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Whether a spider naturally produces a web.

Further one inquires about the spider, and first one asks whether a spider naturally produces a web.

1. It seems not, because an effect attests to its cause. But the web's material is sticky [tenax]. The spider, however, since it has a dry complexion, does not have a sticky nature. Therefore, etc.

2. Moreover, the spider is a venomous animal. But the web is useful for caring for wounds. Therefore, a web does not belong to a spider naturally.

The opposite is clear.

One must reply that the spider makes a web because, just as the Philosopher says, some animals live by hunting and others live by storing food. The spider seeks its nourishment by hunting, and this is why it prepares for itself an instrument suitable for hunting. The web is something of this sort, because the spider remains in the middle or just below the middle of the web, and when a fly enters the web and is held there on account of the web's strength and stickiness, the spider drags to itself the animal that has been caught and sucks out its moisture.

1. To the arguments. To the first, one should reply that it is not necessary that the effect and its cause be similar in every respect, for in animals generated from putrefaction the effects and their cause differ by species and sometimes even by genus. Thus, the [cause of the] spider's production of a web is not univocal but equivocal.

2. To the second argument one should say that two things are required to care for wounds, namely, a drawing together of the sides of the wound and a knitting together of the parts. But the cold and the dry draw together, and the viscous knits together, and the spider's web has a cold and dry nature, and similarly it is sticky, and this is why it draws together wounds by means of its coldness and dryness, and knits together the parts by its stickiness.

Whether the web is produced from the spider's substance or from some superfluity it has.

In addition, one asks whether the web is produced from the spider's substance or from some superfluity it has.

1. It seems that it is not produced from some superfluity. This is because every superfluity in the body either remains in the body, like sperm or milk, or does not remain but must be cast off, and one such as this is not needed. But the spider's web comes from neither of these two types. It does not remain in the body, because if it did it would contribute to the conservation of the species, like milk and sperm, nor is it from the second type of superfluity, because if it were the web would be unnecessary, which nevertheless is not true.

2. Moreover, nature is an internal principle of operation, whereas art is actually an external principle. But the spider works a web by a natural instinct. Therefore, the web's principle is internal. But this would only be so if it came forth from some part of the spider itself, and therefore, etc.

To the contrary, the Philosopher says that the web is related to the spider just as the bark is to a tree or a scale to a fish. But these things are produced from a superfluity, and for that reason, etc.

One must reply that the web comes from a moisture remaining after the third digestion, because part of the spider's nutriment is converted into the substance of the spider, and the superfluous and residual part is retained in its body for the composition of a web. Then, from this material the spider, although it is a multi-pedal animal, first using an extracting motion forms a thread, one end of which it affixes to a given place, and then it draws the thread to another place, and thus, in the manner of a weaver, it produces a web from a superfluous moisture. Thus it is not produced from the spider's substance, as some have pro-posed, because no animal separates off some of its substance without pain. But the spider produces a web without distress. This is why it cannot be from its substance.

1. To the first argument one must reply that the question differs for the spider and for other animals, because other animals strive to expel wholly the moisture remaining from the third digestion. But the spider divides that moisture into two, since it reserves the more subtle part for the web, whereas it casts out or expels the more digested part, just as other animals do.

2. To the second argument one ought to reply that although nature is a principle within, it is not necessary that the principle of a natural action come from the substance of the one generating, but only that it come from a superfluity of the last food. This is clear concerning a man's semen.

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