What is tagmosis??
Why I’m glad you asked :)
Enter the common ancestor of all arthropods. They probably looked a lot like centipedes and millipedes, or even velvet worms, which are super close to arthropods. By which I mean they were long and made of segments.
Super simple! Got that? Good.
Lets go forward through evolutionary time. Now every segment has a pair of legs. Everything is very uniform and unspecialized.
Still with me?
Here’s where things get weird, the segments themselves to change size and shape, and the legs change as well. How these things change depends on the subphylum itself, but I am an insect blog so I’ll focus on them. To make a long story short:
Some of the legs migrated to the top of the segment (antennae)
Some legs in the front were used to shove food into their mouths (mandibles),
Some legs were used to sense the environment around them through touch (maxillary and labial palps).
Some of the legs on the back end elongated (cerci)
Some of the legs reduced until they basically disappeared
And of course some of the legs remained as walking legs, the six legs that we are all familiar with
The first six segments became thinner and thinner until they eventually fused, becoming the head
The next three segments are the ones the walking legs are attached to, making up the thorax
The rest of the segments comprise the abdomen, absent of any legs
Like so.
And woah! My awful generic drawing turned out looking like the most ancestral insect order we know of, the archeaognatha! How interesting!
You can even make the vestigial remnants of the legs that were on the segments that would later become the abdomen!
So there you have it! Tagmosis is the process of an unspecialized body plan separating into specialized regions, which are called the tagmata. The three tagmata in insects are the head, which specializes in sensory of the environment, the thorax, which specializes in locomotion, and the abdomen, which specializes in digestion and reproduction. From there evolution stretched and squashed segments and legs (and mouthparts) into all different kinds of crazy shapes. The number of segments differs too.
Tl;dr everything about insects is legs
