The insect cuticle: (5) Mild synthesis conditions

The insect cuticle: (5) Mild synthesis conditions

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.

In Nature Easy Does It

Most biological materials, including the insect cuticle, are produced at ambient temperature and pressure. In addition, biological material synthesis often occurs in an aqueous environment. This is very different from how engineered materials are fabricated. Engineered materials are too commonly fabricated with the heat, beat and treat method.

Machine Meltdown by Darkday from

Machine Meltdown by Darkday from

A lot of energy is added to the process of fabricating engineered materials. The material is usually forced to conform by adding pressure to it from the top down, and then (not always benign) chemicals are added to give the material additional properties. Nature teaches us that fabrication under mild synthesis conditions is possible too.

In general, most physiological processes related to growth in insects, including the fabrication of new cuticle, occur at temperatures between 40-100°F (5-40°C) and at standard atmosphere (~760 mmHg). This fabrication occurs from the ground up, no big machinery is needed, and only benign chemicals are used.

In my bioinspiration courses I ask students to create an Ashby plot (comparing tensile strength vs Young’s modulus) for engineered and biological materials.


Engineered materials plot.

Ashby Plot Complete

Biological materials overlaying engineered materials plot. (By P. Karnstedt)

The activity reinforces that, at least for these two variables, biological materials are often as good, if not better, as engineered materials. Yet biological materials are fabricated at far milder synthesis conditions than engineered materials.

Some notes on how “extremophile insects” are able to survive, and maybe even grow, in extreme conditions.

Insects live in just about every habitat on Earth, from hot deserts and thermal springs to cold caves and frigid mountain streams. Clearly insects and other arthropods can survive some of the most inhospitable conditions on Earth. But some of extremophiles listed below will not molt under these extreme conditions.

  • Desert ants such as Cataglyphis bicolor, which live in extremely hot deserts (Sahara), use behavioral tactics to function at midday temperatures. The stages that will have to produce new cuticle in order to grow, the larval stages, are present underground where temperatures are not as extreme.
  • The flat bark beetle (Cucujus clavipes) larvae and adults survive arctic northern Alaskan winters by way of some clever blood chemistry that results in anti-freeze properties and prevents inter- and intra-cellular fluids from freezing. The larvae do not molt until the warmer periods and will do so under bark.
Red Flat Bark Beetle by Tom Murray via BugGuide.

Red Flat Bark Beetle by Tom Murray via BugGuide.

Cucujus clavipes larva by Lynette via

Cucujus clavipes larva by Lynette via

  • The largest purely terrestrial animal on Antarctica is the tiny midge Belgica antarctica. This insect can survive extreme freezing temperatures, and dry habitat, by using its immediate environment as a barrier and through the management of body water as temperatures drop and come back up. Most of the larval growth however, including molting, only occurs during the warmest weeks when temperatures are well above freezing.
Belgica antarctica larva by Richard Lee. Courtesey: National Science Foundation

Belgica antarctica larvae by Richard Lee. Courtesy: National Science Foundation.

  • Other extreme conditions that some arthropods deal with are lower pressures or very acidic water. The Himalayan jumping spider (Euophrys omnisuperstes) is able to survive at 20,000 feet (6000 meters) despite the lower air pressures (video). But the range of pressures that are encountered on Earth are not as great as the pressure difference humans create during manufacturing of engineered materials, when materials are often pounded into the desired shape.
Himalayan jumping spider by Gavin Maxwell via Arkive

Himalayan jumping spider by Gavin Maxwell via Arkive

  • In one of the most inhospitable places on earth, Cueva de Villa Luz, a cave located in the southern Mexican state of Tabasco, arthropods thrive despite the fact that the waters have an extremely high concentration of hydrogen sulfide. Insects such as predatory belostomatid bugs, have been shown to live within these caves during all life stages. How physiological mechanisms, such as molting, are possibly altered in these acidic waters has not yet been studied in detail.
Quorum Sensing On Capitol Hill

Quorum Sensing On Capitol Hill

Last Fall I was selected to be a member of the inaugural class of the Entomological Society of America (ESA) Science Policy Fellows. Last week came our first big test – be advocates for entomology and entomological research on Capitol Hill. And guess what? We had a very productive time.

I’ve wanted to write about our Congressional visits from before I even left on my trip, but was struggling to find a hook that would pull together government, politics and bioinspiration. I could go with a story about dominance hierarchies (=pecking order) in both human and animal societies, or one about gift giving and the influence it can buy, or an essay about insects that steal from other insects. Or was this finally going to be the blog post where I could discuss my favorite topic in all of biology: parasitism?

Actually, turns out that the Congressional visits were challenging, exciting, beneficial, fun, inspiring, etc. The most appropriate bioinspired analogy I can make is to quorum sensing seen in social insects.

Now what? We better find the best new nest site quickly. Picture by Eran Finkle via

“Now what? We better find the best new nest site quickly.”
Picture by Eran Finkle via

Honeybees, for instance, use quorum sensing to find a new nest site. A swarm comprised of 10,000+ bees decides on the best spot to start a new nest, not by letting just the queen decide, but by gathering information from different scout bees. A couple of hundred scout bees convey information to others in the swarm about the quality of about a dozen possible nest sites. Nest sites can be superb, mediocre or lousy, or somewhere in between. Based on the information about the possible site’s quality conveyed by the scout bees the swarm as a whole decides which site is most suitable. It is important that all information is freely shared, no bee’s opinion is stifled. Coalitions of scouts that have discovered a certain site will try their best to convince other scouts to go check out a potential site. The better the potential site, the more vigorous the bee will waggle-dance. A more vigorous dance will convince more uncommitted scouts to go check out the site. So there is competition between the coalitions, but what is important is that the uncommitted scout does not blindly follow the information. She will go check out the site but will decide for herself if she will advertise the site when she returns to the swarm. In other words, acceptance of a poor site (through cheating or through the creation of mass hysteria) is impossible. Through quorum sensing many opinions are heard and evaluated, yet this is done rather quickly so that the swarm is not vulnerable for long. Options are not debated “to death”.

Probably not the best nest site.  Picture by By Nino Barbieri (Own work) [CC BY-SA 2.5 (], via Wikimedia Commons

“Probably not the best nest site.”
Picture by Nino Barbieri (Own work) [CC BY-SA 2.5 via Wikimedia Commons]

Good group decisions, the bees show us, can be fostered by endowing a group with three key habits:
1. structuring each deliberation as an open competition of ideas,
2. promoting diversity of knowledge and interdependence of opinions among a group’s members
3. and aggregating the opinions in a way that meets time constraints yet wisely exploits the breadth of knowledge with the group.

Seeley, Visscher and Passino

American Scientist May-June 2006

What we can learn from honeybee democracies (yes, there is even a fantastic book on this topic) is that any democratic government can make the best decisions, within a reasonable time period, when it goes through phases of collective fact-finding, vigorous debate, and consensus building. Quorum sensing is a type of decision-making process in any decentralized system – one without a clear boss. Any decision-making group should rely on information of individuals with knowledge about the topic, shared interests (stakeholders) and mutual respect. Debate should be relied upon, diverse solutions should be sought, and the majority should be counted on for a dependable resolution. Sound familiar? Too idealistic?

Probably also not the best nest site. Picture by MSgt Todd E. Enderle, 309 AMU/MXACW, 13 Oct 2005, submitted by Wayne Fordham, HQ AFCESA/CESM, Tyndall AFB, FL.

Probably also not the best nest site.
Picture by MSgt Todd E. Enderle, Tyndall AFB, FL. via

I may seem a little idealistic here but while visiting the Senate’s and House of Representative’s offices I definitely had the feeling that constituents, experts, stakeholders and staffers were coming together to share information in an effort to help inform the legislators. All our meetings with staffers, and in my case in person with a Sen. Dick Durbin: D-IL and Rep. Rodney Davis: R-IL, were worthwhile. Staffers seemed interested, if not always knowledgeable about entomology (though many of them were). And it was clear to us that staffers were keen to have access to the best scientific information, preferably written/conveyed by experts using clear terminology and in a concise manner.

The leadership of the Entomological Society of America has put resources into the science policy fellowships, science policy committees and communication about science policy with the understanding that the pay-off may not become obvious until a few years from now. Even what this “pay-off” may be is not clear – it can vary from ESA name recognition, more familiarity with the field of entomology among legislators (beyond pesticide-spraying- or butterfly-net-waving-scientists), more expert entomological input into important policy decisions, job opportunities for entomologists with a science policy interest, more state and federal funding for entomology, etc.

  • In principle the United States government encourages collective fact finding with an open sharing of ideas. Again, I felt that our message was welcomed, but we, ESA members, need to become more proactive in sharing our message. We need to continue to visit State and Federal government offices and explain to them why research on insects is important – why sustained funding is important. We need to be more proactive about sharing our entomological science and expertise. ESA needs to set the agenda, not react to it. If you, the scout bee, are not showing up, or do not have the credibility, then your opinion will not be considered. Over time, with name/research-field recognition, your expert science-based opinion may become more valued (of course, this does not happen in honeybee swarms, and neither does the importance of money in buying name recognition – this is where the bioinspiration model (or is it our current democracy model?) breaks down).
  • On paper a strong democracy should promote diversity of knowledge and vigorous debate among stakeholders. There is, however, a difference between scientific evidence and pseudo-science and the two should not receive equal consideration. Scientific evidence may also not line up with constituent (stakeholder) economic, social and cultural interests. And, again, money and power may outshine solid scientific evidence and stifle debate about the science. By asserting ourselves as the premier source of entomological science we should be able to be part of the debate.
  • The strength of quorum sensing in relation to honeybee swarm nest finding is that in a relatively short time period a consensus is reached. Currently consensus building in Congress on many issues is, let’s just say, difficult. But when it comes to a topic such as Pollinator Health then I am pretty hopeful. (On the issue of the importance of pollinator health we have generally strong bipartisan support.) One of the greatest weaknesses that I see is that even within the ESA itself consensus building is taking too long. For more than 6 months members have been working hard on position statement regarding Pollinator Health and Tick-borne diseases. Meanwhile Congressional hearings have been held, and White House “national strategies” have been set in motion – sometimes with little or no input from ESA. Again, we should help set the agenda, not just react to it!

Our message as entomological experts (7000+ ESA members) will be heard depending on if our information is backed up by good scientific evidence, is supplied in a timely manner and communicated properly. In addition, our representatives in government need to be open to receiving this information. Their willingness to incorporate our advice may depend on their committee assignments, ability to understand the topic, their home districts and states, their constituents, etc. ESA has to start somewhere in gaining more influence and securing sustained research funding for their members. I believe this month’s congressional visits by the science policy fellows was a great beginning.


Now we may be on to something. Picture by Ontheway2it via

Note: I want to thank Lewis-Burke Associates, the government relations firm that is working with ESA to train the Science Policy Fellows. We learn so much from them. Thank goodness for their collective sense of humor and endless patience!

Further reading:

Ariel Rivers also blogged about our DC trip: Entomological Society of America (ESA) Science Policy Fellows do DC!

or the crib notes version

Some more tweets related to our DC trip. Sadly we were unable to self-organize to take a group picture – guess the ESA Science Policy Fellows are more similar to solitary bees.


I met personally with Rep. Rodney Davis (R-IL) (below) and Senator Dick Durbin (D-IL). Pictures provided to me by their offices. Also pictured is Karen Mowrer from Lewis-Burke Associates.


The insect cuticle: (4) hydration

The insect cuticle: (4) hydration

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.

Hydration: Slippery when wet-ish

Traditional manufacturing of engineered materials tries to avoid, at all cost, incorporating water. Yet, the properties of biological materials are determined to a large degree by the absence or presence of water molecules located within the extracellular spaces (=hydration level). The presence of water affects mechanical properties such as toughness or strength. It serves as a plasticizer, keeping a material flexible and resilient. Once an engineered material is manufactured we don’t really want to be able to manipulate these types of properties; materials are to remain static. But wouldn’t there be advantages to having bioinspired materials that are more adaptive depending on their surroundings? Certainly materials that are able to alter their function within a structure at different times will result in some very innovative applications.

Insects go through different life stages, most pronounced in holometabolous insects; egg, caterpillar, pupa, adult. All these life stages exhibit different cuticle characteristics, maybe even on the same body (e.g. tough thorax, soft abdomen).

By Rob Mitchell

Various Sphingid moth life stages and species – with different cuticle characteristics. Art by Rob Mitchell, used with permission.

It is not well understood how hydration influences one of the most impressive features of the insect cuticle: the ability to sclerotize (also called tanning or hardening). The chitin fibers that are a major component of the cuticle are hydrophylic and are thus surrounded by water molecules. Chitin becomes dehydrated when it becomes part of the protein matrix since the proteins within this matrix are generally hydrophobic. This dehydration contributes to cuticle stiffening and de-plasticization. The most commonly referenced mechanism that results in sclerotization is the cross-linking between proteins and phenols that are present in the cuticle. There is certainly some experimental evidence for covalent interactions between proteins and phenols that help stabilize the cuticle, but it is not clear if these interactions directly result in a stiffening of the cuticle. Now, all these interactions between chitin, proteins, phenols and water result in a type of hydrophobic coating of chitin fibrils that increases the mechanical load the proteins can carry. Softening of the cuticle also becomes less of an issue. Just which of these interactions have an hardening effect on the cuticle AND are actually resulting in sclerotization, has yet to be precisely determined

It has been shown that water content of the cuticle has an important role in determining cuticle mechanical properties. At the same time it also clear that sclerotization is not just simply dehydration. In other words, the cuticle minus water becomes hardened, no matter if the cuticle is sclerotized or not. Sclerotization is an irreversible pathway, the bonds cannot be broken. The presence or absence of water in an unsclerotized cuticle means that some fine-tuning of mechanical properties can still occur even after the cuticle has been fully formed.

To me this seems like a feature that we might want to consider using in engineered materials. Fabricating materials using chemistry in water is a different approach, but this is how biological materials are synthesized in nature. To be able to create materials that can adapt to their environment, or are multifunctional, just by varying the degrees of adsorption and absorption of water, could lead to exciting innovations.

Art by Rob Mitchell

Various beetle life stages and species (plus some of their natural enemies) – all with different cuticle characteristics. Art by Rob Mitchell, used with permission.

The insect cuticle: (3) self-assembly

The insect cuticle: (3) self-assembly

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.

Self-assembly: manufacturing the future

The materials and structures that we humans create for ourselves generally involve large factory manufacturing systems where we “Heat, Beat and Treat” materials into the structure we require.

Motor Manufacturing Gardiner

Motor Manufacturing” By Clive Gardiner for the Empire Marketing Board. c1930 National Archives UK

In contrast to most engineered materials, biological materials are assembled from the bottom up, rather than from the top down. The formation of materials like chitin, which is the main component of the insect cuticle, is the result of self-assembly and well ordered biosynthesis.

In engineering the material used is relatively cheap but the shape of the structure is expensive, since it results of very energy-intensive processing (the heating, beating and treating parts).

A Blast Furnace” By Clive Gardiner for the Empire Marketing Board. c1930 National Archives UK

In biology it is actually the other way around. The instructions for manufacturing processes are stored in the genes of all living things. The information contained in an organism’s DNA determines the sequence of molecules that will be expressed by the organism’s cells. It is the characteristics of these molecules that will determine how they interact and ultimately determine the shape of the final material and resulting structure (=self-assembly). In other words, energy is replaced by information, which is cheap. In biology it is the building blocks that make the structure that are relatively expensive to obtain (through capture, consumption, assimilation, etc.) and organisms cannot afford to waste these building blocks.

The biological manufacturing process is constrained by a range of external factors, which then may result in different structures under different circumstances. These external factors are, for instance, temperature (variable on different time-scales), mechanical loading (variable at different life stages and during different seasons) and the presence or absence of water, light and appropriate nutrition (which all three vary over the lifetime of an organism).

“Kreeping It Real” (Graffiti art on boxcar). Picture by Rob Swatski.

A characteristic of biological materials is that they grow relatively slowly. At first this may seem like a disadvantage, but since these materials as they grow are exposed to a range of forces they can adapt to situations that may alter over the course of the structure’s lifetime. Engineers, using traditional manufacturing techniques, do not have this luxury, they have to preempt and anticipate changing conditions – or ignore them resulting in a reduced effective lifetime.

Self-assembly happens at all scales in nature – it starts at the molecular scale at which molecules are formed into globs which are formed into fibers. Manufacturing at the nano- or micro-scale may still be in its infancy but great strides are being made. And in the meantime we can apply nature’s lessons regarding biosynthesis and self-assembly at a larger scale – creating hybrids of multiple materials with the functional variety that is required of the material.

As Carl Hastrich points out on his Bouncing Ideas blog:

[the] future is fibre. Everything in nature is a fibre. From beetle exoskeletons, to the incredible structures found in the botanical world, they are all fibres. The results are complex, malleable, repairable materials that integrate a vast array of functions. Have a look at a cross section of human muscle and you’ll see what I mean. Humans on the other hand build with large brittle mono-materials and achieve complex function by mechanically integrating multiple components.

Over the past few decades we have tried to incorporate more manufacturing techniques that are non-toxic, water-based, and do not require high-energy inputs – life-friendly chemistry. 3-D printing techniques open up such an avenue since it is by definition builds from the bottom-up, rather than top-down, thus already mimicking nature in that regard, and new non-toxic base materials for these printers are being developed.

 The future of manufacturing. (Art based on pictures by M. Alleyne,  Yogendra Joshi and MKZero)

The future of manufacturing biological materials. (Art based on pictures by M. Alleyne, Yogendra Joshi and MKZero)

Further Reading

For this post I looked at a lot of graffiti/street art since I wanted my insect images to have an “industrial” feel. I came across some great art. For examples click here and here and (my favorite) here.

Self-assembly lesson for high-school level students.

The Flea Song

The Flea Song

Today I gave the “entomological interlude” during a talk about Ludwig van Beethoven‘s arrangement of Johann Wolfgang Goethe‘s poem “Flohlied” which is a part of “Faust“. The seminar was part of a Cultual Creativity series through the Musicology Division at the University of Illinois. (NB: I am that person who can not read music, yet can recognize Beethoven’s 9th Symphony as the “Die Hard” theme song.)

Das Flohlied (Flea Song) is part of the scene “Auerbach’s Cellar in Leipzig” in Faust I (First published around 1790). The song is about a king who loved a flea on which he lavished many riches. The flea is fitted with fine clothing and is made Head of State. The flea’s family members are also awarded high positions in the government. The other members of the King’s court did not dare speak up and complain. Instead they tried to cope with the biting and the itching, but what they really wanted to do was kill the little critters.

William Kinderman presented a talk on the political satire conveyed by Beethoven in the composers correspondence and in his work around the time Beethoven wrote the music for the Flea Song. For the performance Prof. Kinderman was joined by tenor Jerold Siena.


I was to provide a little bit of background about the life history of fleas – the entomological interlude. There is much to tell about fleas, but I only had 10 minutes, and was speaking to a non-science audience that was there for the music. But I felt up for the challenge!

I decided to have Goethe’s words dictate the organization of the presentation. Below are my slides, the translation of the poem and my notes.


Thank you very much for inviting me to give a little bit of background about fleas and help you connect this insect with both Goethe and Beethoven. (As an aside: This is an iconic drawing of a flea by Robert Hooke from his 1665 book Micrographia, which can be viewed here on the campus of the University of Illinois at the Rare Books and Manuscripts Library)

Slide02A king there was once reigning,
  Who had a goodly flea,
  Him loved he without feigning,
  As his own son were he!

Lets picture ourselves in late 18th Century Western-Europe. Goethe opens das FlohLied, with a description of a King actually being quite fond of a flea. We may consider that odd, and of course it is, but I think it is important to keep in mind that at that time society viewed ectoparasites such as fleas, lice, ticks and bedbugs more favorably, merely as a nuisance. This view of ectoparasites did not really change until the mid-19th Century when it was discovered that arthropods such as mosquitoes could vector horrible diseases. And not until about 1900 was it known that fleas vectored the bacterium that causes the plague.

In other words, a poem or song about a highly regarded flea is not as strange in 1760 as it may seem now.

Slide03Some basic facts about fleas. Fleas are very small. The body of the flea is about 3mm long. Fleas belong to the insect order Siphonaptera, and there are about 2600 described species of fleas. Adult fleas feed on the blood of their mammalian or avian hosts. Only about a handful of the flea species live in close association with humans, and can use humans as a host but none are specialized on humans.

Fleas do not have wings, they are famous for their ability to jump – they have specialized legs. They have also physiological adaptations that help in dispersal. For instance, fleas are able, through the bloodmeal, to determine when a host rabbit is pregnant. In response female fleas will then start producing eggs. As soon as the baby rabbits are born, the female fleas make their way down to them and once on board they start feeding and laying eggs. After 12 days, the adult fleas make their way back to the mother. They complete this mini-migration every time the mother rabbit gives birth. So they don’t have to be able to fly or even jump very far to be able to disperse their offspring.

Slide04His tailor then he summon’d, 
  The tailor to him goes;
  Now measure me the youngster
  For jerkin and for hose!

In the Flea Song the King calls upon dressmakers to make clothing for the fleas. Funny concept, of course, but not really that odd for that time period.

Around the time that Goethe wrote das FlohLied watchmakers tried to harness fleas, with tiny gold wires, to demonstrate their skills in fine manipulation.

In other parts of the world people also dressed up fleas. In Mexico there is a tradition of Pulgas Vestidas, where fleas are dressed and painted to represent people – such as brides and grooms. They are very very small – and probably only contain the head of the flea.

Slide05In satin and in velvet 
  Behold the younker dressed;
  Bedizen’d o’er with ribbons,
  A cross upon his breast.
Prime minister they made him, 
  He wore a star of state;
  And all his poor relations
  Were courtiers, rich and great.

The late 18th century was the start of the flea circus mania in Europe. It is not clear to me if by 1790 this had reached Germany. But it again shows how enamored people (including the King) were with fleas. This is a picture from the famous flea circus in Copenhagen’s Tivoli Gardens, which was open until the mid 1960s. Fleas would be caught and rigged up in a harness made of thin gold wires. These harnesses could then be attached to props. Fleas were made to pull relatively large objects. Or they were given a ball to juggle or kick. [Video]

Slide06The gentlemen and ladies 
  At court were sore distressed;
  The queen and all her maidens
  Were bitten by the pest,
And yet they dared not scratch them, 
  Or chase the fleas away.

In the poem Goethe mentions that the people at the King’s court are getting bit and that the bites start to get itchy.

This is a very good description of a flea bite. Fleas bite…and they suck!

They have mouthparts, that are basically a pair of sharp lancets with serrated edges and a hard, sharp, awl-like instrument. They make a puncture in the skin, opening up blood vessels, and then suck up the blood by creating a tube with their mouthparts.

The flea’s saliva may cause an allergic reaction that results in welts and itching. It is the itching that usually sends people to the doctor and pets to the vet.

Slide07As Professor Kinderman explained the representation of the flea’s jump is obvious in Beethoven’s music. The flea’s jump is an almost unbelievably fast, precise, and reliable motion.

Fleas can jump about 200 times their own body-length. The jump happens so fast that only about 5 years ago the high-speed camera technology was sophisticated enough to capture the jump in such a way that it could conclusively be shown what parts of the leg a flea uses to jump. [Video]

Slide08The energy for a flea leap comes from a spring inside the flea’s leg. This spring stores and then releases the energy needed to jump. Fleas first lock the joint between body and hind leg, and then they contract muscles within the body. This muscle contraction compresses part of the exoskeleton of the flea, most importantly a part of the body that contains the elastic protein resilin. So in the end, not just the muscle, but also the relatively rigid exoskeleton acts as a tensed spring. The lock on the hind legs is then released, and the rapid expansion of the spring releases the stored energy. The forces in the spring are transmitted through the leg, through the feet, to the ground, which propels the jump.

The resilin material is very interesting since it is very resilient, far more so than rubber we use in engineered devices. The flea can repeat this jump many times without suffering material fatigue.

Slide09This brings me to the all important insect cuticle, or exoskeleton. Insects do not have a skeletal system like we do. Insects are covered with this, at least in my eyes, amazing material, made from pretty simple building blocks, that, depending on the species and life stage of the insect, can be hard (think of a beetle) or soft (think of a caterpillar).

In this picture you can see that the cuticle of the flea is divided up into different segments, and that it may have some sensory hairs and glands – so it is not completely one rigid structure. This makes movement and interaction with the environment easier despite having an exoskeleton.

The cuticle is arranged hierarchically, and built from the bottom up – at atmospheric pressure and moderate ambient temperature – using materials that are readily available in nature. This manufacturing technique is not common in human manufacturing when we use lots of pressure, and heat, and nasty chemicals.

The cuticle is made up of different layers and these layers may not line up perfectly. This is a good thing. Sometimes a crack may appear but not be propagated further down, because the different hierarchical layers “stop” the crack. Again, we do no really engineer our materials with this level of resiliency.

(For more detail about the insect cuticle see posts on this blog here, here and here)

Of course, that fleas have such an amazing exoskeleton also has disadvantages…

Slide10If we are bit, we catch them
  And crack them without delay.

You can’t just squash a flea. Similar to bubble wrap if you push on a flea it compresses, it does not pop.

Since the flea’s cuticle is also made up of these different layers, and the layers are not very stiff it makes it that fleas are very difficult to kill. You cannot just step on them, stomp them, or crush them. You actually have to puncture the cuticle, maybe with your nails, and then bend them until they snap – “KNICKEN” as Goethe called it.

And then if you hold your finger on the flea you can also suffocate it “ERSTICKEN”, but this will not be so easy since fleas have multiple entry points for air (not just the mouth area as in mammals).

(Actually the best way to kill a flee is by rubbing it between your fingers so that the legs fall off…then it will not be able to find a new host and continue feeding

Slide11At the risk of going slightly off topic here I want to point out that much of the research done on fleas during the twentieth century was done by Dame Miriam Rothschild. Yes, she was a member of the famous bankers family. Miriam had become interested in fleas because her father Charles had started an impressive flea collection, and her uncle Walter a Natural History Museum.

Miriam was the one who first described the endocrine regulation of reproduction in fleas, as I described, and was also the driving force behind figuring out how fleas jump – the biomechanics of the jump.

I cannot resist these wonderfully inspiring pictures of Rothschild in her (privately funded) lab with her children. She certainly is an inspiration to female scientists like myself.

Thank you so much for giving me the opportunity to share the wonderful lives of fleas with you.

Slide12FleaSongFlyer (Art work by Nils Cordes)

The insect cuticle: (2) hierarchical structure

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.

Structural hierarchy, and thus strength, is intrinsic

The insect cuticle can have great strength and flexibility, much greater than if it were just made up of stacked sheets of chitin and protein. The cuticle also exhibits energy absorption properties. This is all due to its ingenious structure. It has a complex hierarchical micro-architecture that spans multiple length scales from the nanoscale to the macroscale, which exhibit a remarkable combination of stiffness, ability for crack deflection, low weight and strength. The cuticle’s special self-assembly characteristics, as those of other biological materials, have attracted interest from materials scientists for the development of laminated composite materials and from bioengineers interested in molecular scale self-assembly.

Many biological materials exhibit structural hierarchy, and some have been studied in more detail than insect cuticle: nacre (mother-of-pearl) and bone. There is often hierarchy within hierarchy, at the nano-, micro-, all the way to the meso-level.  Hierarchical structure is primarily due to the outcome of developmental pathways of biological systems that create the material. Having hierarchy within a biological material is intrinsic. The only forces acting during material synthesis are intermolecular, and are thus very weak and work on only a short range.

This can result in very beneficial properties.

  • areas and layers that are softer than the rest can greatly affect the failure properties of a material because these interfaces can stop cracks, or divert them.
  • The hierarchical material’s microstructures can exhibit dramatic increases in compressive strength compared with that of solids of similar density with conventional structure.

Since human-made materials are often made using a top down approach, by forcing structure and shape onto a material, we do not often see structural hierarchies in engineered materials, or only to a limited extend.

Characteristic Figure from "Insects Did It 1st" book by Akre, Paulson and Catts. Illustration by E. Paul Catts.

Characteristic figure from “Insects Did It 1st” book by Akre, Paulson and Catts. This illustration by E. Paul Catts accompanied the chapter on plywood/cuticle. Plywood is a human-made material that is layered – if not quite in a hierarchical manner.

The mechanical properties of biological materials, such as the insect cuticle, may give insights into the design of more advanced engineered composites.

The layers

Meso-level hierarchy

Strictly speaking the insect’s integument is comprised of the epidermal cell layer (epidermis) and the cuticle. The Integument itself is build upon a basement membrane (basal lamina).

The insect integument. Let's call this meso-level hierarchy. The integument is comprised of an epidermal cell layer and cuticle.

The insect integument. Let’s call this meso-level hierarchy. The integument is comprised of an epidermal cell layer and cuticle, all on top of basement membrane. (Drawing by Marianne Alleyne)

The basement membrane is only 0.5 μm thick and separates the epidermal cells from the circulating hemolymph (=insect blood).  The membrane can some proteins through, and nerves and trachea also penetrate it, but when this membrane gets breached alarm bells go off in the insect’s body. Often an immune response is mounted by blood cells in response to the breach and the epidermal cells will help prepare it.

The epidermis is comprised of living cells arranged in a single layer. The cells can be modified into structures such as dermal glands and sensory receptors.

The cuticle is secreted by the epidermal cells and can be incredibly thin (e.g. larval endoparasitoids) or incredibly thick (e.g. adult rhinoceros beetles). The cuticle is non-living. It lines the external surface of the body as well as the lining of the trachea and the anterior and posterior sections of the alimentary canal and even parts of the reproductive system. This means that often even these structures undergo a re-lining during a molt.

Meso/Micro-level hierarchy

Several horizontal divisions of the cuticle are obvious, when using an electron microscope, giving the cuticle at this micro/meso-level also a hierarchical structure. These divisions came about because the sublayers were produced in a certain sequence.

The insect integument. Let's call this meso-level hierarchy. The integument is comprised of an epidermal cell layer and cuticle, all on top of basement membrane. (Drawing by Marianne Alleyne)

The meso/micro-level hierarchy of the insect integument. The cuticle is comprised of the epicuticle and procuticle. The procuticle is in turn subdivided into the endo-, meso- and exocuticle. (Drawing by Marianne Alleyne)

  1. There is a thick inner PROCUTICLE. This is the only layer that contains chitin, it also contains proteins.  As the molting cycle progresses the procuticle becomes horizontally subdivided.
    a)    Exocuticle – Is the first portion of procuticle to become synthesized, and then is pushed outward and becomes the outer layer of the procuticle.  Contains heavily cross-linked (insoluble) proteins and chitin.  This layer cannot be reused and is shed during the molt.
    b)    Endocuticle – This layer is formed just above the epidermis.  Consists of several lamellar layers of protein and chitin.  In soft-bodied insects and in areas of flexibility it is this layer that comprises most of the cuticle.  It is flexible because there is little cross-linking of proteins.  It can also be reabsorbed.
    c)    Mesocuticle – This layer cannot always be identified.  It appears to be a transitional layer.  Proteins are yet untanned (like endocuticle) but impregnated with lipids and proteins (like exocuticle), may also contain chitin.
  2. Thin outer EPICUTICLE consists of several layers, which are produced by the epidermal cells and dermal glands.  This layer does not contain chitin.  Since it is only 1 to 4 um thick it has been incredibly difficult to study. It is lipids in the wax layer on top of the epicuticle that play protective and communicative roles in the life of insects.

Nano/Micro-level hierarchy

The cuticle is a composite consisting of chitin fibers within proteinaceous matrix, all arranged in a layered structure. Several horizontal divisions can be observed. Each layer (or lamina) consists of a layer of parallel chitin chains but the laminae are stacked in such a way that the chitin molecules are arranged in an antiparallel manner.

So what does this mean? Lets see if we can build a cuticle graphically from the nano- to micro-scale.

  • Chitin is one of the two major components of the procuticle, it can make up almost half of the total dry weight of the cuticle – but it probably most likely to be less than 20% for some of the most familiar insects, such as caterpillars. Chitin is a very stable molecule: insoluble in water, dilute acids, concentrated alkali, alcohol, and organic solvents. These features make the molecule a great biological building block, but very difficult to study. Proteins are the other major component of the insect cuticle. It is the interaction between proteins and chitin that provides the mechanical function of the cuticle – giving it strength. (In the future I will come back to interesting proteins, such as resilin.)
  • Chitin is very similar to cellulose. It is an acetylated polysaccharide. A ribbon-like chitin chain is created when N-acetyl-D-glucosamines and glucosamines link together.
A portion of a chitin chain. In most insect cuticles these chains are arranged in an anti-parallel manner.

A portion of a chitin chain. In most insect cuticles these chains are arranged in an anti-parallel manner.

  • Adjacent chains of chitin are linked together through hydrogen bonds. Most commonly the orientation of the chitin chains relative to each other is anti-parallel. This arrangement (already at this nano-level hierarchical scale) allows for tighter packaging and contributes to the strength and stability of the cuticle. The grouping of 18-25 chitin molecule chains then creates chitin microfibrils that are about 3nm thick. These microfibrils are wrapped in protein.

Nanofibrils are created when groups of chitin molecules link by way of hydrogen bonds and are wrapped in protein.

  • At the next hierarchical level the nanofibrils cluster together into long chitin-protein fibers.
Nanofibrils clustered together to form chitin-protein fibers.

Nanofibrils cluster together to form chitin-protein fibers.

  • The microfibrils are laid down in an almost parallel pattern within a single layer. This creates a network or matrix of chitin and proteins, and other components such as minerals. The chitin microfibrils are covalently linked to the proteins.
Chitin-protein fibers are woven into a matrix.

Chitin-protein fibers are woven into a matrix.

  • These layers are stacked on top each other in successive layers that are rotated by a constant angle to produce a helicoidal arrangement that further contributes to the strength of the overall cuticle.
The helicoidal stacking of chitin-protein layers (matrix) creates a twisted arrangement. This creates a arching pattern in the cuticle as a whole. (Pictures by Marianne Alleyne)

The helicoidal stacking of chitin-protein layers (matrix) creates a twisted arrangement. This creates an arching pattern in the cuticle as a whole. (Here layers were created by stacking sushi mats in a twisted manner. Pictures by Marianne Alleyne)

It is the hierarchical structure created at the nano-scale, when chitin molecules link together and form chains in close association with proteins, all the way up to the meso-scale, where different cuticular layers are stacked in non-random orientations, that makes the insect cuticle such an inspiring biological material. It is because of this hierarchical structure that the insect cuticle can take on all the different functions that it has. And ultimately the cuticle is one of the main reasons why insects can be found to be thriving in so many different habitats – air, water, land, even inside horse guts. Modern analysis of small-scale composites and self-assembly-type manufacturing techniques will enable us to make materials equally as versatile as the insect cuticle.

Further reading:

Design and mechanical properties of insect cuticle. J. F. V. Vincent and U. G. K. Wegst (2004) Arthropod Structure and Development V33 pp-187-199. DOI: 10.1016/j.asd.2004.05.006


Side note about those sushi mats:

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


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 (Drawing by Marianne Alleyne)

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


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.


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)