The viscid matter on the spiral line dries up after some hours, so that, even if the web has not been destroyed by insects and stress of weather, this portion of it must be frequently renewed. Commencing a new web is, as has been seen, a troublesome matter, and it will readily be understood that the spider prefers, where practicable, to patch up the old one. This is done by biting away torn and ragged portions and inserting new lines in their place. The part played by the various spinning glands in the construction of the orb-web may be briefly stated.[275] The ampullaceal glands furnish the silk for the foundation lines and radii. The spiral has a double ground-line proceeding from the middle spinnerets, but it is not quite certain whether it proceeds from the ampullaceal or the tubuliform glands. The chief function of the latter, in the female, is to furnish silk for the egg-cocoon. The viscid globules are the products of the aggregate glands. The aciniform and piriform glands provide the multitudinous threads by which the spider anchors its various lines and enwraps its prey. Some Orb-weavers always decorate their snares with patches or tufts of flossy silk. In the snare of the North American _Argiope cophinaria_ the hub is sheeted, and from it extends downwards a zigzag ribbon of silk stretched between two consecutive radii. Vinson[276] discovered a remarkable use for similar zigzag bands in the web of the Mauritian spider, _Epeira mauritia_. It furnished a reserve supply of silk for enveloping partly entangled insects whose struggles were too vigorous to succumb to the rather feeble threads which the spider was able to emit at the moment of capture. The spider was able to overcome a grasshopper much more powerful than itself by dexterously throwing over it with one of its hind legs a portion of the ribbon of silk which it had thus stored up for emergencies. [Illustration: FIG. 191.—=A=, Snare of _Hyptiotes cavatus_; =B=, enlarged view of the Spider, showing the “slack” of the hauled-in line. (After Emerton.) ]