The Proboscis: Evolving from Impregnating to chomping

At some point, the Probiscis went from impregnating to essentially consuming/attacking. How might you explain this evolution? I do not know but can give a brief time line (I include only movies here so I might be missing something from literature).

The first proboscis is seen with the trilobite and it was used for impregnation:

Next, we see the Deacon have a large inner jaw that somewhat resembles the proboscis except it looks like it is meant for chewing:

Later, we see the face hugger with an impregnating purposed proboscis:

followed by the iconic chomper:

Side note- interestingly, the neomorph had only a traditional toothy mouth:

The functions seem to alternate between impregnating to destroying- assuming the inner jaw is seen as a proboscis of sorts. 

This has been a pattern through the series and I don't recall a discussion about it. Any ideas?