Spiders, Slugs & Scorpions, Oh My!
CLASS READINGS STUDY: A Few Common Spiders of Bouverie Preserve Trail Card Common Spiders of Bouverie Preserve
Turret Spider Natural History
California Turret Spider (Pacific Discovery, Leonard Vincent, 1997) 5 Cool Things About Banana Slugs & Earthworms (Gwen Heistand) Love Darts (Carl Zimmer) Scorpions (Jeanne Wirka) Fallen Log Decomposition & Soil Food Web Soil Macro & Microfauna Female false tarantula disturbed In Praise of Spider Silk (Mae Won Ho) near her burrow.
Key Concepts
By the end of class, we hope you will be able to Practice spider observation techniques in the field Make spiders, slugs, scorpions, spiders, snails, (misting, mirrors, tuning forks), ticks and earthworms – oh my! – fascinating to 3rd and 4th graders, Know where to find scorpions at Bouverie and share a few fun facts while reassuring students Understand that some animals have skeletons on that the sting is painful but NOT deadly, the inside and some have skeletons on the outside; some have no skeletons and some use Get students thinking about banana slugs and water to support their body structure (hydrostatic earthworms with some cool facts to encourage skeletons), questions,
Have a good idea where to locate and how to Know what to expect if you kiss a banana slug identify different types of spiders in the field [optional!], including turret spiders on the Canyon Trail, Know what happens if you run an earthworm Become familiar with cool information that you through your lips [also optional], and can share about any spider you find … silk, web construction, prey capture, Know what to do about tick bites.
Resources
In the Bouverie Library Elizabeth Shepard, No Bones (1988). Includes a dichotomous key to common invertebrates, including insects, spiders, ticks, scorpions, pill bugs, etc. Herbert & Lorna Levi & Nicholas Strekalovsky, A Golden Guide: Spiders and
Their Kin (2001 ). This is one of the Golden Guides that is helpful . . . it’s small and includes many of the spiders we have here. Rainer F. Foelix, The Biology of Spiders (1982). Harvard University Press. This is an all round good book on spider biology – it can get technical and it’s a fascinating read.
Alice Bryant Harper, The Banana Slug (1988) Great pictures and good
information.
Paul Hillyard, The Private Lives of Spiders (2007). Large picture book with some good info and great photos. Rod Preston‐Mafham, The Book of Spiders and Scorpions (1991). Another large book with great pictures and good descriptions of mating, spider biology, ballooning, and more.
Roger Drummond, Ticks and What You Can Do About Them (1998). A good
little book that talks about different ticks, what they carry, and what you can do about them.
Online Spider Myths – Myths, Misconceptions, and Superstitions about Spiders. A fun website maintained by the Burke Museum of Natural History and Culture at the University of Washington: http://www.washington.edu/burkemuseum/spidermyth World Spider Catalogue maintained by the American Museum of Natural History at http://www.wsc.nmbe.ch Xerces Society for Invertebrate Conservation: http://www.xerces.org
SEQUOIA CLUB
Trail Tip
Share some new ways to look at spiders, to help students find them more cool than scary. Some cool facts: * Most spiders have venom, not poison. Venoms work by being injected, unlike “poisons”, which are substances that are harmful when eaten, breathed, or touched. * The purpose of spider venom is to subdue the spider’s prey, almost always insects. In brief, it’s an insecticide. Spider venom did not evolve to harm creatures like us humans, who are too big for spiders to eat. In nearly all cases, spider venom has little if any effect on humans. But use a bug box to [gently] capture and examine them! * Although usually associated with webs, not all spiders make a silk web to capture prey. BP resident crab spiders and wolf spiders do not weave webs. * But all spiders can produce silk in one form or another. And most have several glands producing different kinds of silk. Even those that do not weave webs use their silk for other purposes, such as wrapping prey, lining burrows, making egg cases, ballooning for dispersal, building shelters, & more. * Finally, look carefully at orb webs. Their planes are usually not perfectly vertical. Note, too, that the spider usually hangs out on the side facing downward. When threatened or disturbed, it can drop straight down by “bungee jumping” on a strand of silk!
A Few Common Spiders at Bouverie Preserve Mygalomoprh (“primitive”) Spiders Orb Weavers
Turret Spider (Atypoides sp.) •Turret of stiff silk (right) •Added to each year •Up to 8” long •Can go 6 months without f food! •Can molt to get smaller!
egg case False Tarantula Missing Triangle (Calisoga longitarsus) Spider hangs head down in Look for silk-lined (Zygiella sp.) center of web’s burrows in middle of stabilimentum trails; almost perfect Yellow & Black Garden Spider circles (right); after first (Argiope aurantia) rains evidence of “spring cleaning.” Common on Josephine’s Trail.
Cribellate Spiders (produce fine hackle silk that “snaps”) Debris Spider (Cyclosa conica) -line of debris down center of Labyrinth (Condo) Spider web; old prey items, plant parts (Metepiera sp.) -spider is concealed in debris -orb web with messy portion -she hides her eggs there too -retreat is in messy portion -constructs lens-shaped False Widow egg sacs in retreat area Badumna sp. (Steatoda grossa) Cellar Spider These 4 spiders can be found around and on -inside buildings buildings, in electrical and fire hose cabinets and cabinets Triangle Spider (Uloborus sp.) -touch web and •Spider is part of web! watch it vibrate! •When she senses prey, she releases tension thread and triangle collapses Sheet Web Weavers (look for these spiders in the Filmy DomeSpider Black Widow House Spider redwoods under the heronry) (Prolinyphia marginita) (Latrodectus hesperus) (Achaearanea tepoidariorum) -often in large groups -found near creeks Hunting Spiders (no webs) -spider sits under apex of dome
Trail Card originally Bowl and Doily Spider prepared by Gwen (Frontinella pyramiitela) Heistand . Modified Crab Spider (w/ prey) -spider hangs out on for Bouverie by Jeanne Wolf Spider (w/ egg sac) Family Thomisidae) Jumping Spider underside of “bowl” Wirka 02/2010 (Family Lycosidae) Camouflaged sit and (Family Salticiade) -pulls prey through top web to wrap Runs to catch prey wait predator Jumps at prey from above -look on Loop Trail near Redwood Circle ACR Natural History Notes: Turret Spiders Page 1
Turret Spider (Atypoides riversi)
The Turret Spider is native to Northern California and is part of the larger family of "Folding Trapdoor Spiders" that live in self-made burrows dug into the ground. Folding Trapdoor Spiders like the ones we have elsewhere on the preserve, use the door at the entrance to their burrow to make surprise attacks. Turret Spiders do not construct doorways to their burrows and will leave their burrows open all day long (but can pull them shut when it rains).
Turret Spiders are found outdoors and mostly limited to wooded areas that include pine and Douglas fir forests. Some make their homes near banks of moving water sources. At MGP we have some that are at the edge of mixed evergreen and grasslands that use grasses in the turrets as opposed to needles.
Turret spiders have poor eyesight. They capture their prey (millipedes, ants, beetles, and other invertebrates) by sensing vibrations. When vibrations are perceived, the spider lunges from its burrow, gripping the turret lip with its last pair of legs. The spider injects the prey with venom and drags it into its burrow.
Q: What’s the deal with the turrets?
A: Turrets are stiffened silk connected to a long silk-lined burrow in which the spider lives. Douglas fir needles, dirt, leaves, and twigs are incorporated into the turret and act as trip-lines. At dusk, the spider sits at the edge awaiting victims.
Q: How do Turret Spiders make their burrows?
A: Turret Spiders have spines (rastella) on the sides of their chelicerae (the mouthparts where you find their fangs). The spider uses these spines like a digging rake to loosen and move earth. She also uses her fangs and the muscles at the base of her fangs. Once there is enough loose dirt in the burrow for removal, the spider will spin some silk around it and haul it out (another amazing use of spider silk!).
Version: January 16, 2014 Prepared by Gwen Heistand for ACR Education ACR Natural History Notes: Turret Spiders Page 2
Q: How often do these spiders need to eat?
A: As often as they can … AND they can wait six months without eating. They may even shed their skin to get smaller if need be! Wouldn’t it be great if we could molt down a couple of sizes!!!
Q: If turret spiders live in burrows, how do they find each other when they want to mate?
A: A female turret spider may spend her entire life in the same burrow, enlarging it as she grows. When a male is of reproductive age (8-9 years old), he leaves his burrow in search of a gal. He is guided by a pheromone (a special chemical attractant) that the female releases and by the distinguishing pattern of silk around her burrow. Once he has mated, he is not long for the world.
Q: Where does the female make her egg sac and what happens to the kids?
A: The eggs are laid in the burrow, in a cocoon attached to the silk lining. Hatchlings look like miniature versions of their mother and stay with her for several weeks. (picture: female in burrow with spiderlings)
Q: When you find a largeish turret, you very often see a bunch of smaller ones around it. What’s this about?
A: When you find a turret, look around. You will often see other turrets in a range of sizes. Turret spiders have been shown to stay in their burrows as they age and just enlarge them. So the larger burrows are the older spiders. As with the trapdoor spiders, very often the offspring don’t stray too far from home. Some pretty cool population studies have been done with turret spiders. These studies have estimated that the older female turret spiders are 16years old or older. Turret spiders in the laboratory have lived as long as 26 years.
Version: January 16, 2014 Prepared by Gwen Heistand for ACR Education
ACR Natural History Notes: Turret Spiders Page 3
Q: What do these spiders look like?
A: They are mygalomorph (?!) spiders that look a little like tarantulas. They have thick bodies and small eyes. (Mygalomorph spiders are considered a more primitive group. The other grouping of spiders is Araneomorph.)
The male is sized smaller (13mm to 16mm) and the female is larger (16mm to 18mm). Identifying colors of Turret Spiders have been seen as varying depending on part of the body, with green and brown to purple brown. The legs might sometimes take a darker brown appearance. Males are discernibly different in that their abdomens might feature up to 3 plate-looking coverings whereas the female has been seen with just a single plate covering the abdomen. (The one pictured is a male from Garden Club Canyon.)
Q: These spiders seem really different from the ones that make orb webs … Are they?
A: Turret Spiders (and False Tarantulas, real Tarantulas, and Trapdoor Spiders) are part of a major grouping called mygalomorph spiders. These spiders are considered to be more primitive. Their fangs only move up and down instead of side-to-side. So … they need to rear up and come down on their prey. This would be pretty hard to do on an orb web. Fangs of araneomorph spiders (orb weavers, sheetweb, wolf, jumping, etc.) are able to move side to side, so it’s possible for them to “bite” their victims from just about any position.
Version: January 16, 2014 Prepared by Gwen Heistand for ACR Education
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PACIFICDISCOVERY 37 Wl LD L M S (coiltittued_littttrpale 37) FU LLERTON COIIEGE spider'snorv shriveled body. Within rhe silkenchamber of its erstwhilehost. rhe Lconord S. Vinccnl, Ph.D. Entomology- Arachnology larva molts again. tbrms a pupa. and Divisionof NaturalSciences about ten davs lrter emergestiom the t/zr burrorvas an adrrltt1y. 714-992-7112 E The turret spider is Foundrvidely throughout Calitbrni:r'stbothills and Q'ya' mountains.Werrdell Icenogle o[ Win- O.eo, ,4t. chester.Calitbrnia. knorvn as rhe Dern of Z //*, t?r.u q' drof , 7 ,/^ ft 4t , * Mygalomorphs(tarantrrlas and rheir rel- 21. t'' Z^,t t You' 6 - - atives).has tbund Atypoidesrilersi in the t ,{ 4*k CoastRanqes. tiorn the MonrerevPenin- t*./r.uv +?,lh- rl"'t sula to near rhe Oregon border.In rhe ' u4T::1., hca'/aro 1'6nae prye s i,r*, fr""( G7 At 7 Sierrarhev are tbund ar elevationsas high as2.-100 merers. Icenoele has rlso tbund fr ."A1 t r'*? a^'4 thenrin the SurterButres. possiblv a rem- z zq" t" l";/ '7' aLac/"'/ nantofa pastlink belu'eenthe coasral and Aq'n
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(1, llou do +ha1deposir- th<- e't lR lhlnl 7 A: TnUa rno$e bat\qL0*r"s ovtr le 5Latnclt ^1 +Wi,r lonl Spinerefie| I ',U,'th )., Q, n0 \id ,wVturlwppen tl a btqratnctrop fcttt5 in by A, 1L^c,41.Ln p;tl Ivtei,,entranL( ctoseiL fuir, lLar ic+s th'h db5orbe,t' udt" t+t"b'!rt hlb&t 4'|'ur74r6z'e'tbx'' 1,'i)fi*)iZ) ffi"h'*' b 5/'@'/Li'a /*za'oa gah #2: They have a built-in continually growing radula to rasp plant material (thousands of sharp backward pointing teeth). #3: They move on a trail of slime … imagine if your whole body could produce mucus! They also use mucous to discourage predators. snail radula rp #4: They move by making wave-like motions with the muscles of their foot #5: A single critter is both male and female (hermaphroditic) … and … “apophallation” Most Pictures from: The Banana Slug by Alice Bryant Harper, photographs by Daniel Harper 6 Cool Things About Earthworms (Gwen Heistand 10/05) 1. They have 5 hearts. 2. They eat dirt (soil) (Aristotle called them intestines of the soil). 3. In just one acre there can be more than one million earthworms, eating 10 tons of leaves, stems, and dead roots a year and turning over 40 tons of soil. 4. An earthworm has no lungs or gills. It breathes through its thin skin, which is in contact with the air between the particles of soil. What happens when it rains? 6: What is that lump in the middle of them? The clitellum. What is it used for? Sex … it eventually 5. They move through the soil produces the using their hydrostatic cocoon for the skeleton and setae which they eggs. use as anchors. (Take an earthworm and run it gently through your lips!) From Invertebrates by Brusca & Brusca Love Darts in the Backyard Posted by Carl Zimmer Spring is finally slinking into the northeast, and the backyard wildlife here is shaking off the winter torpor. Our oldest daughter, Charlotte, is now old enough to be curious about this biological exuberance. She likes to tell stories about little subterranean families of earthworm mommies and grub daddies, cram grapes in her cheeks in imitation of the chipmunks, and ask again and again about where the birds spend Christmas. This is, of course, hog heaven for a geeky science-writer father like myself, but there is one subject that I hope she doesn't ask me about: how the garden snails have babies. Because then I would have to explain about the love darts. Garden snails, and many other related species of snails, are hermaphrodites, equipped both with a penis that can deliver sperm to other males and with eggs that can be fertilized by the sperm of others. Two hermaphroditic snails can fertilize each other, or just play the role of male or female. Snail mating is a slow, languorous process, but it also involves some heavy weaponry. Before delivering their sperm, many species (including garden snails) fire nasty-looking darts made of calcium carbonate into the flesh of their mate. In the 1970s, scientists sugested that this was a gift to help the recipient raise its fertilized eggs. But it turns out that snails don't incorporate the calcium in the dart into their bodies. Instead, love darts turn out to deliver hormones that manipulate a snail's reproductive organs. Evolutionary biologists have hypothesized that this love dart evolved due to a sexual arms race. When a snail receives some sperm, it can gain some evolutionary advantage if it can choose whether to use it or not. By choosing the best sperm, a snail can produce the best offspring. But it might be in the evolutionary interest of sperm-delivering snails to rob their mates of their ability to choose. And love darts appear to do just that. Their hormones prevent a snail from destroying sperm with digestive enzymes, so that firing a love dart leads to more eggs being fertilized. Recently Joris Koene of Vrije University in the Netherlands Hinrich Schulenberg of Tuebingen University in Germany set out to see how this evolutionary arms race has played out over millions of years. They analyzed DNA from 51 different snail species that produce love darts, which allowed them to work out how the snails are related to one another. They then compared the darts produced by each species, along with other aspects of their reproduction, such as how fast the sperm could swim and the shape of the pocket that receives the sperm. Koene and Schulenberg found that love darts are indeed part of a grand sexual arms race. Love darts have evolved many times, initially as simple cones but then turning into elaborate harpoons in some lineages. (The picture at the end of this post shows eight love darts, in side view and cross section.) In the same species in which these ornate weapons have evolved, snails have also evolved more powerful tactics for delivering their sperm, including increasingly complex glands where the darts and hormones are produced. These aggressive tactics have evolved, it seems, in response to the Love Darts – Carl Zimmer Page 2 evolution of female choice. Species with elaborate love darts also have spermatophore-receiving organs that have long, maze-like tunnels through which the sperm have to travel. By forcing sperm to travel further, the snails can cut down the increased survival of the sperm thanks to the dart- delivered hormones. Sexual conflict has been proposed as a driving force in the evolution of many species, and this new research (which is published online at BMC Evolutionary Biology) supports the idea that hermaphrodites are not immune to it. What's particularly cool about the paper is that all these attacks and counter-attacks co-vary. That is, species with more blades on their love darts tend to have longer reproductive tracts and more elaborate hormone-producing glands and so on. Only by comparing dozens of species were they able to find this sort of a relationship. My wife always tells me that as a science writer, I ought to be well-prepared to give our children the talk about the birds and the bees. But I'm not sure the love darts would send quite the right message. Scorpions! Scorpions are among the fascinating “micro-fauna” of the Bouverie Preserve. They are nocturnal, but hikers regularly find them under rocks and logs along the trails during the day. At night, they “glow” under UV light. The kind of scorpion we have at Bouverie is called the California Forest Scorpion (Uroctonus mordax). How do you ID a scorpion? Most kids know a scorpion when they see one. They are easily recognized by their characteristic pincers and segmented tail and stinger. The tail ends in a small bulb-like organ called a telson, which contains venom. The California forest scorpion is about 1-2 inches long with a dark reddish-brown body and lighter brown legs. Are scorpions insects? No. Scorpions are arachnids, the class of arthropods that also includes spiders, ticks, mites, harvestmen and a few others. Insects are also arthropods (which means “jointed appendages”) but they are in their own class. A “touchy” subject: how do scorpions feel? . Like spiders, scorpions have 8 legs and more than 2 California Forest Scorpion eyes. The number of eyes on a scorpion varies by species, but they generally have two on top of their (Uroctonus mordax) . cephalothorax (the front part of their body) and then 2 to 5 pair of lateral eyes off to the sides. Even with that Kingdom Animalia many eyes, they can’t see very well. Instead, they come Phylum Arthropoda equipped with a set of unique comb-like structures Class Arachnida called pectines. Located on the underside of the Order Scorpiones scorpion, the pectines Family Vaejovidae (photo left) along with hair-like projections on Genus Uroctonus the body and pedipalps Species mordax (the front appendages with the pincers), provide enough sensory Prepared by Jeanne Wirka for ACR Education. information to allow the Photos by Mike Witkowski, Richard Wasson, and Jeanne Wirka. Diagram from Enchanted scorpion to navigate and Learning.com. Used for educational purposes only. hunt by touch. How do scorpions catch and eat their prey? Scorpions are nocturnal hunters. They eat insects, spiders, centipedes, and other scorpions which they capture, literally, by feeling around in the dark. If necessary, the scorpion will paralyze its prey by injecting it with venom from its telson. Interestingly, scorpions don’t always use their venom. If they can overpower their prey with their pincers alone, they won’t bother stinging. This conserves venom, which can take up to 2 weeks to regenerate. The scorpion uses its sharp mouthparts, or chelicerae, to tear off pieces of food. The food is first stored in a pre-oral cavity where it is digested externally before being slurped down into the stomach in semi-liquid form. Awesome. Are Scorpions dangerous to humans? Of the 1,500 species of scorpions worldwide, only A scorpion consumes prey with specialized about 20 to 25 are regarded as dangerous to appendages called chelicerae. humans. In North America, there are about 70 species and only one--the bark scorpion Will it hurt if I get stung? (Centruroides exilicauda)--is considered potentially lethal to humans. Yes, the bark scorpion is found in People who have been stung by the Forest California but only down in the extreme Scorpion report a range of symptoms from an southeastern corner of the state near Arizona. The intense stinging similar to a wasp sting to no California Forest Scorpion is not considered sensation at all. Some report swelling that may aggressive or dangerous. When it feels last 2-3 weeks while others experience little or no threatened, it may “play dead” by tucking its legs in swelling. If in doubt, seek medical attention! and shielding its face with its pedipalps. Or it may scurry away. Stings are rare and the venom—if Scorpion reproduction injected—generally has mild effects. Mating starts with an elaborate courtship dance. The male grasps the female’s pincers and leads her in a series of “dance steps.” If she resists, he tries to get her in the mood by touching her chelicerae. He deposits a spermatophore (a tiny structure containing sperm) on the “dance-floor” and tries to lead her over it. If he succeeds, she will trip a tiny trigger mechanism that catapults the sperm into her genital opening. Once the eggs are fertilized, gestation can last several months to over a year. The female bears 25 to 35 live young that look like tiny white replicates of her. The young climb on her back and hang on through their 1st or 2nd molt. They will grow slowly, molting 5 or 6 times until reaching adulthood, after which they don’t molt at all. It can take anywhere from 1- 6 years to reach sexual maturity. Fallen Log Decomposition Litter/Soil Food Web Soil Macrofauna (some of the “larger” critters you may en- counter) Earthworm cocoons sow bug / pill bug Earthworm Jerusalem cricket Centipede (1 leg per segment, predator) Millipede (2 legs per harvester ant segment, herbivore) silverfish ↓wolf spider ÇOpilione (harvestman or Daddy long legs) Çrove beetle pseudoscorpion ←common ground beetle California slender salamander earwig termites Soil Microfauna Proturan Dipluran Collembola (Springtails) Í Primitive Insects ÇÇ Fungus-eating mite Mites Predatory mite Nematodes ÇBeetle larvae Î Fly larvae Ç Psocopteran The Collembola and mites are usually the most numerous critters you’ll find. You may also find tiny spiders, Feather-winged beetle pseudoscorpions, true bugs, flies, anta, wasps, snails, tardigrades, and others Soil Food Web Who can you find: Of those things you found …what is your guess about what function they might be performing ? In Praise of Spiders & Spiders’ Silk Marvels of nature to be appreciated and not to be abused Dr. Mae-Wan Ho A creature of beauty and grace Spiders are beautiful, graceful creatures, and their silk with which they construct their webs, egg cases, immobilize their prey, etc., is indeed among the marvels of the living world. I have always loved spiders and cannot understand arachnophobia (fear of spiders), an extreme form of which I witnessed in my own mother. If ever she spotted one in the house, she would screech and run for help, remaining agitated until the object of fear was unceremoniously squashed and its crumpled body flung out of sight. I, on the other hand, would spend hours watching the creature if I catch sight of one; just as surely as it must be watching me. There was a spider in my garden one summer a few years ago, which suspended itself upside down in front of my window, in the centre of its huge web some metres in diameter. There we could scrutinize each other to our hearts’ content as I sat at my desk trying to work on my computer. It was in the identical position for two successive summers, and I could almost swear it was the very same spider. Fabled silk robe of the golden orb spider In 2004, British art historian Simon Peers and American fashion designer Nicholas Godley joined forces to create a tapestry from the brilliant golden silk of the Madagascar spiders then on display at the Madagascar National Zoo. The impressive size of Nephila spiders makes it relatively easy to extract their silk and because they are used to building large, densely-woven webs, they can produce over 300 metres at a time [1]. Peers and Godley gathered and silked more than a million spiders for the project, by enlisting helpers to fix the dangling dragline of the spider to a spool to draw out the hundreds of yards of silk that the spider had stored up in its silk gland. Afterwards, the exhausted spider was released back into the wild to replenish its reserves, and could be silked again in a week or so. The resulting tapestry went on display at the American Museum of Natural History in the fall of 2009. It has been on show at the Victoria and Albert Museum in London since January 2012, with an additional golden silk robe [2]. It is stunning. The fabric is a rich burnished gold, light as a whisper yet tenaciously strong. The intricate, geometric design of the silk tapestry representing birds and flowers is based on traditional textile originally reserved for the Madagascan royalty. The robe is a brocade embroidered with a more representational design of spiders and flowers; truly a work of art belonging as much to the golden orb spiders as to the human artists and artisans who participated in creating it. The marvels of spider silk Human beings have used silks produced by a large variety of arthropods for thousands of years. The silk from Bombyx mori has been obtained from the farmed silkworms in China for at least 5 000 years. Spiders cannot be farmed, apparently due to their aggressive nature and cannibalistic tendencies, but the webs of orb weaving spiders (Araneidae) have been used historically for fishing and wound dressing, thanks to their astounding mechanical and biomedical properties [3]. There are many kinds of spider silks. A female orb weaving spider is able to produce 7 different silks, including one for egg cases (see Figure 1). Figure 1 The seven kinds of silks produced by the orb weaving female spider (highlighted in red) from Eisoldt, Smith and Scheibel [3] Spider silks are tailored for different purposes, and have different mechanical properties. The stress required to break the fibre ranges from 0.02 to 1.7 GPa, while the extensibility varies between 10 and 500 %. (Stress or tensile strength is measured as force per unit area in Giga (109) Pascals; a Pascal is 1 Newton (1 kg metre/s2) per square metre.) In comparison, steel breaks at 1.5 GPa, and is hardly extensible. Most spider silks have a combination of strength and extensibility which yields a very high toughness - the amount of energy absorbed per volume before breakage - that exceeds most natural or man-made fibres, making it ideal for bulletproof vests. Also common for all silks is the viscoelastic behaviour, as upon stretching, energy is dissipated in the form of heat, diminishing any elastic recoil. The most thoroughly investigated silk is the dragline silk, the lifeline that a spider drags behind, produced in the major ampullate silk gland, hence referred to as major ampullate silk. It is used for the frame and radii of an orb web, and has a maximum strength of up to 1.7 GPa. Although Keviar carbon fibres have a higher strength and stiffness, spider silk fibres have a much higher toughness due to their greater extensibility. It is the dragline silk of the Golden Orb spider that has been silked for making the gold robe on show. The dragline silk also has other interesting features. For example, a hanging spider hardly ever twists. When a dragline thread is twisted, it does not oscillate around the new position like a Keviar fibre would. Instead, the fibre slowly returns to its initial position, indicating that there is a shape memory in the thread. Another interesting feature is its ability to super-contract on wetting, or when the relative humidity is above 60 %. The silk thread swells in diameter and shrinks in length by about 50 %. (There must be an application for moisture sensing and humidity control here.) Spider silk proteins and genes All spider silks are mainly composed of one or more proteins called spidroins, which tend to be large, up to 350 kDa. They share a common primary structure comprising a large central core of repeated modular units, accounting for 90 % of the amino acids of the protein, flanked by non-repetitive domains of approximately 100 -140 amino acids. The terminal sequences, highly conserved throughout spider species and silk types, are essential both for the storage of spidroins in the silk glands and for fibre formation in the spinning duct, where they trigger crucial steps in the complex assembly of the spidroins as the result of changes in pH or ionic composition and strength. The sequence of the repetitive core is tailored for the individual mechanical functions of the different silks. In general, it consists of modular units each with about 40 – 200 amino acids, and the units are repeated up to approximately 100 times within the core domain. In the major ampullate, minor ampullate and flagelliform silk, the modular units are mainly comprised of a subset of the sequence motifs: (A)4-14.(GA)n.(GGX)n and (GPGXX)n, where A is alanine, G is glycine, P is proline, and X is a variety of different amino acids. In contrast to the termini, the core region is intrinsically unfolded, as long as the spidroins are stored in the gland. The major ampullate spindroins are MaSp1, which has the core modular structure (A)4-14.(GGX)n and MaSp2, with the core modular structure (A)4-14.(GPGXX)n. Dragline fibres, silked from spiders, have been examined under the electron, atomic force and light micrsoc0pes, revealing a core-shell structure. The shell is quite thin and contains lipids, glycoproteins and other silk proteins. The core consists mainly of the two spidroin proteins MaSp1 and 2. A single MaSp1 module usually consists of a polyalanine block and several GGX motifs. In modules of MaSp2 the GGX motif is replaced by GPGXX. Several dozens of repeats of modular units build the complete core region of these spidroins. The polyalanine block forms defined nanosized crystals (2 x 5 x7 nm) based on tightly packed antiparallel -sheets, in which polyalanine stretches from different silk molecules often form non-covalent crosslinks. GGX may form-sheets as well as less ordered helical structures. The GPGXX motif forms -turns, which, when repeated yield a spiral structure similar to elastin. All the secondary structures are due to hydrogen bonds (special bonds consisting of a hydrogen shared between two electronegative atoms, such as oxygen and nitrogen). It is the richness of hydrogen bonds in natural spider silk that gives it all the distinctive mechanical properties, and in addition, enables the silk to self-heal after being stretched, by reforming these hydrogen bonds [1]. In Araneus diadematus, the two silk proteins named fibroins - ADF3 and ADF4 – have the core structures (A)4-14.(GGX)n (GPGXX)n and (A)4-14. (GPGXX)n respectively. It appears important to have a pair of hydrophobic and hydrophilic spidroins: MaSp1/ADF4 are more hydrophobic and MaSp2/ADF3 more hydrophilic [3]. The primary structure of the silk protein alone does not explain the outstanding properties of dragline silk fibre. Fibres technically spun from reconstituted spidroins (chemically dissolved spider silk fibres) or from the original spinning dope taken from the silk gland show completely different mechanical properties compared to fibres spun by spiders. The natural process in the spider involves pre-aligned flow of a nematic (thread-like) liquid crystal solution with 50 % water, elongation through a narrowing S-shaped duct, and a pulling of the fibre out of the spigot, leading to some structural changes and phase separation between the water solvent, which is actively removed by the epithelial cells, and the spidroins. As well as mechanical influences, chemical changes occur especially in the spider’s S-shaped duct. There, the exchange of sodium and chloride ions for potassium and phosphate ions is responsible for exposing the hydrophobic areas within the C-terminal non- repetitive domain. On lowering the pH from 7 to 6, the amino terminal domains dimerize (join up in pairs), acting as additional crosslinks between the spidroins. Once the fibre has left the spider, the spider pulls and stretches the fibre leading to further water evaporation and additional alignment of the molecules inside the fibre. Fibrils oriented along the fibre axis form the core of the fibre, resembling the structure of a rope. Small crystallites (2 x 5 x7 nm) comprising tightly packed alanine - sheets and larger crystalline regions (> 100 nm) based on glycine-alanine sequences with high -sheet content can be found. Both crystalline regions are interconnected by an ‘amorphous’ matrix with loosely arranged helical structures oriented along the fibre axis. The crystalline areas are responsible for strength while the matrix is responsible for elasticity. Uses and abuses Apart from fibres, which have numerous applications in light-weight textiles for military and civilian use as well as in fashion design; spider silk can also self-assemble into non-natural shapes such as spheres, capsules, films, and hydrogels, with numerous potential applications, as carriers for drugs, scaffolds for tissue engineering, cosmetics, and even electronic and biosensing devices [3]. Much effort is currently dedicated to genetic engineering spider silk for commercial production. But it is important to do so ethically, without undue exploitation of animals (see [4] Unspinning the Web of Spider-Goat, SiS 54), and with due regard for safety [5] (Genetic Engineering Spider Silk, SiS 54). References 1. Conahan GS. Bulletproof gossamer: spinning a superfiber, M.Sc. in Science Writing, MIT, September 2011, http://dspace.mit.edu/bitstream/handle/1721.1/68474/769911187.pdf?sequence=1 2. “Silk cape spun by 1.2 m Golden Orb spiders on display at V&A museum. Metro, 24 January 2012, http://www.metro.co.uk/lifestyle/888246-spider-silk-cape-spun-by-over- 1-million-spiders-to-go-on-display-at-v-a-museum 3. Eisoldt L, Smith A and Scheibel T. Decoding the secrets of spider silk. Materials Today 2011, 14, 80-86. 4. Ho MW. Unspinning the web of spider-goat. Science in Society 54 (to appear) 2012. 5. Ho MW. Genetic Engineering spider silk. Science in Society 54 (to appear) 2012.