Cagaster

Cagasters are former human beings that have been transformed into giant insects. They are the focus of the manga/anime series Cagaster of an Insect Cage.

History
Near the end of the 21st century, a mysterious disease began spreading across the world. Though cases were rare, the results would have a devastating impact on humanity as a whole. Those who were stricken by the Cagaster virus would transform into huge, insectoid monsters (also called Cagasters or simply Bugs) and attack humans, consuming them. Within a few short years, human society collapsed as major cities were overwhelmed by Cagasters and converted into dens, which surviving humans would call Cages.

By 2115, all of the world's major population centres had been converted into Cages for the Cagasters. Surviving humans would establish new settlements out in the wilderness and wastelands, or otherwise convert smaller towns previously abandoned by their inhabitants into strongholds where humans could mete out some sort of stable life. Meanwhile, the Cagasters would continue to multiply within the ruins of the old world, largely keeping to themselves apart from the odd outlier such as former human nomads who had succumbed to Cagasterization beyond any settlement boundaries.

Infection and Transformation
The origin of the Cagaster virus remains a mystery. What is known is that the disease has spread globally and affects a significant portion of the human population. Merely carrying the virus does not guarantee transformation into a Cagaster, however, there is no method of determining who will eventually transform or not. What has been determined, both by limited scientific data and eyewitness information, is that human children cannot undergo an 'outbreak'. Only humans older than fifteen years are at risk of transforming, though when a human will change cannot be determined. Likewise, there is no method of detecting people who are immune to the disease and not at risk of transforming. A Cagaster outbreak can occur within any adult human at any time. Metamorphosis from human to insect generally lasts for 20-30 minutes. The transformation begins in the extremities, with the subject's fingers and toes becoming stiff and hardening as the skin morphs into chitin. The change soon spreads up the limbs, but takes longer to convert the organs and bones within the torso. By 15 minutes, insectoid wings will have erupted from the subject's back, though they are not yet developed enough to allow flight. By 20 minutes, the chest area will have hardened and become chitinous as well as having grown additional insect limbs. By 30 minutes, the subject's neck and head will have become insectoid. At this point, the new Cagaster will still be human-sized; the time it takes for a Cagaster to reach full size may vary with each subject.

In the new human society that has been built since the fall of the old world, individuals known as Exterminators are paid to kill anyone who undergoes an outbreak before they go full Cagaster. Within the first 20 minutes of transformation, subjects are still legally recognized as people and have human rights, but these rights disappear after the 20-minute mark. After such time, the transforming subject may be killed and the killer will face no legal repercussions. However, even during the outbreak phase, Cagaster chitin is very tough and not easily penetrated. The most vulnerable part of a transforming subject is the neck, but even that will become impenetrable after 30 minutes, leaving Exterminators only a brief period to decapitate the subject before they become a threat.

Biology
Cagasters can come in a variety of forms, most notably those of gargantuan flies or great, ground-dwelling centipedes. While much of their biology remains unknown, their bodies are covered in extra-dense chitinous tissue that is impervious to conventional weapons. Only artillery-grade weapons and high explosives have any chance of breaching this armour.

Cagasters are also capable of reproduction, most likely in a similar manner to normal insects. Cagaster larvae appear as grub-like creatures that run the length of an adult human's arm.