The Brahmins assert, that the world arose from an infinite spider, who spun this whole complicated mass from his bowels, and annihilates afterwards the whole or any part of it, by absorbing it again, and resolving it into his own essence. Here is a species of cosmogony, which appears to us ridiculous; because a spider is a little contemptible animal, whose operations we are never likely to take for a model of the whole universe. But still here is a new species of analogy, even in our globe. And were there a planet wholly inhabited by spiders, (which is very possible,) this inference would there appear as natural and irrefragable as that which in our planet ascribes the origin of all things to design and intelligence, as explained by Cleanthes. Why an orderly system may not be spun from the belly as well as from the brain, it will be difficult for him to give a satisfactory reason.
What really makes this passage is the parenthetical expression "which is very possible". But Philo arguably overreaches here. A planet of spiders intelligent enough to make the inference that the world must be spun from a spinner analogous to the spiders would be a world of spiders intelligent enough to be considered designers of their webs. For that matter, we tend to think of spiders as doing something like designing their webs already, and certainly Hume, given his view that animals have at least basic rational capacities, cannot rule out such an idea. So the design inference of the spider-planet turns out not to be, as Philo wants to suggest, a rival of Cleanthes' design inference; yes, the spiders tend to think in terms of webs while Cleanthes tends to think in terms of machines, but Cleanthes' understanding of the design inference obviously does not apply only to machines. In fact, we've already seen Cleanthes' openness to thinking in terms of more than just machines in Part III, when Cleanthes gave his Living Library analogy:
But to bring the case still nearer the present one of the universe, I shall make two suppositions, which imply not any absurdity or impossibility. Suppose that there is a natural, universal, invariable language, common to every individual of human race; and that books are natural productions, which perpetuate themselves in the same manner with animals and vegetables, by descent and propagation. Several expressions of our passions contain a universal language: all brute animals have a natural speech, which, however limited, is very intelligible to their own species. And as there are infinitely fewer parts and less contrivance in the finest composition of eloquence, than in the coarsest organized body, the propagation of an Iliad or Aeneid is an easier supposition than that of any plant or animal.
Suppose, therefore, that you enter into your library, thus peopled by natural volumes, containing the most refined reason and most exquisite beauty; could you possibly open one of them, and doubt, that its original cause bore the strongest analogy to mind and intelligence?
(It's interesting that Cleanthes also insists on the possibility of the Living Library.) So whether the contrivance is that of machine, language, or web shouldn't affect the analogical inference in the least.
But there's also good reason to think that Philo is (in this part of the Dialogues) not taking aim at Cleanthes' design inference as such but at Cleanthes' Anthropomorphism. Here too Philo seems to overreach. When Cleanthes makes his design inference, he is not talking about the material of the world -- that is, he's not claiming that the world comes from things that could be considered machine parts, although that's consistent with his reasoning, but that the world comes from a cause that's like the human mind insofar as it designs machines and implements such designs. So why would the spiders on the spider-planet be concluding that the world comes from spinnets rather than from a cause that's like the spider mind insofar as it designs webs and implements such designs? There's no obvious reason why they should focus on "the belly" rather than "the brain"; the orderly system in a web is presumably not designed by spider "bellies" but by spider "brains", especially if the spiders are intelligent enough to know what they are doing, as they must be if they are inferring that there is a Great Spider designing the Web of the Universe. Philo's argument seems to involve an illicit shift.