I think that Howitt herself details the theme at the end of her poem. Once she has finished telling the narrative of the spider and the fly, she breaks from the poetic storytelling and offers up a type of warning as a moral or theme to conclude the poem:
And now dear little children, who may this story read, To idle, silly flattering words, I pray you ne'er give heed: Unto an evil counsellor, close heart and ear and eye, And take a lesson from this tale, of the Spider and the Fly.
It is here where the theme becomes strongest displayed. The weakness to succumb to flattery and false praise could be one's undoing. If we consider how such words play on our ego, causing us to embrace what could be disastrous, the theme of limiting individual pride becomes the driving theme of the poem. The spider is able to "weave a web" to ensnare the fly because of the superficial praise heaped on it. When the fly gives in to this praise, believing it as truth, the fly is destroyed. In this, there is the theme of not giving in to tempting words of praise and adulation, no matter how seductive they may be.
The theme of "The Spider and the Fly" is that people should not allow themselves to be manipulated by others who prey on their vanity. In the poem, the spider first tries to inveigle the fly into entering the spider's house by telling the fly how comfortable his parlor is and how welcoming he will be to the fly. The spider then promises the fly a rest on his bed and treats from his pantry. The fly wisely resists these attempts to get her to enter the spider's house; however, she cannot resist the spider's flattery, as he praises her wings and eyes and offers her the chance to look into his mirror. Motivated by her great vanity, the fly enters the spider's lair and is entrapped. The poet tells the reader not to be tricked by "idle, silly, flattering words" at the end of the poem. The theme is that vanity will make people susceptible to doing foolish things, so people need to be wary of others who flatter them only to deceive them.