"That flies in the night/ In the howling storm"- What flies in the night?What role does the 'howling storm'? why is the worm called 'invisible'?

In William Blake's "The Sick Rose" the invisible worm flies in the night.

        The' howling storm' is a damaging wind making loud noise while making the atmosphere dark at the same moment. It plays a major part in the worm's seduction of the rose. It energizes the worm and adds strength to the worm's evil design. Simultaneously, it adds to the rose's helplessness.

The worm is called invisible in this poem because it has crept into the bed of the rose and also because of the darkness produced by the night and the hurling storm. No one can see this. The rose's sickness can make its presence felt. So to the human eye, the worm remains invisible.