In this article, you will know more about the internal anatomy of scorpions. This is the second part of the article dedicated to scorpion anatomy (the first one was about their external structure).
The internal anatomy of scorpions can be divided into the following systems: the circulatory system, the respiratory system, the digestive system, the nervous system, and the reproductive system.
Without further ado. Let’s look at each of these systems in detail.
Quick Notes about Scorpion Internal Anatomy
| Scorpion’s internal organs | Function |
|
Cephalothorax (prosoma) includes |
|
| Supra-oesophageal ganglia (brain) |
Contains a few nerve cell clusters to control mostly the cheliceral and optic nerves |
| Sub-oesophageal ganglia | Controls the claws, walking legs, and abdomen |
| Ventral nerve cord | The central nervous system of scorpion. It coordinates neural signaling |
| Anterior artery | Supplies hemolymph (blood) to the prosoma |
|
Mesosoma |
|
| Heart | Pumps blood to all organs of the scorpion |
| Cardiac arteries | Supplies hemolymph (blood) to the mesasoma organs |
| Book lungs | Respiratory organ |
| Foregut | Mechanical and extracellular digestion |
| Midgut (and anterior intestine) |
Food digestion and nutrient absorption |
| Digestive glands | Secrete digestive substances necessary for digestion |
| Reproductive organs | In males, production of the spermatophore. In females, development of egg cells and embryo |
|
Metasoma |
|
| Posterior artery | Supplies hemolymph (blood) to the metasoma |
| Midgut (and posterior intestine) |
Collects indigestible matter |
| Anus | Excretes waste that cannot be digested. |
1. The Circulatory System of Scorpions
All scorpion species have an open circulatory system which is typical for arthropods. It includes the heart, hemolymph, aorta, and sinuses.
Note: The open circulatory means that scorpions almost do not have blood pressure and their entire abdominal cavity is filled with hemolymph (blood). It is almost ideal for simple animals (such as scorpions, crabs, crayfish, shrimp, etc.) with low metabolic rates. Scorpions have a particularly slow metabolic rate even compared to similar-sized arthropods.
- Heart. The tubular (single-chambered) heart is located dorsally and runs entirely in the pre-abdominal region (mesosoma).
The heart is perforated by small, seven pairs of slit-like pores (called Ostia).
- Hemolymph (or haemolymph). This is a fluid in the circulatory system of scorpions and is analogous to the blood. The hemolymph is colorless when deoxygenated and light blue when oxygenated.
This is because hemolymph contains hemocyanin, which gets its blue color from the copper that is part of its structure.
- Aortas. When the heart contracts, it sends the oxygenated blood into the spaces around the organs through the aortas (large arteries).
The anterior artery supplies the prosoma, six or eight pairs of cardiac arteries that supply organs of the mesosoma, and the posterior artery supplies the metasoma.
Arteries branch further into smaller arteries that carry blood to each organ, limb, and nerve cord.
- Sinuses. Smaller arteries also branch further and eventually empty into blood sinuses (open spaces).
Basically, the entire abdominal cavity is surrounded and filled with hemolymph that has direct contact with the scorpion’s internal tissues and organs. So, the blood is not segregated from interstitial fluid.
When the heart relaxes, the blood returns back to the heart through the pores (Ostia), to start the cycle again.
2. The Respiratory System of Scorpions
The respiratory system of scorpions consists of four pairs of book lungs. These are rudimentary or primitive lungs used for atmospheric gas exchange.
Note: Book lungs got their name due to their anatomical structure. Each book lung is made of a series of thin membranes (called Lamellae) that are arranged like the pages of a book. Each membrane is hollow which allows hemolymph to flow through them and saturate the body with oxygen.
The book lungs are located in the 4th, 5th, 6th, and 7th segment of mesosoma and can be noticed by the pronounced furrows (called Spiracles).
Depending on the scorpion species, the spiracles can be slits, circular, elliptical, or oval. They are muscular structures and control (open or close) the book lungs.
Scorpions do not breathe in the true sense of the word. Instead of active respiration, they utilize diffusion for gas exchange. For that, each book lung is surrounded by a sinus. So, when the heart pumps hemolymph (blood) it goes into and out of the lamellae.
In scorpions, ventilation occurs passively when animals move.
How long can a scorpion hold its breath? How long can a scorpion stay underwater?If you type these questions on the Internet, you will get the answer that scorpions can live underwater for at least 48 hours (in some cases even up to 6 days!). DO NOT BELIEVE IT. This so-called ‘fact’ is pure nonsense. Unfortunately, it is repeated almost everywhere. Even though scorpions are able to hold their breath for a few hours, most species will die within half hour or so. |
3. The Nervous System of Scorpions
Scorpions have a simple nervous system that consists of:
- Central nervous system. Consists of a supra-oesophageal ganglia (brain), sub-oesophageal ganglia, and ventral nerve cord.
- Peripheral nervous system. Connects to eyes, mouthparts, walking legs, pedipalps, pectines, genital operculum, and internal organs.
- Visceral nervous system. Controls the involuntary (autonomic) functions of the scorpion such as heart rate, digestive secretions, respiration, flight-to-fight responses, reflex actions, etc.
The brain (or Supra-oesophageal ganglia) is located in the back of the prosoma just above the oesophagus and beneath the median eyes. It mostly connects to the cheliceral and optic nerves.
Note: Actually, scorpions have a very rudimentary ‘brain’; it is a very small bilobed mass and consists of only a few nerve cell clusters (cerebral ganglion or ganglia). The ganglia interconnect by bundles of nerve fibers that transport the signals at high speed.
The sub-esophageal ganglion connects to the claws, walking legs, and abdomen.
Although two main ganglia are divided horizontally, the nerves radiate from both and form the peripheral nervous system.
The ventral nerve cord extends along the abdomen of the scorpion and runs to the rear end of the body (to the telson). It also has small nerve clusters at the end of each segment (walking legs, book lungs, etc.).
Even though the nervous system of the scorpions is not completely centralized, it still exhibits cephalization.
4. The Digestive System of Scorpions
The scorpion’s digestive system is essentially a long alimentary canal that is divided into 4 functional parts:
- Foregut
- Midgut
- Hindgut
- Digestive glands
| Scorpions have a complete digestive system. It means that food processing occurs within the alimentary canal, running lengthwise through the body from mouth to anus. |
Foregut
The foregut has functions in both mechanical and extracellular digestion. Basically, it stores, grinds, and transports food to the midgut.
Anatomically, the foregut can be divided into:
- Preoral cavity. It is formed in front of the mouth opening by chelicerae (fangs).
- Mouth.
- Pharynx. It is a suctorial bulb, leading to the oesophagus.
- Oesophagus. A short and narrow tubular passage into the midgut.
Midgut
This is where the main digestion takes place. It looks like a long tube and consists of:
- Stomach,
- Anterior intestine,
- Posterior intestine.
Hindgut
This is a very short part of the alimentary canal. The hindgut starts in the last metasomal segment and extends as far as the anus which is surrounded by 4 chitinous plates which keep it usually closed.
Digestive glands
They secrete digestive enzymes that allow more efficient digestion:
- Salivary glands. The small paired glands can be found on either side of the oesophagus.
- Gastric glands. The large, brownish, and lobulated paired glands are located in the dorsal part of the mesosomatic cavity (filling the whole of the pre-abdominal cavity). They secrete enzymes through 5-6 pairs of hepatic ducts.
The scorpion’s digestive system works in the following way:
- Scorpion uses their pincers to catch, hold, and/or tear apart the prey.
- Next, the food is brought close to the chelicerae (a scorpion’s jaws) to initiate digestion by tearing the food into even smaller particles.
- Inside the pre-oral cavity, the food is mixed with the secretion of the salivary So, the food gets into the stomach in a partly digested condition.
- A well-developed suctorial pharynx starts sucking all fluids from the partially digested
- The digestion process continues in the midgut (directed by the digestive glands) where digestion is completed.
- Next, the food reaches the intestine.
- Indigestible matter collects in the hindgut where it is expelled through the anus (just before the venom vesicle).
5. The Reproductive System of Scorpions
Scorpions are sexually dimorphic.
Generally, to differentiate between males and females, we have to look at the pectins. Males have a larger number of pectinal teeth compared to females.
However, in some scorpion species, there can be other pointers as well, such as:
Nonetheless, many of these traits are only present in mature adults and belong to a particular species. So, they cannot be a reliable factor in this matter. |
In scorpions, the male reproductive organs consist of:
- a pair of testes (to produce the spermatophore),
- a pair of paraxial organs (a glandular function; they are involved in the secretion of the spermatophore),
- a pair of flagella (to increase reproduction rates).
- a pair of seminal vesicles (to store the mature spermatophore),
- a pair of vasa deferentia (to transport spermatophore to genital chambers where it leaves the body),
- a pair of genital chambers (to serve as claspers during mating).
The female reproductive system consist of:
- a single ovary (development of egg cells),
- two oviducts (transport the oocyte from the ovaryand facilitate the transport of spermatophore),
- a median genital chamber (serves as a copulatory pouch during mating),
- a gonopore (an external genital opening; it is located on the 2nd abdominal segment and protected by a pair of small plates called the genital opercula).
Note: In females, the genital opercula are often fused, but not in males. However, in some species, males have a pair of genital papillae protruding from the posterior part of the operculum.
5.1 Fertilization
The process of mating in scorpions consists of 3 phases:
- introductory phase (‘cheliceral massage’),
- promenade phase (‘the scorpions’ dance’),
- spermatophore deposition phase.
In the introductory phase, the male grips the female’s pedipalps and approaches her at a very short distance. Next, he starts rocking his body forward and backward. After ‘the cheliceral massage’, the female generally became more cooperative and less aggressive.
In the promenade phase, the male starts to move backward and forward with the female following him. At the same time, the male’s first pair of legs sweep the substrate to clear a suitable place for spermatophore deposition.
In the third phase, the male lowers his mesosoma until the genital aperture touches the ground where he deposits spermatophore. Next, he pulls the female onto a proper position over the spermatophore, so it will be touched and collected by the female genital operculum. Fertilization is internal.
The mating process may continue for 30 to 90 minutes.
In Conclusion
In scorpions, the brain is located in the back of the prosoma and the mesosoma (pre-abdomen) contains most of their organ systems, including the circulatory system, the respiratory system, the reproductive system, and digestive glands.