I’m guessing you’ve duplicated your square to get individual animations??
If that’s the case, then it should be pretty easy to create a second small size compostion, with one static sqaure in it. Add a new solid, slightly larger than your square. Draw in a tail shape using the path tool. You can set keyframes to animate it by hitting the M key and clicking on the shape setting. Then apply Stroke to make it visible.
Now then easy bit. In your original comp, highlight one of your sqaures and hold down the ALT key. Now drag the square-tail comp on top of that layer. The sqaure is replaced by your tail comp and retains all the movement keyframes.
For a swarm, think about trying Particle Playground, or Foam.
Hope that helps
Graham
https://www.quinceweb.com – web design
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https://www.shiveringcactus.bravehost.com – Free FX for amateur films