mutexes and const_iterators

Hello

I am using a map which holds a list of client connections (to a server).
When a client connects a client gets added to the map and also when a
client
disconnects.

In various parts of the code the map gets updated - the key is an int and
the value is a class.

As the map may be accessed by multiple threads I use a mutex to control
access - ie locking whenever an item is added or removed.

I have these questions:

1. Do I also need to lock if I just update and item?  I access the item
via
an iterator.

2. Do I need to lock if I simply access the map via a const_iterator?

Angus