FOOD OF INSECTS. 



353 



In this renewal, as above hinted, the geometrical spiders are 

 constantly regulated by the future probable state of the 

 atmosphere, of which they have such a nice perception, that 

 M. Q. D'Isjonval, to whom we are indebted for the fact, has 

 proposed them as most accurate barometers. He asserts that 

 if the weather be about to be variable, wet and stormy, the 

 main threads which support the net will be certainly short ; 

 but if fine settled weather be on the point of commencing, 

 these threads will be as invariably very long.^ Without 

 going the length, with M. D'Isjonval, of deeming his dis- 

 coveries important enough to regulate the march of armies, or 

 the sailing of fleets, or of proposing that the first appearance 



yet remain to be explored, when points at once so curious and yet obvious in the 

 economy of a spider, found in every garden, had so long remained unnoticed. 



Another reason for directing attention to this fact is to recommend strongly 

 to comparative anatomists and microscopical observers an investigation of the 

 mode in which the geometric spiders are enabled to spin two different kinds 

 of silk, one gummy and the other not, and whether the spinners noticed by 

 Leeuwenhoek, as suggested in a preceding note, are concerned in the process 

 ^ points to which Mr. Blackwall, in his examination of the spinning apparatus 

 of spiders {Linn. Trans, xviii. 219.), has not adverted. It is obvious that these 

 spiders must either have two distinct sets of spinners, of which one spins the 

 gummy and the other the unadhesive threads, or else, if all the threads proceed 

 from the same spinners, the spider must have the means of passing the threads 

 of the concentric circles through a reservoir of gum so as to stud them with the 

 globules of this substance which give them their fly-catching viscidity. There 

 is, however, a considerable difficulty in the way of this last supposition, for as 

 the threads at their issuing from the spinners are, as has been already explained, 

 so numerous, it is not easy to conceive how, after being united into one, they 

 can be passed through any gum reservoir, nor how, if they were so passed, the 

 gum, instead of being applied to the entire surface of the threads, should come to 

 be divided in the process into distinct and bead-like globules. The subject is 

 certainly highly curious and interesting, and well deserves investigation for an 

 additional reason originally noticed above and confirmed by Mr. Blackwall, that 

 the circular lines differ from the radii and main lines of the net, not only in 

 being studded with gum globules, but in l)eing far more elastic, which elasticity 

 (as well as the viscidity of the gum globules) he found remained unimpaired far 

 more than seven months in a net of Epeira diadema constructed in a glass 

 jar which was placed in a dark closet. {Linn. Trans, xvi, 479.) 



Before concluding this long note, an omission in the account of the geometric 

 spiders' forming their nets, in the text, which has been supplied by Mr. Black- 

 wall, should be given, namely, that in the process of spinning the concentric 

 gummy circles, the spider, as she proceeds, destroys the first made distant un- 

 adhesive circles which had served her as a scaffolding in placing the former. 

 {Zool. Journ. v. 18S.) A curious calculation, also, of Mr. Blackwall's, as to the 

 number of distinct globules of gum in a geometric spider's net, should be noticed. 

 These he found to be 87,360 in a net of average dimensions, and 120,000 in a 

 large net of fourteen or sixteen inches diameter ; and yet Eperia apoclysa will, 

 if uninterrupted, complete its snare on an average in forty minutes, (p. 478.) 



1 Brez, La Flore des Insectophiles, 129. 



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