The display of "Running Ants" is Off by Default in version 5.8.

To reduce resource load when nearing operating limits, running ants do not appear on links by default.

You will find this in the 5.8 Release Notes:

Note: To see the release notes you will need to be signed into your account on the web site.

To re-enable the ants, use the Edit > Preferences > Animations tool.