Entomology III - Cuticle and molting

In Entomology I and II, we’ve explored the external anatomy of the head and the diversity of mouthparts, we’ve also explored flight, leg types and cerci modifications. Here, we'll explore the cuticle, its chemical make-up, its colors and explain how insects molt. By the end of this post, the cuticle won't have any secrets left for you. THE CUTICLE IS STRATIFIED The cuticle is made of chitin - a polymer of glucosamine, which forms most of the integument. It can be soft in juvenile larvae such as in caterpillars and hard in adults. It has several layers, making it complex and interesting. what I'm about to tell also applies to crustaceans, arachnids and myriapods. Here’s a representation:

Let’s start from the bottom. The cuticle rests on a thin membrane called basal lamina. On top of it lays a single layer of epidermal cells. This layer is important. It contains gland cells that release wax to protect the insect from dehydration and release chitinase, an enzyme that breaks the chitin during molting. Another type of cell is the trichogen cell which produces a sort of hair called setae, useful for mechanical and chemical perception. On top of the epidermis lays the endocuticle which is the thickest cuticle layer. It’s mostly made of soft chitin (none-sclerotized chitin) and doesn’t vary in thickness. On top of the endocuticle lays the exocuticle which is hard chitin (sclerotized chitin) and varies in thickness. Less thick in juveniles than in adults. Okey cool,  but what's the purpose of the cuticle❓ 🙄 ⏱tic toc.. Any ideas? The most important role of the cuticle, is to provide anchoring for the muscles. As humans, we have an endoskeleton for muscle anchoring, insects use their exoskeleton for the same purpose. The cuticle also offers rigidity and protection against potential predators, protects insects against dehydration thanks to the wax and against parasites. It can be highly modified for mimetics and combat!

Rhinoceros beetle battle

ECDYSIS Now that we know the anatomy of the cuticle, we can take a look at the molting process. Insects molt in order to grow! The molting process – ecdysis, begins with the separation of the old cuticle from the underlying epidermis, this is known as apolysis. To allow the old cuticle to separate, the underlying epidermal cells replicate in response to the release of ecdysteroid molting hormone (synthesized and released into the insect’s system by the prothoracic glands). Apolysis has created an open space between the cuticular layers into which a digestive fluid containing chitinase is secreted. This breaks down the inner endocuticle into basic metabolites that can be reabsorbed by the epidermal cells to form the new cuticle.

AND NOW WHAT? The insect has shed its old cuticle to grow, how does it grow a larger cuticle so that it doesn't prevent its own growth? It needs to grow a new cuticle, larger than the previous one to allow its own growth. First, the insect contracts its muscles to increase the fluid pressure in different parts of its body. Air or water is often swallowed in combination with controlled muscle contractions to inflate the gut and the body. This makes the insect appear larger, allowing for a larger cuticle to form around it. As the new cuticle forms, the H-bonds between the polymers of glucosamine (the molecules that make up the chitin) are replaced by phenolic bonds, making the cuticle hard and stiff, this process is called sclerotization or tanning.

WHERE DO INSECTS' COLOR COME FROM? Does the cuticle in itself have pigments, is it a result of light scattering? Physical or chemical? Well often both mechanisms occur together to produce a different color than if they were produced individually. Insect coloration is the result of many factors. One of them is the interaction of light with the cuticle or the underlying cells or fluid. This is what we call physical structural colors. They result from light scattering, interference and diffraction. Then we also have pigmentary colors, which result from the absorption of visible light by chemicals. There are many different pigments that produce different colors and hues. Carotenoids, ommochromes, papillochromes, pteridines (petrins) mostly produce yellows to reds. Flavonoids give yellow, tetrapyrroles (including breakdown products of porphyrins such as chlorophyll and hemoglobin) create reds, blues and greens. Quinone pigments occur in scale insects as red.

From left to right, top to bottom Stag beetle (cuticle is very melanized) Blue morpho (structural color due to microscales) Firebug Leef beetle

These chemicals can be sequestrated from a plant source, produced from their metabolism and more rarely by microbial endosymbionts. Many adult insects have a dark pigmentation because darkening easily happens through sclerotization (unrelated to pigmentation) or the exocuticular deposition of melanin (a group of polymers that can give a black, brown, yellow or red color). COLOR ROLES Colors are used for intraspecific communication, such as sexual and defensive display, but also protection against harmful light rays and conversion of light into heat thanks to melanin, or be part of the main visual pigments in the eye (ommochromes). Btw, did you know that insects are very resistant to radiation? This is linked to everything we've seen in this post. Animals are most vulnerable to radiation during mitosis, which occurs continuously in most vertebrates but is largely discontinuous in insects! In insects, mitosis mainly occurs right before and during the molting process. Insects that are not growing are very tolerant to high levels of radiation but are vulnerable during ecdysis.

SOURCES - My Uni lectures (won't mention the name of my uni for privacy reasons) - Lectures from another uni - University of Alberta: Bugs 101: Insect-Human Interaction. Can be found on Coursera - 2 Books on entomology which names I don't remember. They're in my uni library, I'll add the titles when this whole covid mess is over and I can go back to uni. - Lifeyard