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. 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 barometrieal 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 
rove to have been so, deserves being held out to the young entomologist in proof 
ar 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, ‘here 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 Zpeira 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. Blackwall’s, as to the number of distinct globules 
of gum in a geometric spider’s net, should be noticed. ‘Ihese he found to be 87,360 
ina net of average dimensions, and 120,000 in a large net of fourteen or sixteen 
inches diameter; and yet Lpeira apoclysa will, if uninterrupted, complete its snare 
on an average in forty minutes. (p. 478.) 
1 Brez, La Flore des Insectophiles, 129, 
