Christina didn't have much time to think. She couldn't afford to let the male spider take advantage her, apart from being totally gross it could trap her in this form for a horrifically long time according to the witch. And the fly did look very tasty. And she was a spider, who needed to eat insects to live.
Losing all sense of reason, Christina hovered over the injured fly and quickly devoured it. Once she had finished, she could hear the familiar sound of the witch cackling. Turning round, she could see a familiar toad sitting at the feet of the witch. She realised with relief that she had been tricked and eaten a common fly rather than an ex human. And to her surprise it had tasted good.
"Well, whose a good little spider?" the witch smiled, gleefully aware that she was now completely in control of the situation. The male spider enthusiastically approached Christina again and the witch looked pensive for a minute.
"Oh, that's right we had a deal didn't we. Well, fair is fair I suppose"