How did the spider know that the fly would return to his den?

Three reasons explain how the spider knew the fly would return to his den in Mary Howitt's poem "The Spider and the Fly." First, the spider has a great deal of experience attracting creatures to its web. The fly states repeatedly that she has "often heard" many tales of poor victims that go up the spider's stair or lie in the spider's bed and never return. The spider has been quite successful in the past and has developed a relatively foolproof system for catching prey. Second, the spider knows the power of flattery. The spider tries several enticements in order to get the fly to come into his parlor, such as offering the "pretty things" that are up "the winding stair," suggesting a rest on the curtained bed, or offering a tasty morsel from the pantry. The spider saves the best enticement for last, skillfully tossing out the flattering words that implant in the fly's brain and bring her back again. Finally, the fly's own parting words indicate that she will return: "And bidding you good-morning now, I’ll call another day.” Instead of closing the door firmly on the spider's advances, the fly leaves herself open to hearing more of the wily creature's seductive words. At that point, the spider knows he has successfully deceived another "poor, foolish thing."