FOOD OF INSECTS. 237 



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 in- 

 debted 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. 1 Without going the length, with M. D'Isjonval, 

 of deeming his discoveries important enough to regulate the march of 

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

 of these barometrical spiders in spring should be announced by the sound 

 of trumpet, I have reason to suppose from my own observations that his 

 statements are in the main accurate, and that a very good idea of the 

 weather may be formed from attending to these insects. 



The spiders which form geometrical nets differ from the weavers also 

 with respect to the situation in which they watch for their prey. They 

 do not conceal themselves under their net, but either place themselves 



prove to have been so, deserves being held out to the young entomologist in proof 

 how wide a field of discovery must 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 noted 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 being far more elastic, 

 which elasticity (as well as the viscidity of the gum globules) he found remained 

 unimpaired for 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. Blackwall, 

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

 circles, the spider, as she proceeds, destroys the first made distant unadhesive circles 

 which had served her as a scaffolding in placing the former. (ZooL Journ. v. 183.) 

 A curious calculation, also, of Mr. BlackwalPs, 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 Epeira apoclysa will, if uninterrupted, complete its snare 

 on an average in forty minutes, (p. 478.) 



1 Brez, La Ftore des Insectophiles, 129. 



