@Goodlyay i think the problem of the stroke is because i am more illustrator than a pixel artist, and i use the tablet a lot, i will try to correct this problem, being more prudent with the tablet and trying to use more the mouse. Thanks for the examples, i will improve my corners whith your trick. Thanks for the tips Goodlyay  !

Thanks @irneaart,  I try to correct the problem of colors in the export .GIF now has 32 colors. (in the next practice, i will try to use less colors). you're right with the caterpillars, i will try to give more time to animation phase. Thanks a lot for the tips  !  

First of all there are too many colors in this image; 127 which is far too much. This kind of image shouldn't have more than 16 colors. Many of this colors go to the smoke and debris which you've made with transparent brushes. That's not in the spirit of pixel art. Magic of the pixel art lies in the fact that every pixel is important (well, at least in my opinion).
The black color is too dark compared to other colors. For the outer edge it's okay, but for the inner lines you should use some lighter color.
Maybe for the first time would be better if you make complitely side view animation. So it will be easier to make animation and it will be easier to animate the proper objects rather than the objects thinned by rotation.
The inner parts of the caterpillars look static. There is no real indication of the rotation of these parts. And you should  darken bottom part of the background caterpillar.
Well, all in all not bad. It just needs a little more refinement. Or you can completely ignore my tips and work as you like. 

Hi! There are a couple of  big issues with this piece, the color count is out of control (probably due to blending tools used) and the outlines are a bit messy.

These shouldn't be too hard to fix as long as you're aware of them.

Few examples of the messy lines I mean

Mostly making sure the corners aren't inconsistently touching does the trick: https://dl.dropbox.com/s/7rgdf8lw8u6dmm7/lines.png

I am voting against this being accepted into the gallery because of the tools used resulting in non-pixel art effects (the dust and smoke), but I encourage you to go through and resubmit the piece with a more controlled color count.