272 Mr. Frederick Pollock on the 



to tolerably near the centre, and the inadhesive centre part ; 

 and I have frequently seen the spider scramble through this 

 opening, when frightened or anxious to get hurriedly from one 

 surface of the web to the other. Various opinions and doubts 

 have been advanced by arachnologists, upon the specific pur- 

 pose which this open space is meant to subserve ; but in this 

 case the object appears plain enough — namely, to give a short 

 cut, near the centre, between the opposite surfaces of the web. 



But there is another reason which, I think, may account for 

 spiders of the geometrical kind leaving the space just alluded 

 to ; and I will endeavour to explain it. 



When any comparatively large insect is caught in their 

 webs and carried away to be eaten, the spider generally bites 

 away the lines surrounding the insect (in order to get it free) ; 

 and thus a rent or hole must, of necessity, be made where the 

 insect became entangled. 



Now the strength of these webs depends mainly upon the 

 radial lines, which are, of course, much closer together near 

 the centre than near the circumference. 



If a fly is caught near the latter, perhaps no radial line, or 

 only one, need be broken, to get the creature away ; whereas if 

 it were caught very near the centre, two or three, at least, of the 

 radial lines might have to be cut, and the web would be 

 greatly weakened thereby. This may be the reason why the 

 spiral adhesive line is not carried nearer to the centre. 



The central space, which is devoid of adhesive lines, and in 

 which it is undesirable that any thing should be caught, is 

 much larger than is requisite for a resting-place for the spicier. 

 The real resting-place of inadhesive lines does not occupy the 

 whole of this space ; and hence the void ; which there would be 

 no use in filling up, apparently. 



I may here remark that, owing to the adhesive property of 

 the spiral lines so soon disappearing in Madeira, N. Aurelia 

 makes, on an average, about two webs in every three days. 



Lycosa BlachwalUi. 



This spider is described by Mr. J. Y. Johnson in the 

 August and November Nos. of the 'Annals ' for 1863. On 

 the 6th of January, 1870, I dug out of a hole (of its own 

 making, probably), in a soft sloping bank of earth, at an ele- 

 vation above tlie sea of about 2000 feet, in Madeira, a half- 

 grown female of L. Blackwallii ; and none were ever found by 

 me at a lower elevation, though I repeatedly searched for them 

 in every direction. On the 9th of March it changed its skin ; 

 and in April I brought it to England, where it changed again 



