Spiders

These are slides from a basic public talk I gave on spiders. It’s not zoology course-level, but good if you know nothing about spiders. PDF version.

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On the arthropod tree of life, spiders are just one group within the Chelicerata, a group of animals characterised by the possession of a chelicera (and, more informally, by having 4 pairs of legs). The phylogeny above names all extant chelicerate orders; chelicerates evolved in the oceans, but only the basal Pycnogonida and Xiphosura remain marine animals.

Except for the Acari and the Araneae, these orders are rather small. The Acari are the most species-rich with 55000 species, followed by the Araneae with 42000+ species.

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The main innovation of spiders is the splitting of the body into two parts: the prosome at the front and the opisthosome at the back, separated by a petiole. The prosome is the head, contains the brain, limbs, mouthparts, and eyes, and is covered by a carapace, the peltidium.

The eight legs are fundamentally identical – there is no differentiation as with crustaceans. The chelicerae end with a fang, and is actually hollow with venom flowing into it. We’ll see its importance later. The pedipalp is actually homologous to the pincers of the scorpion. They serve as sensory structures and, in males, as reproductive organs. Spiders have no antennae.

The opisthosome contains most of the organs except the brain and stomach. At the back, next to the anus, are the spinnerets, the structures that shoot silk out.

The book lungs are only present in some basal spiders. They’ve mostly been lost and replaced by a tracheal system similar to the insectan one described in this post.

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As mentioned already, the opisthosome contains most of the organs. The names should be self-explanatory, here are the ones that don’t have the same names as in humans:

  • Malpighian tubule: The equivalent of a kidney.
  • Silk gland: Where silk gets produced (this will be examined in more detail further down).
  • Spinneret: Where silk comes out. Will also be exmined in more detail later.
  • Stercoral pocket: Temporary storage site for feces.

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There are 42000+ extant species of spider in 111 families (one was discovered recently and I haven’t updated the slide). Going into their detailed systematics would require its own article, so I will just outline their most basic divergence.

The basalmost extant spiders are the Mesothelae, of which there is only one extant family in SE Asia. If you look at the picture above, you’ll see why they’re considered to be quite ancestral. The pedipalps are almost like an extra pair of legs, and the opisthosome still has a faint segmentation, unlike the Opistothelae.

The Opistothelae are the spiders we all know, and they fall into two groups: mygalomorphs and araneomorphs.

The Mygalomorphae include tarantulas and trapdoor spiders – the big hairy spiders, if you will. A more accurate distinction is to be made with their fangs: they are orthognath, i.e. their fangs bite straight down, as opposed to labidognath araneomorphs whose fangs cross when biting, like scissors.

Araneomorphae are characterised by several innovation in their silk apparatus, most importantly the acquisition of a cribellum, a plate over which silk is laid out, and the calamistrum, hairs on the back pair of legs that sculpt the silk. Accordingly, araneomorphs have greatly expanded architectural skills.

Picture sources: Mygalomorphae (Robert Whyte), Aranaeomorphae (e_monk).

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Besides a single species, Bagheera kiplingi, all spiders are strictly carnivorous. The diagram above showcases the tight correspondence between evolutionary history and predation style in spiders, the latter of which is directly linked to silk-weaving capabilities. Mygalomorphs, with their rudimentary toolkit, are mainly burrowers, their use of silk usually limited to lining their burrows and to make simple traps or alarm lines (a line of silk that vibrates when touched by a prey, alerting the spider to its presence).

With the evolution of the araneomorphs, new types of traps could be built, from basic sheet webs to messy space webs to the highly-elaborate orb webs.

As always with evolution though, some spiders eschewed such silly things and dumped silk use in prey acquisition altogether, preferring instead to hunt. This is the most basic dichotomy when discussing spider ecology: is it a hunter or a web-builder? It should be noted, however, that hunting spiders have not lost the ability to use silk – many use draglines as stabilisation help when hunting, for example.

Diagram source: Vollrath F & Selden PA. 2007. The Role of Behavior in the Evolution of Spiders, Silks, and Webs. Annual Review of Ecology, Evolution, and Systematics 38, 819-846.

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Useful for both types of spiders are an array of senses, of course. Spiders can taste and they can sense chemicals in their environment. On the last segment of the pedipalps, they have a tarsal organ, which is used for sensing temperature and humidity, and most probably for chemosensation as well.

Tarsal organ SEM source: Huber BA & Wunderlich J. 2006. Fossil and extant species of the genus Leptopholcus in the Dominican Republic, with the first cases of egg‐parasitism in pholcid spiders (Araneae: Pholcidae). Journal of Natural History 40, 2341-2360.

Where hunting spiders tend to differ from the web-builders is in vision. All spiders can see (except those unique species that have gone blind), and the basic toolkit is to have six eyes, although some can have as many as eight, and others have four, and others only two. The primary ones are at the front of the head, the anterior median eyes. They’re the only eyes that can move; the others are immobile.

As such, they often have a division of labour. For example, in the salticids, the jumping spiders acknowledged as having among the best eyesight in all spiders, the immobile eyes are used to detect motion. If something is detected, then the spider turns its head and uses the anterior median eyes to focus and judge the distance to the prey.

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But obviously, the biggest innovation in spiders is their use of silk to build traps – even hunting spiders will use silk to make draglines or help stabilise themselves or a prey. Spider silk is made of fibroin proteins, and comes in 6 major types, each produced by a different spinneret type. Different silks are often used together.

Each silk category has very different properties. Consider a typical orb web as an example. The spider uses two types of silk to build that: major ampullate silk (red in the diagram) and viscid silk (blue).

Major ampullate silk is extremely strong, and so it makes up the frame of the web. In fact, when humid, MA silk is around 5 times stronger than steel – this is why spider silk is under intense research for biotechnological and biomimetic pruposes, with the major stumbling block to industrial application being that we can’t get any spider to overproduce its silk on demand.

Viscid silk that forms the capture spiral, on the other hand, is more of an elastic product, able to handle a lot of strain without breaking.

This is why large insects can fly into an orb web at full force without the web breaking: the viscid silk absorbs the impact, and its rebounding from the impact is supported by the major ampullate silk of the frame. This is also what enables the orb weaver defence mechanism of shaking its web so violently that the spider becomes more or less invisible (try it next time you see an orb weaver: make your presence known and watch the spider vibrate the web!).

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As already mentioned, spiders have different spinneret types for producing the various silks. Going into how each one works would go outside the scope and level of this article, but suffice it to say that differences between spider families arise from the arrangements of each spinneret type – like eyes, their positions can also vary.

Spinnerets diagram source: Eberhard WG. 2010. Possible functional significance of spigot placement on the spinnerets of spiders. Journal of Arachnology 38, 407-414.

How silk makes its way to the spigot is simple. Silk is first made as a liquid in the silk glands, where the tail is, and stored in the lumen. When silk needs to be used, it gets squeezed out through the funnel and into the duct, where water and ions get absorbed out of it, similar to how ureine is processed in the kidneys. By the end of it, you have solid silk that gets ordered throught he valve and sculpted by the spigot that shoots it out. It’s at the last stage that final chemical modifications are made, from changes to the protein structure to adding glue droplets as needed.

Silk production diagram source: Blackledge TA, Kuntner M & Agnarsson I. 2011. The Form and Function of Spider Orb Webs: Evolution from Silk to Ecosystems. Advances in Insect Physiology 41, 175-262.

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Taking the phylogenetically-determined variations in silk types, spinneret types, spinneret arrangements, and spigot modifications leads to the large diversity of spider webs that are observable, from the messy sheet and space webs (Eresus, Linyphia in the diagram above, respectively) to the highly-elaborate and infinitely customisable orb web (Araneus). See my Quora answer here – which is also featured in the Best of Quora book! – for more on these types of webs.

Webs diagram source: Vollrath F & Selden PA. 2007. The Role of Behavior in the Evolution of Spiders, Silks, and Webs. Annual Review of Ecology, Evolution, and Systematics 38, 819-846.

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Regardless of how they gain their prey – by hunting, by ambushing, by trapping – all spiders have the same fundamental feeding mechanism (even that weird vegetarian one). The first step is to incapacitate the prey. In order to do this, they bite the prey and inject a venomous cocktail through a hole in the fang.

Hole SEM: Foelix R & Erb B. 2010. Mesothelae have venom glands. Journal of Arachnology 38, 596-598.

The venom can contain many components but the active ingredient is a potent neurotoxin (or several, which is why it’s hard tog et an antidote sometimes). As you can see from that table of contents from a review paper, they can affect any possible section of the nervous system. There are other parts to the cocktail too, and I recommend reading the review linked below.

Venom categories source: Rash LD & Hodgson WC. 2002. Pharmacology and biochemistry of spider venoms. Toxicon 40, 225-254.

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All that said, however, the majority of spiders are not strong enough to pierce human skin and are thus completely harmless. At most, these spiders can just bruise you – common examples of this type of spider are the only aquatic spider, the diving bell spider Argyroneta aquatica and the yellow sac spider Cheiracanthium punctorium.

However, there are some spiders that do pose a threat by being able to pierce skin and thus are able to inject venom and be lethal. The Sydney funnel web spider Atrax robustus and the Brazilian wandering spider Phoneutra fera are probably the most dangerous spiders, while the widow spiders of the Latrodectus genus and the brown recluse Loxosceles reclusa are dangerous for being rather common to find in urban habitats. So if you see those, you might want to run. Everything else is good for you (they keep your house clean of insect pests!).

Non-Wikipedia picture sources: Brazilian wandering spider (Óscar Méndez); yellow sac spider (Eran Finkle); brown recluse (Oakley Originals).

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That said, some spiders, notably tarantulas, have another way to hurt you: urticating hairs. These are sharp, needle-like hairs that they can eject at high speeds and that become embedded in your skin. As the diagram on the right shows, they cause at least an inflammation (a rash), which can last up to a week (personal experience). In case of allergy though, there may be more severe symptoms requiring medical care.

This is why pet tarantulas should generally be handled with gloves.

Diagrams source: Battisti A, Holm G, Fagrell B & Larsson S. 2011. Urticating Hairs in Arthropods: Their Nature and Medical Significance. Annual Review of Entomology 56, 203-220.

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Besides the neurotoxins, spiders also inject a bunch of enzymes into their prey. These are digestive enzymes, and they basically turn the insides of the animal into a pudding or a slushy. This is a feeding method called extra-oral digestion, as opposed to typical internal digestion where the food is broken down into its nutrient parts inside the digestive system.

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This is then sucked in through the oesophagus with the help of their sucking stomach. There is no chewing invovled – this is why you will find spider webs or spider burrows full of empty insect exoskeletons.

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The point of feeding is to have enough energy to survive and to reproduce. Reproduction in spiders is interesting because while the female has a vagina, the male has no penis. Imagine that. Instead, the male transfers his sperm to the female with his spoon-shaped pedipalps.

After any mating ritual is done, the male puts some silk at the bottom of the pedipalp, then dips it in his gonads to fill it with sperm. He makes a nice silk coccoon to make a sperm ball (technical term: spermatophore), and shoves this into the female’s oviduct, as pictured above.

The human equivalent is ejaculating into your hand and inseminating your partner by fisting her.

Diagram source: Alberti et al.. 2007. Chelicerata. In: Westheide & Riger (eds.). Spezielle Zoologie Teil 1.

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However, mating is not as simple as that in many spiders, as the male runs the risk of being eaten in an act of sexual cannibalism. As I explained in my post on natural selection, in some cases, males have evolved a behaviour where they actually jump into the mouth of the female and get eaten willingly, and this increases their chance at fathering offspring, therefore conferring a selective advantage to this seemingly destructive behaviour.

Picture source: Latrodectus hasselti (Klaus Stiefel);

Study: Andrade MCB. 1996. Sexual Selection for Male Sacrifice in the Australian Redback Spider. Science 271, 70-72.

That’s an extreme example though. In most cases of post-copulatory cannibalism, the male will try to run away and fail. The advantage is mainly for the female in that she gains extra nutrition.

There is also pre-copulatory cannibalism where the male gets eaten during the mating ritual. This has the same advantage for the female, but means that the opportunity for reproduction is gone, which is risky since another one may not come again.

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Once the eggs are fertilised, they are ejected and develop in a coccoon that is either hidden somewhere (e.g. inside layers of bark), or placed in view of the mother. Some spiders carry their eggs on their backs or in their mouths, and some rarer ones even carry the first couple of instars on their backs too, as in the picture above.

Picture source: Lycosid (stonebird).

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Such extensive childcare is a stepping stone towards sociality, which a few spider species have achieved – 25 as of last count. These social spiders share a familial web, cooperate in foraging for food and in raising offspring. The picture at the bottom of the slide shows an example of a social spider web, from Anelosimus eximius.

Picture source: Anelosimus eximius social web (Arthur Ankel).

There is another brand of sociality in spiders, termed coloniality. This is when a web is shared but among unrelated aggregations that may not even be of the same species. Besides sharing the web, no cooperation takes place, and the web is split into territories, but wars generally don’t break out. It’s just a culture of tolerance. A bit more than 50 species have achieved this.

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A word on the fossil record of spiders to round this off. The fossil record exists both in rocks and in amber, and is actually surprisingly full. Above are some examples, but there are over 1150 fossil spiders known already.

Palaeoperenethis thaleri source: Selden PA & Penney D. 2009. A fossil spider (Araneae: Pisauridae) of Eocene age from Horsefly, British Columbia, Canada. Contributions to Natural History 12, 1269-1282.

Korearachne jinju source: Selden PA, Nam K-S, Kim SH & Kim HJ. 2012. A Fossil Spider from the Cretaceous of Korea. Journal of Paleontology 86, 1-6.

Eoplectreurys gertschi source: Selden PA & Huang D. 2010. The oldest haplogyne spider (Araneae: Plectreuridae), from the Middle Jurassic of China. Naturwissenschaften 97, 449-459.

Spider in amber source: Saupe EE & Selden PA. 2011. The study of fossil spider species. Comptes Rendus Palevol 10, 181-188.

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It’s worth mentioning Megarachne at this point, since you’ve probably seen the otherwise excellent Walking With Monsters series where it was featured. Megarachne as a giant spider never existed. At the time of production, Megarachne was interpreted as a spider based on the original fossil, pictured above in the lower left – it is admittedly very spider-like.

However, new fossils like the one in bottom center made it clear that Megarachne was not a spider at all, but a eurypterid, a sea scorpion, reconstruction at the bottom right. Unfortunately, the reinterpretation was published too late to make changes to the series. While it’s not unlikely that there were giant spiders around as shown in the series, we don’t have any fossil evidence for them.

Reinterpretation of Megarachne: Selden PA, Corronca JA & Hünicken MA. 2005. The true identity of the supposed giant fossil spider Megarachne. Biology Letters 1, 44-48.

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And just so you get an overview of the completeness of the spider fossil record, here is the most up-to-date diagram. As you can see, their earliest fossil record goes back to the terrestrialisation period in the Devonian, and no truly large gap exists, compared to the fossil record of insects. Some families have no fossil record at all, but most of them do.

Source: Penney D, Dunlop JA & Marusik JM. 2012. Summary statistics for fossil spider species taxonomy. ZooKeys 192, 1-13.

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