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Scorpion

Scorpiones

Scorpions are a diverse group of terrestrial arthropods belonging to the order Scorpiones. There are approximately 2,500 described species within this group distributed worldwide. Often terrifying in appearance, they can be easily recognized by a conspicuous pair of grasping claws (pedipalps) and a piercing, segmented tail ending with a venomous stinger (telson).     

Although they are primarily found in deserts, scorpions can thrive in various habitats across all continents except Antarctica.

Description

Size

On average, scorpions are typically around 6 cm (2.5 inches) long. Some scorpion species stand out as giants, such as the black emperor scorpion (Pandinus imperator) of Guinea, reaching lengths of about 18 cm (7 inches) with a weight exceeding 60 grams (more than 2 ounces). The largest scorpion in the world is the Giant Forest Scorpion (Heterometrus swammerdami), which measures about 23 cm (9 inches) in length and weighs as much as 56 grams (2.0 oz). In contrast, one of the smallest scorpions, Microtityus fundorai, measures barely 12 mm (0.5 inches) in length.

Body Plan

A scorpion’s body is divided into two segments or tagmata.

1. Cephalothorax (Prosoma)

This segment comprises the eyes, chelicerae (mouth parts), pedipalps (which have chelae, commonly called claws), and four pairs of walking legs, all covered by a thick carapace.

2. Abdomen (Opisthosoma)

It is further divided into a broad anterior mesosoma or pre-abdomen and a narrow posterior metasoma or post-abdomen.

Mesosoma is the broad part of the opisthosoma, constituting segments from 2 to 8 due to the disappearance of the first segment during embryonic development. Each of the anterior seven somites is covered dorsally by a sclerotized plate called a tergite, while ventrally, somites 3 to 7 are covered by corresponding plates called sternites. Somite 1 on the ventral side features a pair of genital opercula covering the gonopore, while sternite 2 bears the basal plate supporting the pectines (sensory organs). The following four somites, 3 to 6, each bear pairs of spiracles for respiration, while the last somite bears no such notable structures. The mesosoma also contains the heart (dorsal vessel) and the components of the reproductive system.

Metasoma, commonly called the ‘tail,’ comprises five segments, including the telson. These segments, resembling body rings, lack distinct sterna or terga and possess keels, setae, and bristles, which are often significant for taxonomic classification. At the distal and ventral end of the final segment lies the anus, surrounded by four anal papillae and the anal arch.

The telson encompasses the vesicle with a symmetrical pair of venom glands. It also bears the curved stinger, or the ‘hypodermic aculeus,’ equipped with sensory hairs. Each venom gland has its own duct, conveying secretion along the aculeus from the gland’s bulb to the tip, where each duct opens through its own venom pore.

Organ System

Circulatory System

Scorpions possess an open circulatory system with a tubular heart located dorsally in the abdomen. Body fluid (hemolymph) flows through blood sinuses and spaces surrounding the internal organs within the hemocoel.

Respiratory System

They have four pairs of ‘book lungs,’ housed within a pulmonary chamber formed by an infolding of the exoskeleton. Around 140 to 150 parallel leaves, known as lamellae, are found within this cavity, providing a large surface area for gas exchange.

Digestive System

Excretory System

They possess two pairs of Malpighian tubules, which are tubular structures collecting nitrogenous wastes, such as xanthine, guanine, and uric acid, derived from protein metabolism.

These wastes are transported to the gut, from where they are expelled through the anus along with feces.

Nervous System

Scorpions possess a centralized nervous system comprising a brain and a ventral nerve cord. The upper lobe of the brain, known as the protocerebrum, processes optic information and governs complex behaviors, while the lower lobe, the tritocerebrum, regulates basic bodily functions. The nerve cord extends along the ventral side of the body, consisting of seven ganglia interconnected by pairs of fibers.

Sensory Organs

Reproductive System

In females, the gonads consist of three or four parallel tubes that serve as sites for oocyte formation and embryonic development. They are connected to two oviducts, which join to form a single atrium leading to the genital orifice.

In contrast, male scorpions have two gonads with cylindrical tubes arranged in a ladder-like configuration containing cysts responsible for spermatozoa production. Each tube terminates in a spermiduct, located on either side of the mesosoma, and connects to glandular structures known as paraxial organs, leading to the genital orifice.

Taxonomy

Scorpions are pulmonate arachnids (characterized by book lungs) placed under the subphylum Chelicerata. They are often considered a sister group to Tetrapulmonata, a clade comprising other arachnids like spiders and vinegaroons. However, recent research indicates that pseudoscorpions are the closest relatives to scorpions, forming the group Panscorpiones within the clade Arachnopulmonata.

Recently, scientists have classified around 22 different families of scorpions, including more than 2,500 extant species and 100 taxa of fossil scorpions based on Soleglad and Fet (2003.)

Scorpions (Scorpiones)

Distribution

Scorpions are found everywhere except in Greenland and Antarctica, with their greatest diversity in subtropical regions. Their range extends from Canada and central Europe to South America and Africa, while humans have unintentionally introduced them to countries like New Zealand, England, and some of the islands in Oceania.

Since the late 19th century, five colonies of Euscorpius flavicaudis have thrived in Sheerness, England ( 51°N), while Paruroctonus boreus (a species belonging to the family Vaejovidae) has been documented in Alberta (52°N).

Habitat

Although these arachnids are xerocoles that prefer living in deserts, they exhibit remarkable adaptability and can inhabit nearly all terrestrial habitats, including mountains, caves, and intertidal zones. However, they are nearly absent in boreal (subarctic) ecosystems like the tundra, taiga, and mountain peaks.

They generally thrive in microhabitats like ground, trees, rocks, or sand. While some species, such as Vaejovis janssi, live in all habitats on Socorro Island, Euscorpius carpathicus only occupy specialized niches in the rivers of Romania.

Diet

They are primarily insectivorous, feeding on insects like grasshoppers, crickets, termites, beetles, and wasps. They also consume other arachnids like spiders and solifugids and crustaceans like woodlice. These arachnids also hunt small vertebrates, including lizards, snakes, and even mammals like rodents. Sometimes, they may also feed on invertebrates like earthworms and mollusks.

While most scorpions are opportunistic feeders, some exhibit specialized dietary choices. For example, spiral burrow scorpions (Isometroides vescus) specialize in hunting burrowing spiders.

Behavior

Defense

When a scorpion feels threatened, it adopts various defensive strategies to protect itself from potential harm. Some common defensive postures include raising its claws and tail or rubbing the claws together (stridulation) to produce a warning sound, signaling its readiness to defend itself.

Moreover, some scorpions, like those in the genus Parabuthus, Centruroides margaritatus, and Hadrurus arizonensis, can spray venom in a narrow jet that is a potent deterrent to predators. Some species of the genus Ananteris can also shed their tails to escape a predator’s grip, while

others make direct, swift strikes with venomous stinger tails. A few other species employ slower, more circular motions that enable easy return to their strike position. For example, Leiurus quinquestriatus delivers rapid strikes with its tail whipped at an impressive speed of up to 128 cm/s.

Lifespan

Depending on the species, these arthropods survive between 5 to 25 years.

Reproduction and Life Cycle

Although most scorpions reproduce sexually, certain species, including those in the genera Hottentotta and Tityus, Centruroides gracilis, Liocheles australasiae, and Ananteris coineaui, have been found to reproduce through parthenogenesis, where unfertilized eggs develop into embryos.

During mating, receptive females emit pheromones to attract wandering males, who then engage in various courtship behaviors to communicate and bond with females. These include juddering, a rhythmic movement that produces ground vibrations sensed by females, and a courtship dance known as the ‘promenade à deux,’ during which both the male and the female move back and forth while searching for a suitable location to deposit the spermatophore (a packet containing sperm).

Once the male finds a suitable substrate, he deposits the spermatophore and navigates the female over it, thus allowing the sperm to transfer to the female’s genital operculum and fertilize her eggs. After mating, a mating plug forms in the female, preventing further mating until the offspring are born.

The gestation period in scorpions is long, sometimes lasting over a year in some species.

They exhibit two main types of embryonic development: 

  1. Apoikogenic development is primarily observed in the Buthidae family of scorpions, where embryos develop within yolk-rich eggs contained within follicles.
  2. Katoikogenic development is documented in families such as Hemiscorpiidae, Scorpionidae, and Diplocentridae, where embryos develop within a diverticulum, a teat-like structure serving as a feeding apparatus throughout development.

Unlike many other arachnids, scorpions exhibit viviparity, giving birth to live young instead of laying eggs. Before giving birth, the female elevates her body and joins the pedipalps and front legs to create a ‘birth basket’ to catch the emerging offspring.

The newborn scorpions emerge one by one from the female’s genital opercula and are placed on the mother’s back, where they remain until they undergo their first molt. During this pro-juvenile stage (5 to 25 days), the young cannot feed or sting but have suckers on their tarsi to cling to their mother for protection.

The brood molts simultaneously for the first time, marking the transition to the juvenile stage in which the scorpions closely resemble smaller versions of adults, with well-developed essential body parts, like pincers and stingers. They continue to ride on their mother’s back for protection until their exoskeletons harden fully, allowing them to hunt prey independently.

The number of molts required before reaching maturity varies among species, with some scorpions undergoing about six molts. Sexual maturity is usually achieved between 6 to 83 months, depending on the species.

Predators

They are preyed upon by owls, lizards, snakes, frogs, toads, and mammals like bats, rodents and shrews. Some animals, like meerkats, also specialize in feeding on these arachnids.

Adaptations

Interesting Facts

  1. As of 2021, the oldest known scorpion fossil is of Dolichophonus loudonensis, which lived during the Silurian Period in present-day Scotland (about 437 to 419 million years ago).
  2. They can increase their body weight by up to one-third during feeding periods. Despite their voracious appetite, they possess remarkably low metabolic rates and energy requirements and can easily survive six to twelve months without food.
  3. Scorpions belonging to the family Buthidae, including Leiurus quinquestriatus, Hottentotta spp, Centruroides spp, and Androctonus spp, bear deadly venom that could potentially kill humans.

References Article last updated on 9th April 2024
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