The insect cuticle: (1) multi-functionality

This post is part of a series on the insect cuticle as a biological material that can inspire novel engineered materials. The characteristics of the cuticle, setting it apart from most synthetic/engineered materials, will be discussed in this series. The introduction to the series can be found here.

Cellular and acellular layers make up the insect cuticle. With the most interior layer being comprised of living epidermal cells that secret the outside layers, which then completely cover the insect – even some of the interior surfaces such as the trachea, foregut and hindgut – are lined with cuticle. (The hierarchical characteristics of these layers will be discussed in the next post.). One of the characteristics that makes the insect cuticle, an “inspirational” biological material is its multi-functionality – something that is rarely seen in engineered materials.

Living materials, including the insect cuticle, often exhibit novel properties that are difficult to incorporate (all at once) into engineered materials. Unique physical and chemical interactions of the biomolecules that make up the cuticle (building blocks) at the nanometer scale convey characteristics such as high strength, energy absorption, and flexibility. Currently multi-functionality in engineered materials is limited to different functions due to a hybrid of several distinct phases the material can attain.

Below I have sketched out some of the functions of the insect cuticle. (If I forgot some important functions please leave a comment below and I will add them to the diagram)

insect cuticle functions

Insect Cuticle Functions – probably not complete – click on diagram to enlarge.

Insect Cuticle Functions

Protective Barrier

The insect’s exoskeleton/cuticle/integument is doing the functions of both our skin and our bones. The cuticle completely covers the insect (~armored skin), while at the same time serving as a supportive skeleton (~bones).

The protective covering creates a barrier. Precious water and ions are prevented from freely moving outward, while pathogens, parasites and dangerous chemicals are prevented from moving inward. This function is not at all trivial for insects. Since insects are relatively small they present a large surface area to the outside environment so that loss of water is a greater problem than it is for larger animals such as mammals.

Structure and Form

The insect’s exoskeleton gives the insect structure and form. And over an individual’s lifetime that form can change. In the case of holometabolous insects, such as flies, wasps, bees, beetles, butterflies and moths, this form change is striking. As an immature caterpillar a moth has a cuticle that stretches and is relatively soft, as a pupa the same individual (using the same building blocks, or biomolecules) has a cuticle that is extremely tough and does not change shape easily. Then as an adult moth the cuticle, including the wings, has yet other features that make the insect successful.

The change in structure and form seen in holometabolous insects, and to some extend the growth strategies employed by ametabolous and hemimetabolous insects, enables the animal to exploit different habitats and diets – even during its lifetime.

This is one of the most striking things about the cuticle. During the separate life stages the cuticle has different functions, it therefore has distinct characteristics and appearance. Yet, the biomolecules that make up the cuticle are pretty much the same, and one individual can synthesize all these seemingly very different types of cuticle.

How different is the cuticle from life stage to life stage? I asked family, friends and colleagues to describe, in non-scientific terms, what the cuticle of each form of the hornworm (=moth) feels like. Here are their responses.

Caterpillar or Larva

CaterpillarDrawing

Caterpillar (Drawing by Marianne Alleyne)

Soft, rubbery, squishy, velvety, muscular, cold. Feels like pleather, like a writhing rubber pickle, like play dough that I rolled in my hands to make a snake.

Pupa

PupaDrawing

Pupa (Drawing by Marianne Alleyne)

Hard, smooth, leathery, acrylic, slick. Feels weird & dead.

Adult

Adult Moth (Drawing by Marianne Alleyne)

Adult Moth (Drawing by Marianne Alleyne)

Fuzzy, hairy, soft. Feels like perfumed talc from my grandmother’s vanity. Holding it will probably feel like a fluttering beakless bird but since I haven’t held a fluttering bird, with or without a beak, I can’t be sure.

Movement

The rigidity that the exoskeleton exhibits makes it possible that insects can make rather precise muscle movements since those are due to the insertion of muscles to the integument wall. It is often the cuticle that has important biomechanical features that enables an insect to run, jump, dig, fly or swim. These precise movements are also essential for respiration (creating flow of air in and out of the tracheal system), food manipulation, excretion and osmoregulation.

Sensing the Environment

Maybe surprisingly a rigid integument is not necessarily limiting awareness of the surroundings. The cuticle has been modified in many insects into structures that can sense most of the modalities that we can sense with our skin. Some examples are the trichoid sensilla, the campaniform sensilla and chordotonal organs. Note that all three of these sensors use the cuticle as an integral part of their structure.

The functionality of the trichoid sensilla - a mechanosensory hair - is dependent on the rigidity of the cuticle.

The functionality of the trichoid sensilla – mechanosensory hairs – is dependent on the rigidity of the cuticle. (Drawing by Marianne Alleyne)

The campaniform sensilla is a mechanoreceptor found in insects. When the exoskeleton bends the resulting strain stimulates the sensilla.

The campaniform sensilla are mechanoreceptors found in insects. When the exoskeleton bends the resulting strain stimulates the sensillum. (Drawing by Marianne Alleyne)

The chordotonal organ is a stretch receptor that senses to what degree the cuticle is being deformed. This deformation can then give information about movement of body parts, gravity (proprioception) and vibrations of the surrounding air (hearing).

The chordotonal organ is a stretch receptor that senses to what degree the cuticle is being deformed. This deformation can then give information about movement of body parts, gravity (proprioception) and vibrations of the surrounding air (hearing).

Energy Storage

For many insects the cuticle also represents a temporary food store. The basic building blocks, to some extend, can be withdrawn during times of nutritive stress. Having to molt to be able to grow in size is one of the drawbacks of having an exoskeleton. Molting consumes time, energy and metabolic resources, and makes the insect vulnerable against pathogens, predators and water loss.  Reabsorbing much of the cuticle during molting minimizes some of these costs. (The closed-loop cycle characteristics of the cuticle will be discussed in part 8 of this series.)

Behavior Modulation

The single layer of epidermal cells that secretes the cuticle also secretes and deposits within or on the cuticle hydrocarbons that are involved in behavioral sequences that are important in recognition and mating. Pheromones and pigments are also deposited. The cuticle may also have modified structures that are important in mating or other behavioral processes – such as bumps,  hairs, nano-scale structures that create structural colors.

By further studying the insect cuticle it is my hope that new materials can be created, using similar “manufacturing” steps as employed by insects, that can provide increased function through integrated or further-integrated systems. Insects do it, we should too.

Next: the structural hierarchy of the insect cuticle.

(My apologies to dumpy grey-brown moths everywhere. That is a terrible drawing of an adult moth. Oh, well)