Seen above are parasitic wasp embryos, which apparently inspired the "alien birth" in Alien. From National Geographic's photo gallery of "'Extreme' Animal Embryos Revealed'":
In a biological attack unique in the animal world, the unassuming embryos (injected by their mother into a caterpillar) use a virus in their DNA to paralyze their host. They bite their way out of the caterpillar and begin spinning cocoons."IN THE WOMB: 'Extreme' Animal Embryos Revealed"
As a final insult to the injured host, the caterpillar--apparently brain-addled by the virus--builds a silky blanket over its attackers and defends them against predators until the wasps emerge, fully formed, and take to the skies.