We do not recollect that naturalists have ventured to assign any cause for this very remarkable multiplicity of the spinnerules of spiders, so different from the simple spinneret of caterpillars. To us it appears an admirable provision for their mode of life. Caterpillars neither require such strong materials, nor that their thread should dry as quickly. It is well known in our manufactures, particularly in rope-spinning, that in cords of equal thickness, those which are composed of many smaller ones united are stronger than those spun at once. In the instance of the spider’s thread, this principle must hold still more strikingly, inasmuch as it is composed of fluid materials that require to be dried rapidly, and this drying must be greatly facilitated by exposing so many to the air separately before their union, which is effected at about the tenth of an inch from the spinnerets. In Fig. 10. Plate IV. each of the threads shown is represented to contain one hundred minute threads, the whole forming only one of the spider’s common threads. In the figure the threads are, of course, greatly magnified, so that, for the small space represented, the lines are shown as parallel. The threadlets, or filaments as they come from the papillæ, are too fine to be counted with any degree of accuracy, but it is evident that very many are sent forth from each of the larger papillæ. This fact tends to explain the power possessed by the spider of producing threads having different degrees of tenuity. By applying more or less of these papillæ against the place whence it begins its web, the spider joins into one thread the almost imperceptible individual filaments which it draws from its body; the size of this thread being dependent on the number of nipples employed, and regulated by that instinct which teaches the creature to make choice of the degree of exility most appropriate to the work wherein it is about to engage. Réaumur relates that he has often counted as many as seventy or eighty fibres through a microscope, and perceived that there were yet infinitely more than he could reckon; so that he believed himself to be far within the limit of truth in computing that the tip of _each_ of the five papillæ furnished 1000 separate fibres: thus supposing that one slender filament of a spider’s web is made up of 5000 fibres! Leeuwenhoeck, in one of his extraordinary microscopical observations on a young spider, not bigger than a grain of sand, upon enumerating the threadlets in one of its threads, calculated that it would require _four millions_ of them to be as thick as a hair of his head!