December 1, 1894.] 



KNOWLEDGE. 



285 



what their first appearance no doubt suggests, a series of 

 concentric circles, or ratlier polygons, but they are con- 

 stituted by a single thread wound round and round in a 

 spiral fashion. This first proceeds from the centre to the 

 circumference in such a way that the distances between 

 the successive turns is double what it will ultimately be, 

 and then the coil is carried backwards from circumference 

 to centre, half-way between the previously formed con- 

 volutions. 



There is a considerable difference between the radii and 

 the cross lines. The former are stronger, less elastic, and 

 much less sticky than the latter, and are composed of a com- 

 pound strand proceeding from more or fewer of the spinning 

 tubes which open on the spinnerets. The cross lines are 

 somewhat finer, and, when freshly formed, have a number 

 of globules of viscid silk all along them, like glittering 

 beads on a string. This beaded appearance can be traced, 

 with the help of a lens, in one of the threads in the right- 

 hand top corner of the photograph, but it can hardly be 

 seen elsewhere, as the web was not a freshly-formed one, 

 and these globules of viscid matter dwindle on exposure to 

 the sun, apparently by evaporation, whence the necessity 

 for frequent repair, and possibly even the construction of 

 a new web. Since these beaded threads do not harden on 

 exposure to the air as the radii do, it is they chiefly that 

 are the agents of detention when an insect flies into the 

 snare. 



The method of construction of such a web is interesting, 

 and we will follow it in detail. The outline enclosing the 

 area to be filled is first formed, and for this purpose the 

 wind is made to lend some aid. The spider sits upon a 

 leaf or other support, exposing its spinnerets to the wind, 

 and by this means a thread is gradually drawn out and 

 floats behind the creature till the free end comes in contact 

 with some object, to which it at once adheres. Others 

 will be formed and fixed in the same way, or by the spider 

 dropping from point to point and carrying its thread with 

 it. This goes on tiU an irregular area is enclosed. The 

 radii are the next parts to be made. Proceeding to the 

 centre of the upper boundary, the spider attaches a thread 

 to it and then drops down to the opposite one, where the 

 thread is again fastened, and thus we have the first dia- 

 meter. This thread can now be used as a climbing pole, 

 and the spider swarms up it, and attaching a new thread 

 at its middle point, climbs up the rest of the distance, 

 drawing out the thread as it goes. On reaching the 

 horizontal boundary it turns aside and travels a short 

 distance along it, thus stretching the newly-made thread 

 across in a sloping direction from the first point of 

 attachment to the point it has now reached, where a new 

 attachment is made. Keturning to the centre, the spider 

 carries another line up in the same way, fastening its free 

 end a little further on, and so on till all the radii have 

 been laid down. 



The next business is to put in the spiral thread. This, 

 as already mentioned, is of difierent texture from the radii, 

 and commences at the centre. Fastening the beginning of 

 its new and more viscid thread at the centre, the spider 

 crawls from radius to radius, going round and round in a 

 spiral direction, and as it crosses each radius it fastens 

 there the thread that trails behind it, using its hind legs 

 to assist in the process. Having reached the outermost 

 limit of its web, it retraces its steps, but threads the maze 

 backwards between the lines already laid down, fastening 

 each strand as it advances, just as it did on the way out, 

 till it reaches the centre. Thus the cross lines of the web 

 are in the form of a double spiral, the coils of the return 

 half lying alternately with the rest. As a snare, the web 

 is now practically complete ; but the spider will not unfre- 



quently improve its elasticity and powers of resistance by 

 biting out a small part in the centre, thus severing the 

 connection of the radii with one another, and throwing 

 the burden of resistance upon the cross strands. If any 

 object now gets entangled in the web, the highly elastic 

 cross strands yield to its struggles, carrying the stiffer but 

 now disconnected radii with them, and thus there is less 

 likelihood of the main threads snapping, a catastrophe 

 which might be followed by the escipe of the prey. Any 

 entanglement of cross threads that may take place can 

 soon be set right again, and the damage, so far from being 

 a drawback, may actually be advantageous, as it will lead 

 to the substitution of new and more viscid threads for the 

 older ones that have lost a good deal of their original 

 viscosity. 



A newly-formed web will usually be found to be very 

 regular, but as time advances irregularities soon begin to 

 appear, through the interference of wind and weather, as 

 well as of insects flymg into it. One of the commonest 

 irregularities is that which is exhibited in many parts of 

 the web represented in the photograph ; two consecutive 

 threads of the spiral have coalesced to form a single one 

 for the greater part of their length, separating from one 

 another only near the ends when they slope oft' to their 

 respective attachments to the radii. This evidences both 

 their elasticity and viscosity, for being stretched they have 

 met, and then have adhered together. 



The web, as just described, is intended to serve as a 

 snare for the capture of prey, and not as a place of abode, 

 even though the spider may often be seen at its centre 

 where the web is not viscid. The spiders of the family 

 Epeiridce usually construct a cell or nest in some concealed 

 spot near the web, and this is the true place of abode. It 

 is connected with the snare by a strong line running to 

 the centre of the latter, which forms a kind of tight-rope 

 along which the spider passes when the sudden appearance 

 of a desirable victim in the snare demands its immediate 

 attention. If the insect is not too large, it will be at once 

 twirled round and enswathed in silk, whereby its struggles 

 will be prevented, and yet it will be kept alive and fresh, 

 so that its captor can at any time obtain for itself a supply 

 of fresh provisions. 



THE FACE OF THE SKY FOR DECEMBER. 



By Herbert Sadler, F.R.A.S. 



BOTH spots and faculse are stiU very numerous on 

 the Sun's disc. Conveniently observable minima 

 of Algol occur at 7h. 32m. p.m. on the 3rd, 4h. 20m. 

 P.M. on the 6th, 9h. 13m. p.m. on the 23rd, and 

 6h. 2m. P.M. on the 26th. 

 Mercury is a morning star, and but for his southern 

 declination would be well situated for observation during 

 the first half of the month. He rises on the 1st at 5h. 50m. 

 A.M., or Ih. 56m. before the Sun, with a southern declina- 

 tion of 16° 6', and an apparent diameter of OO', tVs*'^^ 

 of the disc being illuminated. On the 9th he rises at 

 6h. 20m. A.M., or Ih. 36m. before the Sun, with a 

 southern decUnation of 19^ 84', and an apparent diameter 

 of 5^", Tco''^^ °f "^6 '^i^'' being illuminated. On the 

 14th he rises at 6h. 45m. a.m., or Ih. 15m. before the Sun, 

 with a southern declination of 21° 28', and an apparent 

 diameter of 5-0", y''oths of the disc being illuminated. On 

 the 19th he rises at 7h. 7m. a.m., or about one hour before 

 the Sun, with a southern dechnation of 23° 0', and an 

 apparent diameter of 4f", xVo^hs of the disc being 

 illuminated. After this he is too near the Sun to be easily 

 observed. While visible he pursues a direct path through 



