558 



NATURE 



[April 13, 189^ 



I stopped and examined it. Instead of being a flower, I 

 found it was the web of a spider, and it was hanjfinij between 

 the branches of a shrub about three feet from the ground. 



The outer lines of the web were of considerable strength and 

 were stretched between points from eight to ten inches apart. 

 From these lines, supported by a few radii, hung a beautiful 

 rosette-shaped centre, much resembling a delicate pattern in 

 white silk lace. The central space was open and measured about 

 a quarter of an inch in diameter. The notched space was adorned 

 by three circular zig-zag cords of thick white flossy silk. I did 

 not notice any of MacCook's so-called " ribbon braces." The 

 spiral space was very open and the threads composing it were 

 so slight as to be almost invisible. So thin were they that the 

 ribboned centre appeared to be hanging in the air without any 

 support whatever. The appearance of this web was almost ex- 

 actly similarto that of the web of 6''^^(7r«.f, shown in Fig. 57, p. 58 

 of MacCook's "American Spiders." I did not notice any "fen- 

 ders " or protective wings on the outer side of the web ; there 

 were, however, a few strengthening strands on the side turned 

 towards the bush. 



The web, however, especially bore a strong resemblance to a 

 flower, the more so as in the exact centre of its outward side was 

 stationed a spider with a light blue body. This light bine colour 

 gave one the impression that it was the centre of the flower, 

 while the yellow legs spotted with brown were symmetrically 

 disposed in the shape of an X across the ribboned hub, thus 

 dividing it into the semblance of petals. The illusion was 

 remarkable. 



The spider remained motionless until I touched the web. She 

 then fell into the net which I was holding under the snare. 



As soon as she touched the net (a white gauze one) she changed 

 colour. From blue she became white and then, on being shaken, 

 her body turned a dark greenish brown. I then placed her in a 

 glass tube and gradually she resumed her blue tint. Whenever 

 shaken, however, she turned a greenish brown. I placed her in 

 spirits and her colour remained a grey brown. 



On the same road later in the day, I noticed another strange 

 web which bore even a stronger resemblance to a flower. 



The "foundation space" was the same as in the other, but 

 somewhat larger and stronger. The white silk ribbon, however, 

 instead of being disposed around the centre in circular zig-zag 

 lines, was extended in two thick white ribbons stretched cross- 

 wise along four of the radii. In this instance also the spiral space 

 was very open and the spirals very delicate. 



The spider inhabiting this web was considerably larger than 

 the foregoing specimen, but appeared to be otherwise exactly 

 similar to it. Her body was a very light blue, placed exactly in 

 the centre of the cross-head downwards, while her long legs 

 were disposed in pairs over the four arms of the white silk 

 pattern. The whole thing bore a great resemblance to an 

 orchid, and the legs of the spider gave it just sufficient stability 

 for it to be taken lor a flower. 



When I touched the web the spider immediately darted 

 through two strands in the spiral space and placed herself on the 

 reverse side of her web, being almost completely concealed by 

 the thick flossy white ribbons. 



I captured this spider, and her body, like the other specimen's, 

 immediately turned a dark greenish brown. I did not, how- 

 ever, see her turn white. I placed the insect in a glass tube, 

 and live days later put her in a cage. 



I also took the web and succeeded in fastening the centre of 

 it on to a black card, where it remains in exactly the same shape 

 as when it was hanging on the bushes. I have this web, and 

 also a photograph of it. 



The day after the spider was placed in the cage she made a 

 web. It was spun during the night, and I did not observe the 

 operation. The web was of the same pattern as the one on 

 which I discovered her on the bush. It did not have any cir- 

 cular zig-zag cords. 



This spider remained in her cage for four or five weeks, and 

 then 1 placed her in spirits. She was fed principally with flies. 

 On one occasion I put a very large blue-bottle fly into the 

 cage. The spider seized it imu.ediately, violently vibrated her 

 web, and at the same time rolled the fly round and round 

 betv.'een her legs. In the space of three or four seconds the 

 fly was completely swathed in an envelope of wnite silk, and 

 was motionless. The spider then fastened her fangs into the 

 body, and sucked it for about two hours. 



I have since seen several of these spiders on their webs, and 

 have noticed that the pattern of the snare appears to depend on 



IfO. 1224, VOL. 47] 



the size of the insect, the smaller specimens making the cir- 

 cular rosette-shaped snare, while the larger insects weave the 

 cross orchid-like flower. I saw one small web composed of two 

 little rosettes, joined side by side, but 1 did not notice whether 

 it was inhabited by two spiders. I frequently found wings and 

 other debris of insects hanging to the rosettes of the webs, and 

 in one case saw a wing of what must have been a nutterfly of 

 considerable size. 



When does the spider alter the pattern of her snare? Can it 

 be that, when the spider attains to full growth, finding that the 

 rosette shape, becoming too large, no longer deceives butter- 

 flies and other insects, .she adopts the orchid-like pattern which 

 has more vraisemblance, and over which she can dispose her 

 long legs with a better chance of successful trickery. 



The web of this spider being so like a flower would appear to 

 be intended as a veritable "snare." The insect by assuming 

 its bright blue colour increases the resemblance and the mimicry 

 is probably practised not so much for the protection of the spider 

 herself, but rather for the attraction it presents to butterflies and 

 other flower frequenting insects. 



MacCook in " American Spiders," writing of the mimicry of 

 spiders, and of their perception of colour, says (vol. ii. p. 346) :— 

 "There is indeed another theory which may be suggested, 

 namely, that the colour surroundings of the spider, in some 

 manner not now explicable, so rapidly influence the organism of 

 the creature that a change of colour is produced in harmony with 

 its environment. Cati ite suppose in this case that the spider 

 possesses thepotver to influence at will the chroinatophores or pig- 

 ment bodies, so that she may change her colour with champing 

 site ? " 



The specimen observed by me would seem to be an answer to 

 MacCook's suggestion, and I should be very glad to know, 

 through the medium of Naturk, or otherwise, whether the 

 spider described by me, as above, is already known to 

 naturalists. 



I took the specimens which I possess to the Natural History 

 Museum, at South Kensington, and the spiders were declared to 

 be a species of Argiope. 



H. H. J. Bell, 

 Senior Assistant Treasurer, Gold Coast Colony. 

 20, Sussex Villas, Kensington, W. 



Origin of Lake Basins. 



One of the chief reasons for the prevalence of lake basisn 

 in glaciated countries has n )t been alluded to in the letters which 

 have recently appeared in NATURii on the origin of lake basins. 



Whenever earth movements take place in limited areas sucli 

 movements will tend to form basins, but as the movements are 

 as a rule gradual such basins will only come into existence under 

 exceptional conditions. Water-borne detritus, the growth of 

 vegetation, and erosion will obliterate them in most cases as fast 

 as they are formed by slow unequal movements of subsidence or 

 elevation. 



In glaciated countries, however, basins in the course of forma 

 tion by unequal earth movements will be largely protected from 

 such destructive action by being tilled with ice, and will thus be 

 preserved to appear as lake basms when the ice melts. 



So, too, in countries where the rainfall is very small and the 

 action of the forces destructive to lake basins is accordingly 

 much diminished basins may be and are formed by earth 

 movements. In rainless countries they are probably more 

 numerous than we are aware of, for there is little to attract 

 attention to them, but they will become of more importance as 

 works of irrigation are required in such countries. An im- 

 portant depression, the Raiim basin, has lately been surveyed 

 in Egypt by Mr. Cope Whitehouse with a view to utilising it 

 for irrigation purposes (Proceedings Royal Geographical Society, 

 2nd series, vol. ix. p. 608). 



Wind-borne detritus will tend to diminish the depths of 

 such basins in rainless countries. So, too, the capacity of 

 ice-tilled basins to huld water in the future will be diminished 

 somewhat by the erosion of the sub-glacial river, but on the 

 other hand as the movement of the earth deepens the basin the 

 ever-thickening mass of ice will acquire increased power to 

 grind it deeper still. This grinding action cannot be ignored, 

 and some shallow lake basins may be almost entirely due to 

 it, but there is scarcely a limit to the formation of such basins 

 by earth movements under suitable conditions. 



J. C. Hawks HAW. 



33, Great George Street, Westminster, S.W., March 29. 



