SCIENTIFIC NE^A^S. 



[Mar. 9, i8 



THE VISION OF SPIDERS. 



A CCORDING to " Cosmos," Professor Plateau has 

 ■t\ been examining the organs of sight among the 

 Arachneida. Their development in this respect is far 

 from uniform. Some, like the scorpions, shun the daj'- 

 light, and must depend upon chance or upon their other 

 senses for the capture of their prey. Others, like the ma- 

 jority of the true Araneida, hang out snares for insects, 

 whilst other families in this group seek the light and 

 live by hunting. In these latter the visual perceptions 

 have reached the highest grade of development. 



Thanks to the numerous eyes of the spider, it is 

 popularly credited writh excellent sight. Naturalists, how- 

 ever, have long ago refuted this error, and shown that 

 these eyes, in virtue of their curious dimorphism dis- 

 covered by Grenacher, are very indifferent instruments, 

 or at least that their possessors must be very short- 

 sighted. Muller and Lacordaire find that spiders see 

 distinctly only to a very short distance. Duges thinks 

 that if a spider takes refuge in its hole at the approach 

 of a man, even at the distance of a fathom, it is not 

 necessary to suppose that it has any distinct vision at 

 such a distance, but merely recognises moving masses. 



The researches of M. Plateau have not merely con- 

 firmed these surmises, but have extended them in an un- 

 expected manner. His minute experiments have been 

 extended to various species of spiders, and have led 

 him to conclude that the distance at which vision is 

 sufficiently distinct to induce a spider to capture its prey 

 is about an inch in the most sharp-sighted species, and 

 that the vague perception of movements in Epibleiiiuiit 

 scoiiciiin does not exceed five to six inches. 



The following observations, which the reader may 

 verify in any garden on a fine day in the latter part of 

 summer, fully confirm the statements of M. Plateau. If 

 a blue-bottle falls into the web of a geometrical spider 

 {Epeira diadema) she does not rush right to the spot, but 

 applies her feet in rapid succession to the radii of the 

 web, and thus ascertains by touch the exact position of 

 the struggling prey. If the web is gently touched with 

 a blade of grass or a soft feather she rushes to the 

 attack, and does not at once perceive the fraud, but 

 actually seizes the useless object. 



We have seen a harry-long-legs get entangled in a cob- 

 web in his blundering flight, and just manage to extricate 

 himself in time, leaving a leg behind him as a forfeit. 

 The spider, hurrying up, seized the leg, not noticing that 

 it was bodyless, ran to the other end and then back to 

 the first, only learning by very close examination that 

 the booty was just a leg and nothing more. No creature 

 having a moderate range of vision could be so deceived. 



The conclusions of M. Plateau are that the Amneida 

 in general perceive the movements of bulky bodies from 

 a distance. The so-called hunting-spiders, such as the 

 Lycosae, are probably the only ones which see the move- 

 ments ol small objects. They perceive such movements 

 within a range, according to some observers, at 

 from one inch to a foot, but which, according to Plateau, 

 does not exceed six inches. Even at such small dis- 

 tances their sight is not distinct. Weaving spiders have 

 a very poor sight at any distance, and only delect the 

 preseiice and the position of ihcir prey by the vibra- 

 tions of the web. 



The underground tube-spinning spiders, such as 

 Atypus piceus, which may be observed on Hampstead 

 Heath, are not guided by sight at all, but drive their 



fangs into any substance which comes in contact with 

 the tube. 



As regards scorpions the case is very similar. Their 

 sight is poor, and its mean range does not extend beyond 

 half an inch for the middle eyes and one to one and a 

 quarter inch for the lateral eyes. These animals do not 

 hunt, but ramble about or wait until some prey comes in 

 their way. They judge by feeling where is the right 

 place to plant their sting. 



SULPHUR BACTERIA- 

 FUNGUS." 



SEWAGE 



" C EWAGE fungus " is a name of dread to many persons 

 who when they see it slowly swaying to and fro in 

 the sluggish current of a ditch are haunted by dreams of 

 " previous sewage pollution." In reality this unlovely 

 plant grows merely in the presence of sulphur. 



The bacteria belonging to the group Beggiatoia form 

 long strings, and live together in great numbers, compos- 

 ing white slimy masses which may be found in sewers, 

 ditches, in sulphur springs and in the drainage from 

 certain chemical works. It is a great mistake to sup- 

 pose that they have any necessary connection with 

 putrescent animal matter, as they contain exceedingly little 

 nitrogenous substance. It was formerly believed that they 

 were capable of reducing sulphates present in water 

 and thus generating that unpleasant gas known as sul- 

 phuretted hydrogen. But last year Winogradsky showed, 

 in the Botanischc Zcitiiug, that the presence of sul- 

 phuretted hydrogen in water is not the effect, but the 

 cause, of the Beggiatoa. Sulphuretted hydrogen is formed, 

 as was shown by Hoppe-Seyler, by the action of 

 other bacteria upon sulphates. \{ Beggiatoa is cultivated 

 in waters where other bacteria are excluded, they 

 gradually lose their sulphur completely, even if the 

 liquid holds sulphates in solution. They are consequently 

 unable to reduce sulphates. But if sulphuretted hydro- 

 gen is added, they begin to store up sulphur and 

 flourish accordingly. The sulphur thus collected is of 

 a soft consistence and forms minute drops, each of which 

 is inclosed in a film of plasma so that they cannot flow 

 together. In the cell the sulphur is oxidised to sulphuric 

 acid, which converts the carbonate of lime of the cell 

 into sulphate (gypsum), even within the cell, when it is 

 dissolved away. According to Winogradsky, this process 

 corresponds to the oxidation of the carbohydrates in other 

 organisms and is thus a kind of respiration. By the 

 oxidation in the Beggiatoa, it obtains the energy neces- 

 sary to maintain its life. It is easy to explain why the 

 Beggiatoa should occur in \vaters containing mere traces 

 of organic matter, as they can use this entirely for con- 

 structing their tissue, and have not, like other organisms, 

 to expend it in heating. Some other bacteria are also 

 able to store up sulphur. 



Casting Aluminium. — Dr. C. C. Carroll, of Meadville, 

 Pennsylvania, after years of experimenting, has discovered a 

 method by which aluminium can be cast, soldered, and welded. 

 It is considered by American metallurgists and artisans that 

 this is a very valuable invention, since it ensures the use of 

 aluminium lor many purposes on account of its extrem.e light- 

 ness, strength, and non-oxidation by exposure. In the course 

 of his experimenting Dr. Carroll believes he has also dis- 

 covered the law governing the disintegration of iron-stringers 

 emploj'ed in constructing railway bridges. — Iron. 



