Flying Crab

The Spider Alien Baby is a placeholder name given to a creature shown in the E3 trailer.

Behavior
Usually found in a nest. Nests burst on damage or if engineer gets too close. Spiders fly towards engineers to attack, if they are close enough.

If the nest is destroyed from distance, the Spider Alien Babies alternate between flying around and hovering. They stay in the proximity of the nest.

Loot
The Babies themselves drop nothing, but a destroyed nests drops a reskinned Alien Carapace.

Speculation
They are probably weak hostile enemies which are spawned by the Spider Alien in large swarms.