Introduction

 

The body of a spider is an extremely complex unit with different segments, body parts, and internal and external functions that together can become overwhelming to comprehend. Therefore, I intend to separate each unit to fully visualize and explain each part as is often done when describing these animals. Let us begin with the first segment of the spider’s body which is the prosoma, usually titled the cephalothorax, where the spider's chelicerae (jaws), venom glands, pedipalps, the front section of the foregut, anterior branch of the circulatory system, nerves, the brain and eight walking legs. Next to the prosoma is the second unit of the spider's body called the opisthosoma, commonly known as the abdomen, containing the book lungs, ovaries for female specimens, tracheal opening, the posterior branch of the circulatory system, spinnerets, silk gland, anus, the midsection of the gut with Malpighian vessels, heart and the intestinal branching of the digestive system.

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• Cephalothorax, prosoma

• Abdomen, opisthosoma

 

When exploring the intricacies of arthropods within the phylum Arthropoda, one frequently encounters the term "tagma," particularly in discussions of morphology due to their shared characteristic of segmentation. Tagma is employed to characterize an organism with multiple segments that are either fused or articulated to form a cohesive and functional morphological unit. For instance, insects often exhibit fusion of the head onto the thorax, whereas a spider connects its jointed abdomen to the cephalothorax via the pedicel. Consequently, the prosoma and opisthosoma represent two tagmata, forming a cohesive and functional morphological unit.

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Cephalothorax

Carapace

 

The carapace serves as a protective dorsal shield for the cephalothorax within the exoskeleton, safeguarding the internal anatomy. Positioned at the center of the carapace is a small depression or cavity known as the fovea, serving as an internal anchor point for the thoracic muscles. Its rigid structure reinforces the spider's overall structural integrity, reducing susceptibility to external forces. Morphologically, both color and structure exhibit variation among species. For instance, the Ceratogyrus genus displays intriguing external features, including horn-like protrusions at the carapace's center.

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Chelicerae

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The chelicerae are the initial pair of appendages equipped with two jaws and penetrating fangs. Below these jaws lie two cuticular plates referred to as the upper and lower lips. The upper lip, known as the labrum, is obscured by the lower lip, the labium, and is visible from an external perspective behind the jaws. Both the labrum and labium function essentially as lips, opening and closing during feeding.

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The claw-like structures, commonly known as fangs, are hollow organs connected to spiders' venom glands, resembling the functionality of hypodermic needles. When a spider bites, these fangs puncture the prey and inject venom from the glands. Despite their powerful jaws capable of crushing prey, they are not intended for chewing. The venom's primary purpose is to paralyze the prey and liquefy its internal contents, making it easier for the spider to consume through suction.

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Surrounding the venom glands are specialized muscles that regulate the venom's dosage, enabling spiders to adjust the amount depending on the size of the prey. This ability is crucial for spiders due to the energy expenditure involved in producing new venom. Moreover, it allows spiders to deliver "dry bites" as a defense mechanism, biting without injecting venom when threatened.

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However, some spider families, like the Scytodidae, have evolved alternative hunting strategies. Instead of relying primarily on their fangs, these spiders utilize venomous silk to paralyze their prey, demonstrating the diverse adaptations within the spider kingdom.

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There are three primary types of chelicerae: jackknife, scissor, and 3-segmented chelate. Jackknife chelicerae are exclusively found in Tetrapulmonata, a supra-ordinal group of arachnids that includes spiders (order Araneae). The range of motion of jackknife chelicerae varies depending on the species classification. Species belonging to the infraorder Araneomorphae, also known as Labidognatha, demonstrate lateral jaw movement, gripping prey between their jaws. Conversely, species within the infraorder Mygalomorphae and the family Liphistiomorphae exhibit a downward jaw motion parallel to the body axis, termed Orthognathous movement.

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Pedipalps

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Positioned between the chelicerae, which are the primary feeding appendages, and the first pair of walking legs, the second set of appendages in spiders are known as pedipalps. These pedipalps play a crucial role in sensory perception, functioning as antennae-like structures equipped with highly sensitive chemical detectors. Through these detectors, spiders can discern cues from their environment, aiding in navigation, prey detection, and mating. Structurally, pedipalps bear a resemblance to the walking legs. They consist of several segments including the coxa, trochanter, femur, patella, tibia, and tarsus. While they share this general anatomy with the legs, the pedipalps are often more specialized for sensory tasks, with modifications in certain species for specific functions such as prey manipulation or mating rituals.

 

Overall, the pedipalps represent a versatile adaptation in spiders, enhancing their ability to interact with and respond to their surroundings effectively. They are integral to the spider's sensory apparatus, contributing to its success as a predator and survivor in diverse ecosystems.

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Coxa --> Trochanter --> Femur --> Short patella --> Tibia --> Tarsus

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Within the anatomy of spiders, the pedipalps play a multifaceted role beyond their sensory function. The coxa, which is the initial segment of the pedipalp, serves as a point of interest due to its frequent association with extensions known as maxillae. These maxillae serve to augment the spider's feeding process by assisting in the manipulation of mouthparts during consumption. However, it's noteworthy that the degree of modification of these maxillae can differ significantly among species, particularly across various suborders and infraorders.

 

For instance, in the infraorder Araneomorphae, which encompasses a diverse array of spiders including orb-weavers, wolf spiders, and jumping spiders, the ends of the pedipalps undergo a remarkable transformation. Here, the maxillae are adapted into a serrated, saw-like structure. This specialized adaptation allows spiders within this group to effectively cut and subdue their prey, reflecting the diverse evolutionary strategies employed within the arachnid kingdom.

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In the world of spiders, sexual dimorphism is often pronounced, and one of the most striking examples of this is observed in sexually mature males. These males are easily distinguished by their enlarged pedipalps, which give them the appearance of wearing boxing gloves. These specialized structures, known as the palpal bulb or palpal organ, serve a crucial role during mating by transferring sperm to the female's seminal receptacles. What makes this aspect of spider biology particularly intriguing is the vast diversity observed among different spider groups. Palpal organs can vary significantly in their structure and coloration, reflecting the intricate evolutionary adaptations that have arisen across various species.

 

However, the significance of pedipalps extends beyond reproductive function. Male spiders utilize these appendages for a variety of purposes, including visual displays to attract mates, producing vibratory signals to communicate with potential partners, and even shaking their webs to signal territorial boundaries or prey capture.

 

The complexity and versatility of pedipalps in male spiders provide a rich area for exploration in the realm of spider biology, shedding light on the fascinating intricacies of reproduction and mating behavior. This topic will be further delved into in the upcoming "Reproduction and Mating" section, where we will explore the various strategies and adaptations employed by spiders to ensure successful reproduction.

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Eyesight

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Across the diverse world of spiders, the presence of eyes is a common trait, yet there are intriguing exceptions that highlight the adaptability of these arachnids. For instance, species like Sinopoda scurin have adapted to dark, cave environments where eyesight is less crucial for survival.

 

In general, spiders boast eight eyes, each possessing a single lens, a feature that distinguishes them from the compound eyes characteristic of insects. However, the arrangement, shape, and positioning of these eyes exhibit remarkable variation among species. This diversity provides arachnologists with valuable cues for species identification and classification, aiding in our understanding of spider biodiversity.

 

The upcoming section on "Identifying Species" will delve deeper into the fascinating world of spider morphology, offering insights and techniques for distinguishing between different species based on their unique eye configurations and other distinguishing features. Stay tuned for a closer look at the intricacies of spider taxonomy and identification!

In the intricate world of arachnids, the eyes of spiders offer valuable insights into their evolutionary history and ecological adaptations. These eyes can be categorized into two main types: primary and secondary.

 

Primary eyes, also known as ocelli, occupy a central position within the spider's eye arrangement. They are characterized by their distinct circular shape and are often larger and more prominent compared to secondary eyes. Positioned at the forefront of the spider's head, primary eyes play a crucial role in visual perception and navigation.

 

On the other hand, secondary eyes are typically smaller and located on the periphery of the spider's head. While they may not possess the same level of visual acuity as primary eyes, secondary eyes still contribute to the spider's overall sensory perception.

 

One intriguing hypothesis regarding the evolution of spider eyes suggests that secondary eyes may have originated from compound eyes found in the ancestors of modern spiders. This theory underscores the dynamic nature of spider evolution and the remarkable diversity of visual adaptations that have arisen over time. Understanding the structure and function of spider eyes provides valuable clues for unraveling the complexities of their behavior, ecology, and evolutionary history. In the upcoming sections, we will delve deeper into the fascinating world of spider biology, exploring the myriad ways in which these remarkable creatures interact with their environment.

 

In the realm of spiders, primary eyes possess a remarkable feature absent in many other arthropods: the ability to form images. Unlike their counterparts, which can only detect the direction of light, primary eyes, also known as Anteromedial eyes (AME), hold a prominent position at the front of the spider's head. These eyes excel at gathering detailed visual information about their surroundings through focused vision, facilitated by specialized muscles that enable the movement of the retina to track objects.

 

However, spiders face a limitation due to the fixed nature of their primary eyes, rendering them unable to move their heads like other animals. To compensate for this, secondary eyes play a pivotal role in maintaining awareness of the surroundings. Typically smaller in size, secondary eyes are positioned alongside the primary eyes, aiding spiders in navigating their environment and enhancing their overall sensory perception.

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• ALE: Anterolateral eyes; top row on the side of the head

• PLE: Posterolateral eyes, second row on the side of the head

• PME: Posteromedial eyes, in the middle of the head

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Secondary eyes, unlike anteromedial eyes, lack the capability to move the retina. Consequently, the anterior lateral eyes (ALE), posterior lateral eyes (PLE), and posterior median eyes (PME) primarily serve the function of analyzing motion. This specialized role aids spiders in detecting potential prey or predators within their environment.

Species within the Caponiidae family exhibit some peculiar characteristics, notably possessing only one pair of eyes initially, with the potential to grow additional eyes as they mature. For instance, some species may have six eyes, such as those within the Scytodidae or Sicariidae families, while others, like those in the Symphytoganthidae family, have only four eyes.

 

However, there are families renowned for their exceptional vision, such as the Salticidae jumping spiders and Lycosidae wolf spiders. These spiders are adept hunters, relying primarily on their eyesight to locate and capture prey. Unlike many other species, which number over 40,000 as of 2019, these active hunters utilize their keen vision rather than relying heavily on other senses.

 

Nevertheless, for the vast majority of spider species, eyesight is not their primary tool for hunting. Instead, they rely on extremely sensitive sensory organs that detect vibrations. This aspect will be further explored in the upcoming appendages section.

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Appendages

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Spiders are typically recognized by their eight walking appendages, which serve as a fundamental identifier. This distinction is often one of the initial "keys" used for identification purposes: if an animal possesses six walking appendages, it belongs to the class of insects; however if it has eight, it is classified as a spider. These walking appendages are counted in pairs, totaling four pairs attached to the underside of the cephalothorax. Each appendage is comprised of seven segments, contributing to the spider's agility and mobility.

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1. Coxa: The first segment, attached to the cephalothorax.

2. Trochanter: The second segment, between the coxa and femur.

3. Femur: The third segment, between the trochanter and patella.

4. Patella: The fourth segment, between the femur and tibia.

5. Tibia: The fifth segment, between the patella and metatarsus

6. Metatarsus: The sixth segment, between the tibia and tarsus.

7. Tarsus: The seventh segment, the last segment.

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At the tip of the tarsus, one can observe small claws that exhibit variations in size and number across different spider species. Species that predominantly inhabit webs typically possess three claws, with the middle one being smaller. This arrangement aids them in gripping their webs, facilitating easier movement. Conversely, species equipped with two claws are often hunting spiders, relying on active pursuit for prey capture. Interestingly, some species, such as Thomisus spectabilis, commonly known as the Australian crab spider, lack claws entirely.

 

The spider's body is covered with small, highly sensitive hair-like structures called setae. These structures are adept at detecting vibrations, wind, and sound, providing the spider with awareness of its surrounding environment. Further exploration of setae will be available in an upcoming publication, delving into the intricacies of these sensory adaptations.

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Abdomen

Pedicel

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Positioned between the cephalothorax and abdomen is a slender, tube-like structure known as the pedicel, which serves as a crucial bridge linking these two body segments in spiders. Functioning as a conduit for essential anatomical components such as nerve cords, blood vessels, parts of the digestive system, and in some cases, the tracheae, the pedicel plays a pivotal role in facilitating internal communication and physiological functions.

 

The presence of the pedicel affords spiders a distinct advantage: the ability to maneuver their abdomen with remarkable freedom without necessitating significant adjustments in their overall position. This flexibility is particularly advantageous in various aspects of their lives, from foraging and mating to evading predators and constructing intricate webs.

 

Interestingly, while the pedicel may be discernible in certain spider species, its visibility tends to be more pronounced in those considered to be more primitive. Within the Mygalomorph infraorder, characterized by species with less differentiated body designs and structures still undergoing developmental stages, the pedicel often exhibits a more prominent presence, highlighting its evolutionary significance and adaptive utility across diverse spider taxa.

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Book lungs

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The term "book lung" aptly describes the internal structure of the lungs in spiders, as it resembles a folded book with small layers of air pockets compressed together by tissues filled with hemolymph. These hemolymph-filled tissues, also known as plates, play a crucial role in the breathing process by facilitating the circulation and exchange of gases within the book lungs.

 

However, in some spider species, book lungs may be absent, and gas exchange occurs through walls inside the cavity, which branch into the body as tubes called tracheae. This alternative respiratory mechanism demonstrates the remarkable adaptability and diversity present within the arachnid world.

 

Situated on the ventral side of the abdomen are two hardened covers known as epigastric plates, which serve to protect the book lungs. Adjacent to these plates lies a fold called the epigastric furrow, which acts as a boundary separating the epigyne from the posterior part of the abdomen. In female specimens, the oviducts are typically located in the middle of this furrow, positioned close to the crevice.

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Heart

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In the abdomen's midsegment, lies the spider's heart, rhythmically pulsating against the dorsal cuticle. Unlike vertebrates with closed circulatory systems, spiders possess an open circulatory system, where hemolymph (spider blood) flows freely through vessels from the heart, bathing the organs directly in an open cavity.

 

This arrangement allows for a gradual exchange of nutrients, gases, and metabolic waste products between the hemolymph and the surrounding tissues. While this method lacks the rapidity of closed circulatory systems found in mammals, where blood is confined to vessels, the spider's open system ensures efficient circulation over time. Eventually, the hemolymph returns to the heart, completing the circulatory loop and maintaining essential physiological processes throughout the spider's body.

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Maphigian vessels

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Nutrient absorption is a vital process for all living organisms, including spiders. In spiders, this crucial function occurs in the midsection of the gut, where specialized excretory organs known as Malpighian tubules play a pivotal role. These tubules serve as excretory and absorptive structures, opening into the gut to filter and process the nutrients obtained from the spider's diet.

 

As spiders consume their prey, the digestive enzymes break down the food into smaller molecules that can be absorbed across the gut wall. The Malpighian tubules then assist in the absorption of these nutrients, ensuring that essential compounds such as sugars, amino acids, and fats are efficiently assimilated into the spider's body.

 

Furthermore, the Malpighian tubules also play a crucial role in removing waste products and maintaining the spider's internal balance by regulating the concentration of ions and water in the hemolymph. Overall, these excretory organs are integral to the spider's digestive and excretory processes, ensuring optimal nutrient uptake and waste removal to support the spider's overall health and survival.

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Spinnerets

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Nestled at the posterior end of the abdomen, typically on the underside, are the spinnerets, remarkable organs that enable spiders to produce their silken threads. These threads serve a multitude of purposes, from building webs for prey capture to constructing egg sacs and safety lines for movement.

 

While six spinnerets represent the most common configuration across spider species, the diversity of spinneret arrangements is notable. Some species deviate from the norm, possessing four, two, or even eight spinnerets. Despite these variations, spinnerets generally share common features: segmentation and the ability to move independently and in coordination with each other.

 

The intricacies of spinneret structure and silk production are captivating subjects in the field of arachnology. For a deeper dive into these topics, including insights into the chemical composition of silk and its varied uses among different spider species, I will provide a separate post dedicated to exploring spinnerets further. Stay tuned for more detailed information!

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