Answer:
The participle is "weighing" and it modifies "snail".
Step-by-step explanation:
In the structure "A snail weighing a third of an ounce can pull eight pounds", the subject of the main clause is "a snail". Interrupting the flow of the main clause we have an adjective phrase. Adjective phrases have the same subject as the main clause and, instead of being set off by a relative pronoun, they begin with a participle. In this case, it is a present participle - verb + ing - "weighing". The purpose of the adjective phrase is to modify the noun. "Weighing a third of an ounce" is modifying snail, giving it a characteristic. It's not just any snail, it's a snail that weighs a third of an ounce.