standard deviation of worm lengths

Just a little fragment from the two-hour exam I took yesterday. Really. 

The problem went something like this: "Tank A has one large worm, two medium worms, and four small worms. Tank B has one large worm, two medium worms, and nine small worms.  If one medium worm is moved from tank A to tank B, how does that affect the standard deviation of worm lengths in tank B?"

It was accompanied by a nifty little diagram, which I have attempted to replicate:

(Of course, my worms are smiling.  The ones on the exam were not and can you blame them?)

The correct answer? Hmmmmmm.  But the true answer?  That's another story.

With the extra space created by the departure of a medium worm, the inhabitants of tank A decided to remodel and convert the space to a home gym.  However, all is not so happy in tank B, where the medium worms have declared that while two is company three is definitely a crowd, and the small worms are suing the tank owner, claiming discrimination on the basis that because they are horizontally challenged, they are not allotted the same amount of space as the of other worms and forced into population dense accommodations.   The large worm has declined to comment at this time but will issue a formal press release after his agent has completed negotiations for his latest book deal, "My Life Behind Glass: A Megadrile's Search for Self".

I hear a major motion picture studio is interested in optioning the story - stay tuned.