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Spiders are air-breathing arthropods, did you know they are not insects as many people usually say?
Figure 1: Taxonomy classification of Class Arachnida and Insecta
They may not look pretty or friendly, but the truth is they are essential and fascinating beings.
The largest order of arachnids (order Araneae) is one of the most diverse among all organisms' orders. They are found on every continent except for Antarctica, being well established in almost every land habitat.
Anatomic characteristics
As you noted they are slightly different from other arthropods.
Figure 2: Arthropodes
The usual body segment we see in arthropods in spiders is fused into two tagmata, the cephalothorax/prosoma, and a small pedicel joins the opisthosoma/abdomen.
Figure 3: Spider anatomy
Figures 4: Spider anatomy
They do not present antennae. All spiders, except for the most primitive, present a centralized nervous system since all other ganglia formed one mass in the cephalothorax.
Spiders move their eight legs through hydraulic pressure, yes!!!! they don't have extensor muscles, and this is the reason they curl their limbs when dead.
Check this: WHY DO SPIDERS CURL UP THEIR LEGS WHEN THEY DIE?
https://www.youtube.com/watch?v=FlKago05Lxg
Eyes
Spiders present four pairs of eyes on the top-front arena on the cephalothorax. The distribution patterns vary between families.
The principal pair at the front are called pigment-cup ocelli, these eyes allow spiders to form images, a capacity that other Anthrops do not have.
The second pair of eyes detect light reflected by a reflective tapetum lucidum.
Figure 5: Primary pair of eyes
Figure 6: Second pair of eyes on the top of the cephalothorax
Spider Webs
They vary in size, shape and even in the amount of silk used.
Spiders use a wide range of strategies to capture prey:
trapping it in sticky webs,
lassoing it with sticky bolas,
mimicking the prey to avoid detection,
running it down.
Curiosity
While the venom of a few species is dangerous to humans, scientists are now researching the use of spider venom in medicine and as non-polluting pesticides.
References:
https://www.scientificamerican.com/article/sticky-science-the-evolution-of-spider-webs/
World’s Largest Spider. (2013). Animals. https://www.nationalgeographic.com/animals/invertebrates/facts/spiders
aracnologia Vetor Premium. (2020, June). Freepik. https://br.freepik.com/vetores-premium/a-ilustracao-de-uma-anatomia-da-aranha-inclui-todo-o-nome-de-partes-de-animais-especies-de-birdeater-na-mao-desenhada-ou-gravada-estilo-aracnologia_8525183.htm
Spidentify. (2020, January 22). Spider anatomy - Spidentify. Spidentify. https://identify-spiders.com/spider-anatomy/
Lindzi Wessel,Knowable Magazine. (2018, October 31). Sticky Science: the Evolution of Spider Webs. Scientific American. https://www.scientificamerican.com/article/sticky-science-the-evolution-of-spider-webs/
(1965, March 31). Guinnessworldrecords.com. https://www.guinnessworldrecords.com/world-records/largest-spider
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