One feature that sets insects apart from many other creatures is their hard exoskeleton. This is the shell-like structure that gives them a tougher exterior to protect their softer tissues. All invertebrates rely on this system instead of an internal skeleton. They also do so in very different ways.
Cuticles are made up of three layers. The epicuticle, exocuticle, and endocuticle form to create an exoskeleton that is attached to the epidermis. This provides structure, balance, and protection to the insect.
Not all insects have the same cuticle. For example, insects like butterflies are much softer than beetles, which can have incredibly elaborate structure and amour on their backs. So how does this structure work?
With so many variations, it helps to understand the structure of this “shell.” It is far more complex than we might imagine. It is easy to assume that there is one hard layer covering the creature and protecting it from harm. However, that would be problematic when it comes to getting around. There is also the fact that different insects need to adapt their shells to their needs and undergo a process of molting and regrowth.
That is why there are so many layers to the exoskeleton, all with different properties and functions. Three layers form the cuticle, making a non-living layer of material sitting on top of the living layer of skin and cells’. This non-living layer of the exoskeleton is called the cuticle.
There are three layers in the cuticle of an insect.
The epicuticle is the outermost layer. It comprises of a thin protein, often in a series of layers. Although thin, it provides the perfect protection for the insect. The inner epicuticle is a layer, although entomologists are still unsure how it works. One theory is that it acts as an enzyme reservoir for the other layers.
The outer epicuticle is more functional. This layer contains attachments to muscles and other properties that may control the molt of the insect. For example, some layers may have the capacity for stretching, which can impact the creature’s growth after their molt. This critical layer needs some protection, so the wax layer mostly acts as a barrier against dehydration. A further cement layer strengthens the wax.
This is a much thicker protein structure that determines the strength of the structure. This larger layer is essential for the rigidity of the structure and the protection offered to the insect. Armored insects have a different degree of tanning and structural support. Some will also grow additional horns to look more impressive and help them in battles.
We also see this sort of strong, thick structure in crustaceans. An insect exocuticle is strong for an insect, but they pale in comparison to much tougher crabs shells or lobster claws. That is because this part of the exoskeleton of a crustacean is impregnated with calcium carbonate. There is no need for this in the insect world.
The endocuticle is the innermost layer of the cuticle. It is the most flexible structure and in complete contrast to the dark, hard exocuticle above. This layer is important to act as a balance. If the entire cuticle were as hard as the exocuticle, then the creature wouldn’t be able to move. The endocuticle is transparent and has the sole purpose of providing this movement.
This is where we find the living layer of the cuticle and the similarity between insects and humans. We also have an epidermis, our skin. It just happens to be the outer layer on our bodies as we didn’t evolve the need for any additional protection. We develop calluses to harden the skin and use other methods to protect it from damage.
The purpose of this layer is vastly different, however, in insects. Here, the epidermis secretes components needed to create the cuticle and continue the process of growth and molting at the same time. It can dissolve and absorb the cuticle during the molt.
This epidermis also contains the dermal glands and the start of any bristles that come through the cuticle. These glands allow the insect to secrete substances directly onto the external surface of the exoskeleton. This could be a type of cement used to harden the new cuticle or another substance used as a defense mechanism. The bristles can be touch-sensitive to provide a warning about their surroundings.
These exoskeletons aren’t permanent and there is an ongoing process of growth, molting, and regrowth.
Larval and immature creatures can change their form and grow, requiring a new exoskeleton. They can secrete, absorb the old cuticle, secrete the new one, and shed the structure that no longer serves them any purpose. Insects are vulnerable during this time as they need time for the new cuticle to harden and for that protective outer layer to get the wax and cement coating. Many insects cannot also breathe during this process.
Tanning or sclerotization is important for both the strength of the structure and its color. You may notice that a lot of larvae have a pale color with no obvious shell. This is so that they can move freely and develop into a more mature state. Many insects in their mature state have a much darker exoskeleton. In the case of many tougher beetles, it is black. This pigment comes through the thinner epicuticle and looks shiny due to the waxy layer.
We cannot overlook the cuticle’s complexity within the exoskeleton of insects because it is so much more functional and important than we realize. This is more than one tough protective barrier to save them from harm. It is a multi-layer structure that adapts and heals to provide the best possible protection and adaptations for that species.