Position movieclip on top attached movie clips

[Vadersapien]

Hello,
I would like to have one movieclip over the top of other attached ones. I have tried getNextHighestDepth on the movieclip, but it still doesn't work.

Here is my code for attaching the movieclips:

function growSpider() {
    spiders = 0;
    while (spiders<5) {
        spiders++;
        randomX = Math.floor(Math.random()*(745-55))+55;
        spider = attachMovie('Spider', 'Spider'+this.getNextHighestDepth(), this.getNextHighestDepth(), {_x:randomX, _y:-32});
        spiderArray.push(tree._name);

        spider.onEnterFrame = function() {
            this._y += car.speed/2;

            if (!_parent.canGo) {
                stop();
            } else if (_parent.canGo) {
                play();
            }

            if (this._y>=632) {
                this.removeMovieClip();
            }
        };
    }
}