Silk is one of the earliest natural fibres discovered by man. The cocoon silk of domesticated silkworm, Bombyx mori (Lepidoptera: Bombycidae) is prized over millennia for textiles, and most of our understanding about silk production is from this species. However, silk is known to occur in many arthropod classes, and a few molluscs and fishes also. Among the class Insecta, 16 out of 30 orders produce silk for a variety of purposes, which include reproduction, shelter, protection from predators, etc. Insect silk possesses extraordinary mechanical properties in terms of strength, extensibility and stiffness. The obvious example of the use for silk is cloth, which also takes up the highest proportion of silk consumption. The versatility and sustainability of silk based materials attract its use in food packaging, medicine, automobile industry, dietary and cosmetic supplements, optics, art, craft, etc. Though the term ‘silk’ encompasses a wide range of distinct materials, it is remarkable that certain features are common among silk production systems in insects. Today, insect silk has taken on new importance to society beyond fabric. Mechanically enhanced silk is expected to open up possibilities for numerous novel applications.

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Silk
Cocoon silk , silk worm Dragline silk , spider

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Insect silk: one name,
many materials
ATHIRA G. MENON
Ph. D Scholar
Department of Agricultural Entomology

3.  Silk - an introduction
 Serigenous animals
 Insect silk and its significance
 Silk glands and classification
 Chemical composition and molecular structure
 Mechanical properties
 Applications
Content
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4. “Silks are fibrous proteins containing highly repetitive
sequences of amino acids and are stored in the animal as liquid, and
configure into fibers when sheared or ‘spun’ at secretion”
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(Craig, 1997)
Silk
Silk yarn

5. • Proteinaceous secretion - specialized ectodermal cells - silk gland
• Stored as hydrated jelly within cells or in multicellular cavities
• Secretions polymerize into solid fibres in the external environment
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Tent caterpillar Cocoon
…Silk

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1. Arthropoda
a. Arachnida
Spider mite PseudoscorpionSpider
Catching prey, nest making Protection from predators,
dispersal
Moult into silk chambers
(Craig, 1997)
Serigenous animals

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b. Diplopoda
Peramphithoe femorata ChrysalisPrimitive millipede
(Craig, 1997)
c. Crustacea d. Insecta
...Serigenous animals
Weaving webs,
Protecting eggs
Making nest Building shelter, cocoons,
protective structures

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Pinna nobilis - sea silk Carp - Cyprinus carpio
To attach to the sea bed
3. Pisces
To attach eggs to substratum
2. Mollusca
(Craig, 1997)
...Serigenous animals

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• 16 / 30 insect orders produce silk
Distribution of silk production in the orders of Hexapoda
Lepidoptera
Insect silk
(Sehnal and Sutherland, 2008)
• Diverse material properties and
chemical constituents
• Crucial role in survival and
reproduction

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1. Pupal protection
Neurotoma edwardiHyalophora cecropia Mallada signata
Lacewing - Neuroptera
(Rudall, 1962)
Sawfly - Hymenoptera
(Warwicker, 1960)
Cecropia moth - Lepidoptera
Insect silk - significance
(Weisman et al., 2009)

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• Adult males
• Indirect sperm transfer
Lepisma saccharina
(Sutherland et al., 2007)
Empis snoddyi
• Adult males
• Silk-wrapped nuptial gift
Dance fly - Diptera
2. Reproduction
Silverfish - Thysanoptera
(Walker, 2013)
…Insect silk - significance

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4. Dispersal
• “Moulting pad”
• Larva can hang onto leaf
Tiger swallowtail - Lepidoptera
3. Moulting
Leaf roller caterpillar - Lepidoptera
• “Ballooning”
• Also an escaping mechanism
(Warwicker, 1960)
Parotis marginataPapilio glaucus
…Insect silk - significance

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(Warwicker, 1960)
Prey catching webs
Glow worm - Diptera
Sticky silk strands
• Collect food that comes through
water current
• Beads - droplets of glue
• Attract small insects at night
5. Catching prey
Caddisfly - Trichoptera
(Rudall, 1962)
…Insect silk - significance

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Raspy cricket - Orthoptera
Silk chambers
• All stages
• Tunnels
(Walker et al., 2013)
• All stages
• Building nests
Nest
6. To hide/live in
Webspinners - Embioptera
(Okada et al., 2008)
…Insect silk - significance

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Objective: To investigate the function of silk structures
Silk production in Hemiptera

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Materials and methods:
• Centre for Agricultural Innovation - Western Sydney University
Eucalyptus robusta Kahaono montana Mallada signata
• Infected leaves collected from field
• M. signata larvae were purchased from commercial supplier
(Gurr and Fletcher, 2011)
…Silk production in Hemiptera

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1. Silk (+), predator (+)
2. Silk (+), predator (-)
Treatments:
Observations:
 Live K. montana nymphs after 24h; nymphs within structures
Silk produced by K. montana Nymphs beneath the refuge constructed
3. Silk (-), predator (+)
4. Silk (-), predator (-)
…Silk production in Hemiptera
(Gurr and Fletcher, 2011)

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a. Proportional survival of K. montana
nymphs after 24 h
b. Proportion of K. montana nymphs
inside refuges after 24 h
Conclusion: Silk produced by K. montana - protection from predators
…Silk production in Hemiptera
(Gurr and Fletcher, 2011)

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 Ectodermal organ
 Found in variety of locations
 Secrete different substances
 Simple and uniform morphology
Three components,
A) Secretory cells
B) Internal reservoir
C) External opening
Silk glands
(Julien et al., 2005)
Structure of silk gland

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 Based on location
1. Labial gland - accounts for most silks
2. Malpighian silk gland - least common
3. Dermal gland - less numerous
(Sehnal and Akai, 1990)
Classification of silk glands
Labial gland Dermal glandMalpighian silk gland

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Semi-schematic drawing of
salivary gland
…Contd.
• Salivary gland
• Tubular with epithelium of secretory cells
• Bilateral
• Terminate in specialized structure - spinneret
• Predominant - Lepidoptera
Booklice Glow wormsCricket
Labial gland
(Aoyagi, 1994)

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Silk gland of
lacewing larva
(Aoyagi, 1994)
• Larval Neuroptera, Coleoptera & Ephemeroptera
• Derived from excretory cells lining lumen
• Silk production - after larva terminates feeding
• Concentrated silk proteins - collect at rectum
• Spun through the anus
Malpighian silk gland
Antlion Mayfly Thrips
…Contd.

23. • Made up of few cells - as isolated units or clustered
• One large secretory cell
• Extracellular lumen to collect secretion
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Epidermal silk glands in the
tarsus of a dance fly
(Young and Merritt, 2003)
Dermal gland
LacewingWater beetle
 Epidermal glands - male dance fly, male webspinner
 Internalized glands - sex accessory glands
…Contd.

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• Epidermal glands - situated in different locations
 Sternum of female sphecid wasp - silks used for nest lining
Psenulus sp.
 Basal tarsomere of prothoracic leg - male dance fly, male webspinner
Hilara sp.
(Young and Merritt, 2003)
Sternum
Basitarsus
…Dermal gland

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• Silk composed of AA linked together in a long chain
• Glycine
• Alanine
• Serine
• Intermediate hydrophobicity
 Interact with water - contribute to protein solubility
 Interactions are weak - allow AA for crystallizing
Amino acid composition
83.4 % (non-essential)
Chemical composition and molecular structure
(Sutherland et al., 2010)

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• Semi-crystalline material
• Hydrogen bond - mechanical strength & stability
• Five different structures have been identified
• Each crystalline structures has specific amino acid
sequences
Silk molecular structures
Crystallites embedded
in amorphous matrix
Molecular structure of silk
(Rudall and Kenchington , 1971)
…Chemical composition and molecular structure

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• Two or more α-helix coil to form coiled coil structure
• Seven AA residue protein sequence - HEPTADS
• The first and fourth residue - hydrophobic
• Proline and glycine absent
1. Coiled-coil structure
…Molecular structure of silk
Apoidea Vespoidea
(Sutherland et al., 2010)

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• Multiple β strands - hydrogen bond
• Alternate AA residues orient towards opposite faces
of β-sheet
• Asparagine and glycine are prominent
2. Extended beta-strand structure
…Molecular structure of silk
EmbiopteraLepidoptera Neuroptera
(Sutherland et al., 2010)

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• Similar to extended β sheet but orientation of strand differs
• 16 AA residue motif - beta sheet `ribbon’
• Glycine present at regular intervals
3. Cross-beta structure
…Molecular structure of silk
Chrysopidae Keroplatidae
(Sutherland et al., 2010)

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• Right handed triple helix formed from 3 left handed
helices
• Three AA residues - glycine present at every 3rd residue
• Core cannot accommodate amino acid side chains larger
than hydrogen
…Molecular structure of silk
4. Collagen structure
Tenthredinidae
(Sutherland et al., 2010)

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• Mirror images of collagen helices
• Protein chain form a right handed helix
• Crystalline region contains only glycine
…Molecular structure of silk
5. Polyglycine-II
Tenthredinidae
(Sutherland et al., 2010)

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Architecture and composition of hilarine silk
Dance fly MatingSilk wrapped nuptial giftMating swarm
Hilarine fly (Diptera: Empididae: Hilarini)

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• Analysis of silk gland protein
 SDS polyacrylamide gel electrophoresis (SDS - PAGE)
 Liquid chromatography-mass spectrometry (LCMS)
• cDNA library construction and analysis
mRNa isolated from basitarsus of foreleg
• Tissue specific expression of silk gene
Total RNA extracted amplified using PCR
Hilara sp.
Foreleg
…Architecture and composition of hilarine silk
(Sutherland et al., 2007)

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SDS PAGE PCR of libraries
1. cDNA of foreleg basitarsi of male
2. cDNA of male without forelegs
3. DNA markers
• Large protein (over 200 kDa) - matched the protein translation of DFibroin
• Expression of DFibroin - specific to basitarsus
…Architecture and composition of hilarine silk
(Sutherland et al., 2007)

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Codon usage patterns of DFibroin
…Architecture and composition of hilarine silk
(Sutherland et al., 2007)
• Composition of silk protein - asparagine (21.9 %) and glycine (13.9 %)

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…Architecture and composition of hilarine silk
Architecture of protein
• β-sheet structured silk
(Sutherland et al., 2007)
• Primary sequence - highly
repetitive
• Low levels of essential
amino acids

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• Silk - viscoelastic solid
• Strength - hydrogen bonds between and within the proteins
• Amorphous fraction - flexibility and toughness
• Crystalline fraction - strength
• Depends on molecular structure
 Cross-β sheet - highest flexibility
 Coiled-coil structure - extensible
Pulling the silk fibre
Mechanical properties of silk
(Walker, 2013)

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Silk stress-strain plots
Silkworm (blue)
Honeybee (red)
(Walker, 2013)
…Mechanical properties of silk

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Fibre
Strength
(MPa)
Extensibility
(%)
Stiffness
(GPa)
Toughness
(MJm-3)
Reference
Lepidoptera 750 22 7 83 Hepburn et al. (1979)
Hymenoptera 400 201 0.38 402 Hepburn et al. (1979)
Trichoptera 221 116 0.3 232 Brown et al. (2004)
Hemiptera 301 7 4.4 11 Chang et al. (2005)
Embioptera 158 14 5.9 14 Collin et al. (2009)
Neuroptera 310 381 0.5 591 Weisman et al. (2009)
Nylon 950 18 5 80 Gosline et al. (1999)
Steel 1500 0.0008 200 6 Gosline et al. (1999)
Mechanical properties of insect silks and some other materials

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Textiles
Domesticated silk Vanya silk
Applications of insect silk
Mulberry silk Eri silk Tasar silk Muga silk
• Export value of silk goods during 2019 - 20 = ₹1498 crores (CSB, 2020)

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Cricula trifenestrata - potential for vanya silk industry in India
(Ragesh et al., 2012)
Larva Cocoon
Spinning Fabric
…Applications of insect silk
Cleaned cocoon
Adult

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…Applications of insect silk
(Marelli et al., 2016)
Edible coating for perishable food preservation
Food packaging
Disposable silk cups
(Omenetto and Kaplan, 2010)

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Dietary supplements
…Applications of insect silk
• For cardiac and diabetic patients
• Astronaut food
(Huang et al., 2018)
Cosmetics
• Tissue regeneration - burn victims
• UV absorption

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Micro needles
Silk sutures
…Applications of insect silk
Biomedical uses
Vein/ bone replacement Drug cards
Implants (Huang et al., 2018)

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…Applications of insect silk
Gears, nuts and bolts Adhesives
Automobile industryOptics
Optical fibre
Flexible optical films
(Huang et al., 2018)

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Art, craft
Necklace DIY craft
Cocoon art
…Applications of insect silk
House building
PP double twisted silk
(Huang et al., 2018)

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• Insect silk - to protect pupae, reproduction, easy moulting,
dispersal, prey catching, to live/hide
Summary
• Silk producing glands - labial gland, Malpighian silk gland and
dermal gland
• What is silk? - fibrous protein
• Amino acid composition - glycine, serine and alanine
• Serigenous animals - Arthropoda, Mollusca, fish

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• Mechanical properties - high strength and extensibility
…Summary
• Molecular structure - coiled-coil, extended β-sheet, cross-β
sheet, collagen structure, polyglycine II
• Applications - fabric, food packaging, diet supplements,
cosmetics, building materials, automobile industry,
biomedical field, art, craft, etc.

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Though the term ‘silk’ encompasses a wide range of distinct
materials, it is remarkable that certain features are common among silk
production systems in insects. Today, insect silk has taken on new
importance to society beyond fabric. Mechanically enhanced silk is
expected to open up possibilities for numerous novel applications.
Conclusion

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Thank you