In this article we will discuss about Scorpions:- 1. Habit, Habitat and Distribution of Scorpions 2. External Features of Scorpions 3. Appendages 4. Body Wall 5. Body Cavity, Musculature and Endoskeleton 6. Digestive System 7. Excretory System 8. Blood Vascular System 9. Respiratory System 10. Nervous System 11. Sense Organs 12. Reproductive Organs 13. Copulation, Fertilisation and Development.
- Habit, Habitat and Distribution of Scorpions
- External Features of Scorpions
- Appendages of Scorpions
- Body Wall of Scorpions
- Body Cavity, Musculature and Endoskeleton of Scorpions
- Digestive System of Scorpions
- Excretory System of Scorpions
- Blood Vascular System of Scorpions
- Respiratory System of Scorpions
- Nervous System of Scorpions
- Sense Organs of Scorpions
- Reproductive Organs of Scorpions
- Copulation, Fertilisation and Development in Scorpions
1. Habit, Habitat and Distribution of Scorpions:
Scorpions are nocturnal animals. During the day they hide under wood, stones, loose bark, in sand, crevices and holes, and in the debris on the ground, etc.
They come out of their abodes at night to hunt for food. They are carnivorous and predaceous, feeding for the most part on insects, spiders and other small animals. They catch their preys by the chelate pedipalpi, kill them with the sting and finally suck their juices. Cannibalism has also been found in them as they devour young ones of their own kind.
In walking, scorpions carry their pedipalps horizontally in front, using them partly as feelers and partly as raptorial organs. As regards the body, the attitude varies considerably. In some cases it is raised high upon the legs and the tail or metasoma is curved forward over the back, but in others the body is held low and the tail is dragged along behind, the end only being slightly curled.
In scorpions, the sense of touch is well developed. The pectines are said to be special tactile organs. Their sight is poor, and they are deaf. The effect of scorpion’s poison is not serious. In man and other large animals, the poison causes pain, local swelling and discolouration. A scorpion’s poison has no effect upon itself.
Scorpions are solitary creatures and never live in groups. If two scorpions are found under the same stone, one is engaged in eating the other. All scorpions are viviparous and the females carry the newly-hatched young on their backs.
Distribution of Scorpions:
Scorpions are inhabitants of warm countries all over the world. However, they are not found in New Zealand, South Patagonia and the Antarctic islands. Scorpions are fairly common in hilly regions of India particularly on slopes of hills. Palamneus is an Indian scorpion found all over from India to Philippines. It has many species, of which Palamneus bengalensis is common in North India.
2. External Features of Scorpion:
Different species of scorpions differ from one another in size, colour, distribution and details of morphology, but the general structure is more or less similar.
(i) Shape and Size:
The body of scorpion is elongated, narrow and dorsoventrally flattened. The size varies from species to species. The smallest scorpion, Microbuthus pusillus is about 1.3 cm in length, while the largest species Pandinus imperator is about 20 cm in length. In India, the adult of Buthus famulus measures about 7 to 9 cm in length, while the largest Indian scorpion Palamneus swammerdami measures up to 15 cm in length.
The colour is variable, usually corresponding with the habitat of the animal which is generally blackish dorsally and slightly light coloured ventrally. The species, which are living in tropical jungles, are of shining black colour, and the species which are found in the sand are pale-yellow in colour usually the dorsal surface is much darker in colour than those of the ventral surface.
(iii) Division of Body:
The elongated segmented body is divisible into two major regions:
(i) Prosoma or cephalothorax
(ii) Opisthosoma or abdomen.
The prosoma or cephalothorax is the anterior short, broad, and flat region of the body. It is formed by the fusion of head and thorax. It is made up of six segments, each of which bears a pair of appendages on the ventral side. Dorsally the prosoma is covered by a single, thick and square-shaped cephalothoracic shield or dorsal carapace.
The dorsal carapace is formed by fusion of terga of this region. The anterior margin of the dorsal carapace has a deep notch forming right and left frontal lobes. The carapace bears a pair of median eyes in the middle and two to five pairs of smaller lateral eyes on the anterolateral margins.
All the eyes are simple in structure. On the ventral surface of the prosoma there is a single median, small triangular plate, the sternum, lying between the coxae of third and fourth pairs of legs. The sternum is greatly reduced due to the enormous development of the coxae of thoracic appendages.
The prosoma is followed by a long opisthosoma.
It is distinguished into two parts:
(a) An anterior broad mesosoma or preabdomen and
(b) A posterior narrow metasoma or post abdomen.
The mesosoma or preabdomen consists of seven segments. It is as broad as the prosoma anteriorly but slightly narrow posteriorly. Each segment of mesosoma is covered dorsally by a tergal plate or tergum and ventrally by a sternal plate or sternum and the two are joined with each other laterally by a pleural or arthrodial membrane.
The sternum of the first mesosomal or pre-abdominal segment is small and bears the median genital aperture which is covered by a plate-like, rounded, bifid and movable lid or genital operculum.
The sternum of second segment bears a pair of comb-like appendages, the pectines. Each pectine consists of a three segmented stem or shaft or handle attached to the median line and along the posterior margin of which is a row of 4 to 36 narrow comb-like teeth.
The pectines are tactile organs. The sterna of third, fourth, fifth and sixth pre-abdominal segments bear a pair of lateral, oblique, slit-like openings or apertures, the stigmata. Each stigmata leads into a pulmonary sac or book-lung. The sternum of the seventh segment has no appendage.
The metasoma or post-abdomen is narrow, slender and consists of five cylindrical segments and a telson or sting. Each segment of metasoma has complete tergal and sternal arcs of octagonal shape formed by the complete fusion of terga, sterna and pleurons.
These segments are flexibly joined one behind the other. The last segment bears a telson which consists of two parts—the proximal rounded ampulla or vesicle and the distal pointed spine or aculeus. Inside the ampulla are two poison glands enclosed in smooth muscles, whose ducts open by a pair of minute apertures at the tip of the spine.
External Apertures of Scorpions:
These are as follows:
The mouth is a small transverse aperture situated antero-ventrally in the prosoma between the cephalic appendages. A small chitinous lobe or labrum hangs above the mouth.
The anus is a small median aperture situated ventrally at the base of the telson or sting.
(iii) Genital Aperture:
The genital aperture is situated mid-ventrally in the first segment of mesosoma and covered over by a genital operculum.
The stigmata are slit-like oblique exits of book-lungs. These are four pairs; one pair each in the mesosomatic segments from third to sixth situated ventro-laterally.
(v) Openings of Coxal Glands:
These openings are minute and paired, one in each coxa of the third pair of legs.
(vi) Openings of Poison Glands:
These are also paired, situated at the tip of spine or aculeus of telson or sting.
3. Appendages in Scorpions:
In scorpions, there are six pairs of appendages in the cephalothorax. These are a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs.
The chelicerae are the anterior-most and preoral appendages, situated close to each other on either side of the mouth. Chelicerae are homologous with antennae of crustaceans. Each chelicera is a small, three jointed and chelate appendage. The first or basal segment is small, triangular piece, lying concealed beneath the carapace.
The two distal segments form the chela or small pincer. The second segment is large, swollen and bears at its anterior end on the inner side a projection which forms the immovable finger of the chela. The third segment or movable finger is articulated on the outer side of the second segment. Both the fingers are armed with teeth. The chelicerae are prehensile and are used for holding and tearing up the body of prey.
Behind the chelicerae are a pair of powerful and clawed appendages called pedipalpi which are post-oral in position.
Each pedipalp is made of six segments, viz., coxa, trochanter, humerus, brachium, manus and movable finger. The coxa is small and situated on the side of the pre-oral cavity beneath the carapace. On each coxa there is a gnathobase towards
The gnathobases of both sides serve as jaws, protruding in the pre-oral cavity and help in squeezing and crushing the body of the prey. The trochanter is also a small segment bearing at right angles to the longitudinal axis, the powerful humerus. Then follows the brachium directed forward. The last two segments are opposed to one another and form the chela.
The manus or hand is the longest and most developed segment. It forms a powerful chela with the movable finger which is movably articulated on its outer in the middle. The biting edges of the movable and immovable fingers of pedipalpi are provided with rows of minute teeth which help in catching the prey. The pedipalpi move in the horizontal plane.
(iii) Walking Legs:
There are four pairs of walking legs attached to the cephalothorax. These are all alike and are used for walking. Each leg consists of seven segments or podomeres, viz., coxa, trochanter, femur, patella, tibia, protarsus and tarsus.
The coxae of the first three pairs of legs of both the sides are comparatively small, triangular pieces, while those of the fourth are longer.
The coxae of first two pairs of legs are movable and provided with forwardly directed triangular maxillary processes or gnathobases. The two coxae of the first pair of legs do not meet each other; those of second pair meet each other, while those of the third and fourth pairs are separated from each other by the sternum.
The coxae of the third and fourth legs of the same side are immovable and fused with each other. The trochanter is a short and stout segment, moving in all directions. The femur is long, straight segment moving freely upwards and little outwards. The patella is also long and moves downwards.
The tibia is elongated and provided with a tibial spur at its lower distal extremity. The pro-tarsus and tarsus are not so much long. The pro-tarsus bears a pair of dark tipped claw-like spurs, the pedal spurs. The tarsus bears three movable curved pointed claws, of which two are dorsal or superior and one is ventral or inferior. The inferior claw is generally worn out and, thus, only two are usually seen.
4. Body Wall of Scorpions:
The body wall of scorpions consists of three layers, viz., cuticle, hypodermis and basement membrane. The cuticle is the outer, non-living chitinous layer, which is brilliant in colour.
It is secreted by the underlying layer of hypodermis and consists of three distinct layers, an outer superficial layer or tetostracum, a middle layer or epiostracum and an inner layer or hypostracum. The cuticle is traversed by numerous canals which open to the outside at its surface. The hypodermis consists of a single layer of columnar cells filled with pigment granules.
Some of the hypodermis cells are modified so as to form hairs which project above the cuticle and are sensitive to touch. The basement membrane is a thin and structure less layer, situated below the hypodermis.
5. Body Cavity, Musculature and Endoskeleton:
In scorpions, the body cavity is haemocoel as in cockroach. It is filled with blood surrounding the heart, alimentary canal with its associated glands, gonads and other internal structures.
In scorpions, there is one pair of muscles in the cephalothorax and eight pairs of dorso-ventral muscles in the preabdomen, extending from dorsal to the ventral surfaces on either lateral sides of body wall.
In scorpions, there is an endoskeleton known as endosternite lying somewhat obliquely at the junction of the prosoma and mesosoma. It lies inside the body dorsally to the nerve cord and ventrally to the alimentary canal. It is triangular in structure and produced into several paired processes, the anterior, posterior and horizontal which provide surface for the attachment of muscles.
Posteriorly, the body of endosternite is fused with a chitinous plate called diaphragm, which is perforated by the nerve cord, alimentary canal, muscles and dorsal aorta.
6. Digestive System of Scorpion:
Alimentary Canal of Scorpion:
The alimentary canal of scorpions is a fairly uniform and more or less straight tube extending from the anterior to the posterior end of the body. It is differentiated into four distinct regions, viz., the pre-oral cavity, the foregut or stomodaeum, midgut or mesenteron and the hindgut or proctodaeum.
1. Pre-oral Cavity:
The pre-oral cavity is a small cavity enclosed between the basal segments of the first four pairs of appendages. Laterally, it is bounded by the coxae of the pedipalpi, above by the rostrum, in front by the coxae of the chelicerae and below by the two pairs of the maxillary processes of the coxae of the first two pairs of walking legs. All these structures bear bristles or hairs which protrude into the pre-oral cavity.
A laterally compressed cushion-like structure called rostrum or buccal appendage protrudes into the pre-oral cavity from the posterior side. Below the base of the rostrum is situated a narrow transverse mouth.
The rostrum is provided with a set of muscles which can roll it into a tube for sucking the fluids into the pharynx. At times, if need be, the rostrum can also be contracted to allow a broader passage into the mouth opening.
2. Foregut or Stomodaeum:
The foregut or stomodaeum includes mouth, pharynx and oesophagus. The foregut is internally lined by the cuticle.
The mouth is a small, narrow and transverse opening situated below the base of rostrum and behind the pre-oral cavity. The mouth leads into the pharynx. The narrow mouth can only admit juices and pulps.
The pharynx is a pear- shaped, bulbous and muscular structure situated obliquely below the mouth. It is provided with a thick chitinous internal lining and numerous muscles outside. The muscles are distinguished into sphincters and dilators. The sphincter muscles keep the pharynx contracted, while the dilator muscles dilate it considerably. Therefore, the pharynx acts as a powerful sectorial organ for sucking the liquid food.
The pharynx is followed by a small narrow and delicate tube, the oesophagus which passes through the nerve ring and opens into the midgut or mesenteron.
It is also provided with an internal chitinous lining. At its posterior end, it protrude a short distance into the lumen of stomach so as to form a sleeve valve which prevents the regurgitation of food into the pharynx. This function is also supplemented by the presence of sphincter muscles at the posterior end of the oesophagus.
3. Midgut or Mesenteron:
The midgut or mesenteron is differentiated into the stomach and intestine with the two digestive glands. The mesenteron is lined internally by the epithelium and devoid of internal chitinous lining.
The stomach is slightly dilated thin- walled part of the alimentary canal situated in the cephalothorax and extending between the oesophagus and diaphragm. It is covered above and on sides by an irregular brownish gland called stomach gland or the gastric gland.
This gland comprises three lobes—two lateral and one median dorsal and opens into the stomach by a pair of short lateral ducts. Huxley regarded this gland to be salivary gland but Blanchard and Pavlovsky have shown that it cannot be considered a salivary gland either by reason of function or position and, therefore, modern workers called it stomach gland.
The intestine is the longest part of the alimentary canal extending from the diaphragm to the last segment of the body where it passes into the hindgut. It is divisible into two regions, the anterior pre-abdominal region or hepatopancreatic region or pars tecta intestine and the posterior post-abdominal region or ileum or pars nuda intestine.
The anterior pre-abdominal region is narrower and has a thin wall lined with glandular epithelial cells. It has the opening of five pairs of short hepatopancreatic ducts arising from hepatopancreas. At the junction of pre- and post-abdominal intestines, arise two pairs of Malpighian tubules.
The posterior, post-abdominal region of intestine or ileum extends from the thirteenth segment to the eighteenth segment. It is comparatively wider, thin-walled tube lined with tall epithelial cells arranged uniformly in the internal epithelium. The walls of the post-abdominal intestine are feebly muscularised, only few circular and longitudinal muscular fibres are present.
(iii) Digestive Glands:
These are largely represented by the hepatopancreas or chylenteron, also referred to as liver. The hepatopancreas is a large, brownish and lobulated gland filling the whole of the pre-abdominal cavity. Its lobules penetrate the various internal organs.
The wall of the lobules is composed of a layer of epithelium arranged on a thin membrane or tunica propria and a peritoneal covering. The cells of the epithelial layer are of two types; the more numerous large absorptive cells and the fewer smaller ferment cells.
4. Hindgut or Proctodaeum:
The hindgut is the smallest and bulbous part of the alimentary canal, situated in the last or eighteenth segment. It is lined internally by chitin and opens externally by the anal aperture situated on the ventral side between the last segment and the telson. The anus is surrounded by four chitinous plates which keep it normally closed.
Food and Feeding of Scorpion:
Scorpions are carnivorous animals and the food consists of small insects and spiders. They live purely on the liquid food derived as juice from the body of the prey. The prey is seized by chelate (pincers) of pedipalpi and stung to death by the telson. The pedipalpi pass the killed prey to chelicerae, one chelicera holds the food and other rips it open.
The coxae of the pedipalpi and first two pairs of legs now press up on the prey so that the entire body fluid is squeezed out into the pre-oral cavity. Meanwhile, the rostrum forms a groove which is applied to the prey and the liquid is sucked into pharynx and then it is driven into oesophagus and stomach.
Digestion, Absorption and Egestion of Scorpion:
Inside the pre-oral cavity, the food is mixed with the secretion of the alveolar glands situated in the maxillary processes of the first two pairs of legs. This secretion contains proteolytic enzymes. The proteins are proteolysed and the food is reduced into pulp.
Thus, the food reaching the stomach is in a partly digested condition. In the stomach the partly digested food is mixed with the secretions of stomach gland containing the enzymes pepsin, erepsin and lipase, etc.
Most of the digestion is, thus, completed in the stomach. When the food reaches the intestine, it passes into the hepatopancreas where digestion is completed by the enzymes already mixed with the food and those secreted here, i.e., amylase and catalase.
The digested food is absorbed in the hepatopancreas and the pre-abdominal intestine, while the undigested part reaches the post-abdominal intestine and hindgut to be expelled out through the anus.
7. Excretory System of Scorpions:
The excretory organs of scorpion are:
1. Malpighian tubules
2. Coxal glands
1. Malpighian Tubules:
There are two pairs of Malpighian tubules arising from the junction of pre-abdominal and post-abdominal intestine. These tubules are tubular, anteriorly directed and endodermal in origin. They possess thin syncytial walls which remove the waste from the blood. The waste is then excreted into the lumen of the tubules as guanine crystals which pass out through the alimentary canal.
The transverse section of Malpighian tubule reveals that a thin peritoneal sheath externally covering a few longitudinal and circular muscle fibres and excretory epithelial cells arranged on a basement membrane. The epithelial cells show striations at their bases.
2. Coxal Glands:
There is one pair of coxal glands situated near the bases of the third pair of walking legs in the fifth segment of the prosoma. The coxal glands are shining white structures and are coelomoducts of coelomic origin. Each coxal gland consists of a large excretory saccule or vesicle or end sac and a much coiled tube, the labyrinth which dilates near its end to form a swollen bladder or reservoir.
The reservoir opens outside by the excretory pore on the posterior face of the fifth walking leg. The saccule and labyrinth of each coxal gland collect excretory nitrogenous wastes from the blood and pass them outside through the excretory pore.
The hepatopancreas also acts as an excretory organ. Pavlovsky has shown that particles of ammonical carmine injected into the body of scorpion are picked up by the cells of hepatopancreas. Pearson believes that the ferment cells of the hepatopancreas also pick up excretory products. Thus, hepatopancreas also assists in the excretion.
Large nephrocytes and lymphatic organs are specialised excretory structures. These are found under the body wall in the mesosoma. These are believed to be excretory as well as phagocytic in function.
8. Blood Vascular System of Scorpions:
Like other arthropods, the blood vascular system in scorpion is of open type.
It consists of:
1. Heart enclosed in pericardium
1. Heart and Pericardium:
The heart is an elongated and muscular tube extending back from behind the diaphragm. It is enclosed in a thin-walled membranous sac, the pericardium. The heart is suspended into the pericardium by a pair of connective tissue bands in each segment called ali cordi. The heart is incompletely divided into seven distinct chambers by means of shallow constrictions in the middle of each segment.
Each chamber bears a pair of ostia (opening) in its dorsolateral side through which it communicates with the pericardial cavity. The wall of each ostium is slightly drawn into the form of a valve which allows the flow of blood from the pericardium into the heart but not in the opposite direction. The heart is able to perform slow peristaltic movements due to the muscular walls.
The heart is continued anteriorly as the anterior aorta and posteriorly as the posterior aorta. A pair of lateral systemic arteries are also given off from the each chamber of the heart.
(i) Anterior Aorta:
The anterior aorta arises from the anterior end of the first chamber of the heart and runs anteriorly along the dorsal surface of the alimentary canal. It is a short and wide vessel which gives off arteries to the different structures of the prosoma.
The main arteries given off from the anterior aorta are as follows:
(a) A pair of small visceral arteries which supply blood to the intestine and hepatopancreas.
(b) Beyond the diaphragm but before the brain a pair of arteries arises from the anterior aorta which encircle the oesophagus and unite ventrally to form a supraneural artery which runs medially above the ventral nerve cord supplying the nerve cord and ventral muscles.
(c) The anterior aorta, also gives off six pairs of arteries which supply the appendages of prosoma, brain and eyes, etc.
(ii) Posterior Aorta:
From the posterior end of the heart arises a posterior aorta or caudal artery which runs backwards on the dorsal side of the intestine up to the last segment.
It gives off a pair of arteries in each segment of the post abdomen and then divides into three branches:
(a) A median branch extends ventrally to join the suprarenal artery
(b) Two lateral branches run into the posterior lobes of the hepatopancreas and supply the muscles of the fourteenth segment.
(iii) Systemic Arteries:
From each chamber of the heart arises on either side a systemic artery. These arteries form a network and supply the various parts of the preabdomen.
All the arteries break up into numerous small branches and sub-branches and finally pour their blood into narrow spaces, the lacunae. From these lacunae the blood is collected into large spaces called sinuses. There are three important sinuses in the scorpion, viz., ventral sinus, dorsal sinus and perivisceral sinus.
The ventral sinus extends along the whole length of the body below the ventral longitudinal muscles. It collects blood from the ventral part of the body. It also sends a diverticulum to each book-lung to supply blood to the latter.
The dorsal sinus runs along the dorsal side of the pericardial sinus or pericardium. It collects blood from the dorsal part of the body. It is connected to the ventral sinus through the lateral sinuses. The perivisceral sinus surrounds the alimentary canal and is formed by the union of various lacunae between different visceral organs. It also communicates with the ventral sinus.
4. Pulmonary Veins:
From the book-lungs the oxygenated blood is sent back to the pericardial sinus by four pairs of pulmonary veins which are continued over the dorsal side of the book-lungs as lacunae.
The blood or haemolymph of scorpion is colourless and it contains non-nucleated amoebocytes or leucocytes. The respiratory pigment in blood is haemocyanin which is found dissolved in the plasma. The blood becomes bluish in colour when oxygenated due to the presence of respiratory pigment (haemocyanin) having copper metallic base in contrast to the hemoglobin with iron metallic base.
Course of Circulation:
By the contraction of the ligaments, the cavity of the heart enlarges and the blood of the pericardial sinus passes on to the heart through ostia. Afterwards, the heart contracts and sends the blood into various organs and parts of the body through the anterior and posterior aortae and the systemic arteries which are connected with the blood lacunae.
The impure or deoxygenated blood from the viscera is collected in the ventral sinus. From the ventral sinus blood passes to the book- lungs, enters the lamellae and is oxygenated. From the book-lungs the oxygenated blood is sent to the pericardial sinus by four pairs of pulmonary veins to enter the heart through ostia.
The course of circulation of blood in scorpion can be diagrammatically represented as follows:
9. Respiratory System of Scorpions:
The respiratory system of scorpions is highly characteristic and consists of four pairs of book-lungs (lung-books) or pulmonary sacs.
Structure of Respiratory System in Scorpions:
The book-lungs are found situated on the ventrolateral sides in the third, fourth, fifth and sixth mesosomal or pre-abdominal segments. Each book-lung consists of a compressed sac-like cavity formed by the invagination of the body wall and lined with a thin layer of cuticle, the opening of which is called the stigma.
The cavity of the book-lung is divisible into two parts, the smaller proximal part is called the atrial chamber which opens to the outside by the stigma, while the larger distal part is the pulmonary chamber. Each pulmonary chamber contains about 150 vertical folds or leaf like lamellae.
These lamellae are attached to the inner posterior side of the chamber and are arranged like the leaves of a book. Each lamella is a hollow structure made of two thin layers of cuticle united at their edges.
The outer side of the lamellae bear ridges and bristles which keep the adjacent lamellae apart and a little air space (inter- lamellar space) is left between them for the flow of air. The roof of the atrial chamber is perforated by many linear slit-like openings, set parallel with another. The atrial chamber communicates with the inter-lamellar spaces containing air through these openings.
The blood flows continuously in the spaces inside the lamellae, while the inter-lamellar spaces are filled with the air so that exchange of gases takes place through the thin walls of the lamellae which separate the blood contained in the lamellar cavities from the air in the inter- lamellar spaces.
The impure blood from ventral sinus is sent to each book-lung by a diverticulum from where it enters the lamellae at their bases. The blood is aerated or purified in the lamellae of each book-lung and is collected into a pulmonary vein which travels dorsally and opens into the pericardium.
Mechanism of Respiration:
The inspiration and expiration of air in the book-lungs is controlled by the action of the dorso-ventral and atrial muscles. When the dorso-ventral muscles are contracted the pulmonary chamber is compressed and the air in between the lamellae is forced out into the atrial chamber then to the outside through the stigma due to the contraction of atrial muscles.
When the atrial muscles relax, the book lungs resume their normal shape and the fresh air enters through stigma first into the atrial chamber and then into the inter-lamellar spaces.
At this time dorso-ventral muscles relax and pulmonary chamber resumes its normal shape. The gaseous exchange takes place through the thin membranous walls of the lamellae and the blood becomes oxygenated and carbon dioxide is expelled out into the air.
10. Nervous System of Scorpions:
The nervous system consists of central nervous system and peripheral and visceral nervous system.
1. Central Nervous System:
The central nervous system includes the brain or cerebral ganglion, circumoesophageal connectives, sub-oesophageal ganglion and ventral nerve cord.
The brain or cerebral ganglion is a small bilobed mass situated in the prosoma just beneath the median eyes. Microscopical and embryological studies reveal that the brain develops from the fusion of eight pairs of ganglia. A pair of optic nerves is given out from the brain to the median and lateral eyes; brain also gives off numerous fine nerves to rostrum, pharynx and oesophagus.
(ii) Circumoesophageal Connectives:
From the brain arises a pair of thick, short and stout circumoesophageal connectives which encircle the oesophagus and unite ventrally in a sub-oesophageal ganglion. In this way a thick nerve-ring is formed surrounding the oesophagus. From the connectives and ganglion six pairs of lateral nerves are given out to six pairs of cephalothoracic appendages.
(iii) Sub-oesophageal Ganglion:
It represents the first ganglion fused with sub-oesophageal mass.
(iv) Ventral Nerve Cord:
The sub-oesophageal ganglion is continued backwards into the abdomen as double ventral nerve cord. It extends up to the fourth segment of the post-abdomen.
The nerve cord bears three segmental ganglia in the preabdomen and four in the post-abdomen. The last ganglion is located in the fourth post-abdominal segment. The nerve cord is rounded and slender in the preabdomen but flattened and ribbon-shaped in the post-abdomen.
2. Peripheral and Visceral Nervous System:
Numerous nerves are given out from the brain and the segmental ganglia of the ventral nerve cord which constitute these nervous systems. As the brain gives off a pair of optic nerves to the median eyes, a pair of nerves which branch and supply the chelicerae and the lateral eyes, a pair of nerves to the rostrum, a pair of nerve trunks to the pedipalpi and four pairs of nerves to the four pairs of walking legs.
Besides these, the brain also supplies nerves to the pharynx, oesophagus and heart. The sub-oesophageal ganglion may send two to four pairs of vagus nerves which run posteriorly into the preabdomen to supply the genital operculum, the pectines and the first two pairs of book-lungs.
The segmental ganglia of the preabdomen send out nerves to the last two pairs of book-lungs, heart, and muscles of abdomen, while those from the segmental ganglia of the postabdomen supply nerves to the segmental muscles. The last ganglion also supplies the sting.
11. Sense Organs of Scorpions:
The sense organs and receptor organs of scorpions are sensory setae or sensillae, pectines and eyes.
1. Sensory Setae or Sensillae:
The whole body of scorpion is covered with sensory setae (sensillae) and hairs which are generally much longer on the legs and tail. The sensory setae aretactile organs of scorpion.
Each of the sensory setae is connected to a sensory cell located at its base which in turn is connected to a nerve fibre. On the podomeres of pedipalpi, the hairs are long and bristle-like and originate from cuticular integumental pits called setigerous, setal or bristle pores. The hairs are extremely sensitive to touch.
The pectines are a pair of modified appendages lying attached to the sternum of the second segment of mesosoma joined to each other in the middle. Each pecten (Fig. 72.16) consists of a shaft or handle made of three pieces which bears a series of movable lamellae or teeth arranged like the teeth in a comb on the posterior margin of the shaft in the outer half.
The number of teeth differs from species to species. The teeth of the pecten are sensitive either to touch or smell. Pocock observed a scorpion walking over a cockroach until the pectines come in contact with it when it suddenly withdrew and preyed upon it.
There are four pairs of eyes in scorpions, a pair of median eyes and three pairs of lateral eyes.
(i) Median Eyes:
Median eye is like a cup covered externally by a cuticular lens or cornea which is continuous with the cuticle but is much thicker. Inside the pigmented cup are rhabdomes, each enclosed inside several retinal cells which receive nerve fibres of an optic nerve. Median eyes of scorpion are intermediate between compound and simple eyes of insects.
They resemble a compound eye in having their retinal cells arranged in groups around each rhabdome as in ommatidia, but unlike the compound eyes of insects and crustaceans the sensory retinal cells both receive stimuli and transmit impulses. They resemble a simple insect eye in not being faceted but having a single lens. Both lateral and median eyes are sensitive to light changes but are incapable of forming images.
(ii) Lateral Eyes:
Lateral eyes are like simple eyes of insects. It is like a pigmented cup covered externally by a biconvex lens formed from transparent cuticle, inside the cup are several longitudinal optic rods called rhabdomes associated with retinal cell or retinulae. The retinal cells receive nerve fibres.
It is also regarded that in some species of scorpions, stridulating organs are found on the coxae of the pedipalpi or first pair of legs, in the form of ridges across which file-like surface can be drawn to produce sound. It indicates that scorpions probably have some perception of sound.
12. Reproductive System of Scorpions:
The sexes are separate and scorpions show some sexual dimorphism. The male is generally smaller, has a narrower abdomen, longer pedipalpi, longer tail and a larger number of pectinal teeth. In male the operculum is formed of two flaps. The female is bigger having broader abdomen, smaller pedipalpi, shorter curved tail and fewer pectinal teeth.
Male Reproductive Organs of Scorpions:
The male reproductive organs are a pair of testes, a pair of vasa deferentia, genital chambers, paraxial organs and common genital chamber.
There are two testes, extending from third to the sixth segment of the preabdomen and lying embedded in the hepatopancreas. Each testis consists of two slender, thread-like whitish longitudinal tubes lying parallel and joined to each other by four transverse tubes in such a way as to form three squares.
The transverse section of each testis tubule shows that it consists of an outer layer of connective tissue, a thin basement membrane and the germinal cells.
The germinal cells are divided into several chambers by septa of connective tissue. In each chamber spermatogonia, spermatocytes and sperms are seen. The sperms are arranged in bundles. A mature sperm of scorpion is filiform and motile having an oval body and a long tail.
(ii) Vas Deferens:
From the outer angle of each testis arises a narrow duct, the vas deferens which runs forward and opens into the genital chamber of its side. The distal portion of the vas deferens is dilated so as to form the terminal ampulla before entering the genital chamber of its side.
The accessory glands (cylindrical and oval glands) and vesicula seminalis open into the anterior terminal part of the vas deferens. The vesicula seminalis is a club-shaped organ of yellow colour. The accessory glands secrete a fluid which helps in reproduction, while the vesicula seminalis serves to store the mature spermatozoa.
(iii) Genital Chamber and Paraxial Organ:
Each genital chamber is a prominent muscular tube and produced behind into a paraxial organ which contains a tightly fitting chitinous rod, the flagellum.
The structure of flagellum is different in different species of scorpions. It consists of a supporting shaft with a groove inside it. Few spines are also found on one side of the flagellum. The two flagella or chitinous rods together form the so-called double penis of scorpion which serve as claspers during copulation.
(iv) Common Genital Chamber:
The two genital chambers continue forwards and open into a small, median common genital chamber which opens to the outside by a narrow male genital aperture, situated beneath the genital operculum on the sternum of first pre-abdominal segment.
Female Reproductive Organs of Scorpions:
The female reproductive organs consist of the ovary, oviducts and common genital chamber.
The ovary is single which extends from the third segment to the sixth segment of the preabdomen and lies embedded in the hepatopancreas. It consists of three longitudinal hollow tubules (a median and two lateral) called the ovarioles which are interconnected by four narrow transverse tubules so as to form three pairs of squares.
The ovarioles and their cross branches are lined by germinal epithelium, the cells of which form the ova. The mature ova project into hollow bud-like processes on the surface called the diverticula or the follicles.
The embryos start development inside the follicles as a result of which each follicle forms an elongated structure now called uterus. The uterus is differentiated into a proximal wider region or embryonic chamber and a distal narrow appendix terminating into an enlarged tip called the head of appendix.
Thus, in a mature fertilised female several such uteri are seen emerging from each longitudinal and transverse tubules of the ovary. The shape of follicles varies in different species of scorpions and the size depends upon the size of the developing embryo within it. The follicles are fusiform in Palamneus and spherical in Buthus.
The two lateral ovarioles continue anteriorly as the oviducts which converge towards the mid-ventral line to open into a small median common genital chamber. The common genital chamber opens to the outside by the female genital aperture situated beneath the bilobed operculum on the sternum of the first pre-abdominal segment.
13. Copulation, Fertilisation and Development in Scorpions:
The fertilisation in scorpions is internal and it is followed by the courtship or copulation.
Fabre has given a good account of the mating habits of scorpions. According to him, male and female scorpions stand face, to face with raised tails which they intertwined. The male grasps the pedipalpi of the female with its own and the two scorpions go on moving hand in hand for an hour or more during which the animals turned several times.
This is a sort of mating or nuptial dance termed as ‘promenade a deux’ by Fabre. At length, the male would dig a hole in the neighbourhood of a stone and both would disappear into the hole under the stone. After mating, the female often devours the male. In India scorpions breed during monsoons.
Development of Scorpions:
In scorpions, the development is internal and very slow and some species take several years to reach maturity. Scorpions are viviparous. The female scorpions give to two to three dozen young ones at a time. The young ones resemble the adults. For some time the mother carries the newly-hatched young ones on its back and takes care of them. The young ones undergo several moults to grow into the adults.