Trapped like flies

Why is it that, in Virginia Woolf’s “The New Dress”, Mabel keeps comparing herself and the others at the party with flies struggling to free themselves from a saucer of milk but unable to do so?

The story centers on this very self-conscious woman who, upon arriving at a certain soiree, notices how all the other invited guests, her supposed friends, are acting very strangely towards her. They seemed pleasant and gentle on the exterior. But these pretenses of manners and politeness were nothing but skin-deep, sneers and mockeries hidden behind false smiles. They were making fun of her dress, of her “outdated” sense of style. Did it matter to them that she did not come from as privileged a family as they did? “But it was not her fault, altogether after all. It was being one of a family of ten; never having money enough, always skimping and paring;…” (p.4)
It was all because of society really, all it’s fault. Considering her status as a woman, the others expect her to have a “better” judgment in clothing, disregarding all fairness towards her. They were all bound… trapped by the tedious little stereotypes “civilization” had established, trapped like flies in a bowl of milk, their wings stuck together by those damn social rules. And she, in turn, coming from a poor upbringing and already being unsure of herself, so much so that she began envisioning the others as elegant insects such as butterflies and dragonflies while she remained “like some dowdy, decrepit, horribly dingy old fly” (p.2), she is trapped by their unsympathetic considered opinion of her. Her fly is on the verge of drowning, as she stated her self in the middle of the passage, struggling out every now and then, but, ultimately, always sinking