quite una- 

 vailing ; it is only on the supposition, therefore, that they have a fibrous structure, 

 that their adhesive property can be satisfactorily explained. When a sufficient 

 quantity of the inflected filaments is produced, the spider again applies its spinners 

 to one of the glossy lines, and attaches the flocculus to it. In this manner it pro- 

 ceeds with its labours, occasionally employing the combing apparatus of both hind- 

 legs, till the web is completed. Should any of the flocculi be destroyed, or rendered 

 almost useless, by having their adhesive property impaired, new ones are constantly 

 added to the snare." — p. 475. 



Another interesting portion of Mr. Black wall's paper consists in an 

 arithmetical computation of the number of viscid globules on the spiral 

 lines of the geometric spiders, which we shall now give : — 



" An estimate," he says, " of the number of viscid globules distributed on the 

 elastic spiral line in a net of Epeira apoclisa of a medium size, will convey some 

 idea of the elaborate operations performed by the geometric spiders in the construc - 

 tion of their snares. The mean distance between two contiguous radii in a net 

 of this species, is about seven-tenths of an inch ; if, therefore, the number 7 be 

 multiplied by 20, the mean number of viscid globules which occur on one tenth of 

 an inch of the elastic spiral line at the ordinary degree of tension, the product will 

 be 140, the mean number of viscid globules deposited on seven-tenths of an inch 



