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In my world mostly composed of arthropods, I wanted one to fill a niche similar to that of a common snapping turtle (an ambush predator that extends its long neck to strike quickly at prey), and similar to a common snapping turtle, I wished for it to have a long, flexible neck that it could retract into the inside of its body, specifically an elongated segment specialized for it (the neck itself would be composed of several small body segments, the legs of which atrophied overtime). The issue with this is that not only were there basically no insects with actual long flexible necks I could find (most having one rigid and elongated segment for a "neck"), the best counterpart I found for what I wanted was a dragonfly nymph's xenomorph-like lower lip, which is simply tucked under its body while retracted. The examples of insects with parts they could retract into their bodies were also not exactly helpful, mostly limited to the stingers of hymenopterans and similarly small structures, even in proportion to the animals' bodies.

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To be more exact: I'm struggling with harmonizing a turtle-like anatomy regarding the neck with an exoskeleton, as while the turtle's body is almost exoskeletal in nature, the neck itself is supported by a group of neck vertebrae, the outer covering being flexible, bendable skin that can easily "accumulate" on the outside without compromising the neck's internal structure. Yes, I could simply use a dragonfly nymph approach without even needing a long neck, but I wished for a more turtle-like approach for the sake of looks, as I wished for these arthropods to have a more vertebrate-like appearance.

That said, based on how exoskeletons function in our world, can an arthropod-like creature have the ability to mostly retract its head and neck into its body similarly to a snapping turtle? If not, I'd appreciate if another alternative for protecting the head and neck, especially while giving an illusion of retracting it, was presented. For the sake of this question, the creature's overall size and neck length are similar to those of an adult common snapping turtle.

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If you consider the actual mechanics of what the skeleton of a turtle does during retraction, that behavior is 100% compatible with having an exoskeleton.

During retraction all a turtle's neck is curving to the side inside the protective armor of the shell. There are two ways that turtles bend their necks to fit inside the shell. The Cryptodirans, like snapping turtles, bend their neck down and back to retract, while the Pleurodirans bend their neck to the side.

This sort of S-curve motion can be observed in the abdomens of mating dragonflies, the bodies of centipedes, and the tails of scorpions, among multitudes of other examples. If you imagine a fantastic critter with the body of a turtle and the neck of a centipede you should be on the right track to building a critter with an exoskeleton and a retractable head.

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Yes, it is possible. In fact, some insects with retractable heads already exist on Earth.

A great example that may even be similar to what you are looking for is the Photuris firefly's larva (research article here).

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The Photuris firefly larva's head can be retracted into its pronotum (plate-like structure), which is similar in functionality to that of a turtle.

Perhaps you could have your creatures have slightly more elongated heads/necks as necessary (as these ones have heads that protrude roughly 1mm out of 12.5mm), or proportional to their actual sizes and lengths.

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Does it have to be an exoskeleton, precisely? How about scales?

I'm thinking of armadillos. Their scales are attached at the base and it gives the armadillo a lot of flexibility in movement while keeping it protected. A neck with armadillo scales could extend, where the scales overlap very little, and then retract, causing the scales to greatly overlap but giving it that flexibility while providing exterior protection.

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  • $\begingroup$ The only issue with this is that my problem was really about making a retractable head and neck working with an external skeletal structure rather than an internal one, as turtles themselves already offer an ideal example of how to make it work with an endoskeleton. $\endgroup$ Commented Sep 9, 2022 at 0:46
  • $\begingroup$ @ProjectApex I think you can actually mix them. Apparently alligators are considered to have both an endoskeleton AND an exoskeleton (consisting of scutes). That is, it's not that alligators just have tough "skin". They have skin and then on their skin is an attached exoskeleton of non-overlapping scales. So I'm thinking you have a critter that is either endo+exo or just exo but where it needs this extendable part there is actually an endoskeleton (or sufficient musculature) and the exoskeleton turns into scales there. $\endgroup$
    – JamieB
    Commented Sep 9, 2022 at 17:44
  • $\begingroup$ That's not an exoskeleton though. Osteoderms are growths made out of bone present on the skin, and serve primarily for protection and sometimes thermoregulation, with little to no participation in muscle attachment for locomotion. An exoskeleton on the other hand, while also very useful for protection, acts as the main attachment point for the muscles used in locomotion. $\endgroup$ Commented Sep 10, 2022 at 14:16

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