Anatomy of a Spider - Different Spider Body Parts

We can generally divide the parts of a spider into two, specifically the front and the back parts. This does not include the legs which we will address separately below. The cephalothorax, also known as the prosoma, is the anterior part of spiders which many people call the ‘head’. This does not have any type of segmentation and is joined to the posterior part by a structure known as the pedicel, the equivalent to a human waist.

The cephalothorax is made up of different parts, both external and internal. We discover more about the anatomy of a spider by looking at the body parts which make up the cephalothorax:

• Simple eyes: spiders commonly have eight eyes, although this number can vary between species. Each eye has a lens, optic rods, and retina. Despite such numerous eyes, their vision is poor, although some species such as hunters and jumpers can form images. Interestingly, some species have specialized eyes that can detect polarized light, aiding in navigation.
  
  
• Brain: in charge of processing the information the animal receives. The spider's brain is relatively small, but it is highly efficient in managing sensory inputs and motor outputs necessary for survival.
  
  
• Pumping stomach: part of the digestive system which has the function of sucking liquid food to take it to other structures of the system. This unique adaptation allows spiders to ingest prey that has been liquefied externally.
  
  
• Anterior aorta: part of the circulatory system, which is open. Unlike animals with a closed circulatory system and its accompanying veins and arteries, open systems means spider hemolymph (equivalent to blood in other animals) is mixed with interstitial fluid and travels all around the inside of their body. Through it, the hemolymph is transported to the different organs of the prosoma. This system is crucial for distributing nutrients and oxygen throughout the spider's body.
  
  
• Subesophageal ganglion: connected to the brain and is involved in nerve functions. It helps coordinate the spider's reflexes and voluntary movements.
  
  
• Coxal glands: part of the excretory system and end in the coxae of the legs, generally in the first and third pair. These glands play an essential role in regulating water balance and excreting waste products.
  
  
• Digestive caecum: they are part of the midgut and are a series of compartments with no outlet. Although there are only a few in the cephalothorax, nutrients are extracted from food here. This system allows spiders to maximize nutrient absorption from their prey.
  
  
• Venom gland: structure specialized in the production of toxic substances. With only very minor exceptions, all spiders have venom^[1]. The strength varies incredibly and most are not harmful to large animals such as humans. The venom is often used to immobilize prey and begin the digestion process even before ingestion.
  
  
• Chelicerae: a pair of appendages with one joint and two sections. Its distal end has fangs, which many spider species use to inject venom. These fangs are often retractable and can vary in size depending on the species.
  
  
• Mouth: located just after the chelicerae and is attached to the esophagus. The spider's mouth is adapted to handle liquid food, a necessity given their pre-digestion of prey.
  
  
• Pedipalps: correspond to the second pair of appendages, similar to the legs, but shorter. They are not used for locomotion. They are used as an extension of the mouth and in males, it also has a reproductive function to transfer sperm. In some species, pedipalps can be quite elaborate and are used in courtship rituals.
  
  
• Legs: we will see them in more detail below.

Learn more about different spider species with our article on the most venomous spider in the world.

More commonly known as the abdomen, the opisthosoma is the posterior part of the spider's anatomy. As stated above, it is connected by a kind of waist known as a pedicel. This allows the abdomen of the spider's body to have mobility on all sides when weaving their webs. Such flexibility is critical for their survival, enabling them to build intricate webs and capture prey efficiently.

To learn more about this body part of spiders, we look at its constituent parts:

• Glands or digestive tubes: produces digestive enzymes that the spider injects together with the venom so that predigestion of the prey occurs and they are able to suck it through the mouth. This external digestion method is highly efficient, allowing spiders to consume prey larger than themselves.
  
  
• Ostiole: a hole that contains the heart, through which the passage of hemolymph occurs. This opening is essential for maintaining the flow of hemolymph, ensuring that all bodily functions are supported.
  
  
• Heart: tube-shaped, has no divisions and is responsible for pumping hemolymph, which replaces blood, through the arteries to the organs. The heart's simple yet effective design is a hallmark of the spider's highly adapted physiology.
  
  
• Excretory system: it is formed by the Malpighian tubules, small structures in charge of extracting the waste in the hemolymph to be taken to the rectal sac and finally expelled through the anus. This system is vital for maintaining the spider's internal environment, allowing it to thrive in various habitats.
  
  
• Silk glands: structures specialized in producing a type of protein that constitutes the silk with which they form their typical cobwebs. Spider silk is renowned for its strength and elasticity, often compared to steel in terms of tensile strength.
  
  
• Spinnerets: modified appendages and organs made up of many microtubes that connect to silk glands. Through these microtubes, the spiders expel the silk to weave their web with which some cover their dens or trap their prey. The diversity in spinneret structures among species reflects the variety of silk functions, from capturing prey to constructing shelters.
  
  
• Reproductive system: females have an oviduct, a seminal receptacle, and an ovary. Males have testicles, a spermatic duct, and a copulatory organ, specifically the pedipalps explained in the previous section. Discover in this related article more detail on how spiders reproduce. The reproductive strategies of spiders can be quite complex, with elaborate mating rituals and sometimes even cannibalism.
  
  
• Respiratory system: spiders can breathe through book lungs which are internal structures arranged in parallel chambers. They are so called as they resemble the pages of a book. Hemolymph passes through the chambers to carry out gas exchange. Some spiders also have tracheae, a system of small tubes that allow them to breathe directly through the body wall.

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