According to Catholicism, do evil spirits need bodies to manifest themselves to people? Can the Devil and his followers communicate to people without the intermediary of someone in a bodily form?   

I would appreciate a response that quotes Scripture, Tradition and examples of the Saints if at all possible.  

If the demons cannot speak directly to people, how can they manifest themselves to mankind?
Demons can communicate with humans without bodily intermediaries

(My source throughout will be the First Part of St. Thomas Aquinas' Summa Theologica)

Although spiritual creatures can take on a bodily form, they are able to influence humans and creation without doing so.  First consider that the assaults of demons consist primarily in temptation and instigation to sin, and that this can be achieved through evil thoughts or near occasions of sin apart from bodily mediation.

Secondly, consider the fact that demons can work miracles (cf. ST Ia, Q. 110, A. 4; and ST Ia, Q. 114, A. 4).  St. Thomas defines the miracles that demons are able to work in a particular sense:


  But sometimes miracle may be taken in a wide sense, for whatever exceeds the human power and experience. And thus demons can work miracles, that is, things which rouse man's astonishment, by reason of their being beyond his power and outside his sphere of knowledge. (ST Ia, Q. 114, A. 4, corpus)
  
  So when the demons do anything of their own natural power, these things are called "miracles" not in an absolute sense, but in reference to ourselves. In this way the magicians work miracles through the demons; and these are said to be done by "private contracts"... (ST Ia, Q. 110, A. 4, ad 2)


Such a miracle could obviously include a communication with man apart from the mediation of a bodily form.  For example, a demon could tempt man by placing an evil thought into his mind.  This is an example of a "miracle" (in the broad sense) that a demon is capable of working, and the demon can do this directly, without the mediation of a bodily form.