According to the introduction, mary shelley wanted to write what type of ghost story

a story like one that evoked the unspoken fear of humans.

Mary Shelley wanted to write one type of ghost story that evoked the unspoken fears of humans.

In her introduction for her novel Frankenstein she stated  “I busied myself to think of a story, —a story to rival those which had excited us to this task. One which would speak to the mysterious fears of our nature, and awaken thrilling horror—one to make the reader dread to look round, to curdle the blood, and quicken the beatings of the heart.”

She aimed to write a story that frightened readers so much they would become suspicious of their surroundings.

what are you asking i dodnt know

cockroaches and rats often are a symbol for things that are disgusting or vile. though i don't know which play you are talking about, i hope this !