236 FOOD OF INSECTS. 



And however repeatedly their nets are injured or destroyed, as long as 

 their store of silk is unexhausted, they repair or reconstruct them without 

 loss of time. 



The web of a house-spider will, with occasional repairs, serve for a con- 

 siderable period ; but the nets of the geometric spiders are in favourable 

 weather renewed either wholly, or at least their concentric circles, every 

 twenty-four hours, even when not apparently injured. This difference in 

 the operations of the two tribes depends upon a very remarkable pecu- 

 liarity in the conformation of their snares. The threads of the house- 

 spider's web are all of the same kind of silk ; and flies are caught in them 

 from their claws becoming entangled in the fine meshes which form the 

 texture. On the other hand, the net of the garden spider is composed 

 of two distinct kinds of silk ; that of the radii not adhesive, that of the 

 circles extremely viscid. 1 The cause of this difference, which, when it is 

 considered that both sorts of silk proceed from the same instrument, is 

 truly wonderful, may be readily perceived. If you examine a newly 

 formed net with a microscope, you will find that the threads composing 

 the outline and the radii are simple, those of the circles closely studded 

 with minute dew-like globules, which, from the elasticity of the thread, 

 are easily separable from each other. That these are in fact globules of 

 viscid gum, is proved by their adhering to the finger and retaining dust 

 thrown upon the net, while the unadhesive radii and exterior threads re- 

 main unsoiled. It is these gummed threads alone which retain the insects 

 that fly into the net ; and as they lose their viscid properties by the action 

 of the air, it is necessary that they should be frequently renewed. 2 



1 May not the spinners mentioned by Leeuwenhoek be peculiar to the retiary 

 spiders, and furnish this viscid thread ? 



2 The accuracy of the fact above stated as to the essential difference between the 

 radii and concentric circles from the presence of globules of gum on the latter only 

 has been denied by the author of Insect Architecture; but as it has been fully con- 

 firmed by Mr. Blackwall, and as any one, who will examine a newly -made spider's 

 net with a common pocket lens, and throw a little dust on it, will see for himself 

 what is here described, it is needless to refute an error that has most probably arisen 

 from the examination of old nets, which, after being exposed to wind and rain, often 

 lose the globules of gum from the circles. ( Vide Spence in London's Mag. of Nat. 

 Hist. 1832, vol. v. p. 689.) 



When the writer of these letters on the food of insects, in examining for himself 

 the whole process, from first to last, of the construction of the nets of the garden 

 geometric spider, observed this remarkable difference between the radii and con- 

 centric circles, he had certainly no idea that he had made any discovery, as he never 

 dreamed that so obvious a peculiarity in objects so constantly in view had not been 

 very frequently noticed, and even described, in books, though he had not himself 

 chanced to meet with any such description. But the denial of the fact itself having 

 subsequently drawn liis attention to the subject, he is inclined to believe (but with- 

 out speaking positively on a question which he has not now an opportunity of in- 

 vestigating) that the existence of these gum globules and their peculiar object were 

 first distinctly made known in the present work * ; a circumstance which, if the fact 



* Dr. Hooke, indeed, in a passage in his Micrographia, p. 202., quoted by Mr. 

 Blackwall (Linn. Trans, xvi. 479.), speaks of the radii of geometric spiders' nets 

 being " all over knotted or pearled with small transparent globules, not unlike small 

 crvstal beads or seed-pearls strung on a clew of silk ;" but, as he immediately adds, 

 "which, whether they were so spun by the spider or by the adventitious moisture 

 of a fog (which I have observed to cover all those filaments with such crystalline 

 beads), I shall hot now dispute ;" it is clear that he had no distinct or correct ideas 

 as to the origin of these globules, nor the slightest conception of their use. 



