Scorpions (With Diagram) | Phylum Arthropoda

In this article we will discuss about Scorpion:- 1. Habit and Habitat of Scorpion 2. External Structures of Scorpion 3. Integumentary System 4. Endoskeleton 5. Digestive System 6. Respiratory System 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Development.

Contents:

1. Habit and Habitat of Scorpion
2. External Structures of Scorpion
3. Integumentary System of Scorpion
4. Endoskeleton of Scorpion
5. Digestive System of Scorpion
6. Respiratory System of Scorpion
7. Circulatory System of Scorpion
8. Excretory System of Scorpion
9. Nervous System of Scorpion
10. Reproductive System of Scorpion
11. Development of Scorpion

1. Habit and Habitat of Scorpion:

The scorpions are usually inhabitants of warm tropical regions. They are nocturnal, cryptic animals well known for their rapa­cious habit. They love shade and avoid heat. Some live in deserts, some in mountainous areas, and several scorpions are seen in rain forests. These organisms spend the daytime within sands, crevices, holes and under stones and logs.

Often the scorpions are seen within the household furniture and clothing’s. At night they become active and prey upon spiders, cockroaches and other insects. While walking it usually raises the body from the ground on the legs. Scorpions can live for a long time without food and water and often are seen to practise cannibalism. It can dig hole by its three anterior pairs of legs and possesses delicate sense of touch.

2. External Structures of Scorpion:

The body of scorpion is elongated, nar­row and dorsoventrally flattened (Fig. 18.107). The colouration of scorpion varies from yellow, orange to black and it depends upon the background of its living place.

The segments are distinct and are organised to form two tagmata—Prosoma or Cephalo­thorax and Opisthosoma or Abdomen. Both these parts have exoskeletal coverings and several structures are present in the two regions.

A. Prosoma or Cephalothorax:

As in Prawn, here too, the head and thorax have fused to form prosoma. It is enclosed dorsally by a single square cara­pace formed by the fusion of terga. Ventrally it is bounded by a single triangular plate, formed by the fusion of sternites. A median notch splits the anterior margin of the cara­pace into two frontal lobes.

Following struc­tures are present in the cephalothorax:

1. Eyes:

The dorsal side of the carapace bears a pair of median eyes. Several lateral eyes are seen along each side of the carapace. Number of lateral eyes on each side varies in different species but usually ranges from 2- 5. Both the median and lateral eyes are simple eyes but differ in their construction. Some scorpions (cave-dwelling forms) have secondarily lost their eyes.

2. Appendages:

Six pairs of appendages on the ventral side of the prosoma include a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs (Fig. 18.108).

(a) Chelicerae:

These small paired ap­pendages are present on the anterior side of the mouth. Each one is composed of three articles. The proximal article remains be­neath the carapace and the other two parts constitute a distal chela or cutting surface. When not in use, the chelicerae remain hid­den beneath the carapace.

(b) Pedipalpi:

This second appendage is postoral and consists of following six articles from proximal to distal end—coxa, tro­chanter, humerus, brachium, manus and movable finger. At the proximal end, the coxa carries an anteriorly directed sharp blade-like structure, called gnathobase. The gnathobases of the two sides work like scis­sors to cut the prey.

At the distal end, the extension of manus forms an immovable finger. The inner borders of the movable and immovable fingers carry denticles and serve as chela for holding. The pedipalpi are strongly developed and are used for captur­ing prey, feeling the environment and to hold the partner during nuptial dance.

(c) Walking legs:

Each of the four pairs of walking legs is composed of seven articles and at the distal end bears a pair of sharply pointed claws. The seven parts from proxi­mal to distal end are—coxa, trochanter, femur, patella, tibia, pretarsus and tarsus. The en­tire surface of each leg is beset with bristles and spurs.

In each of the first and second pair of legs, the coxa is movable and has anteriorly directed gnathobases which operate during feeding. In the remaining legs, the coxae are immovable and devoid of gnathobases. The sternum, in between these two pairs of legs, bears a specific outline, which is of consid­erable importance in classification.

B. Opisthosoma or Abdomen:

It is flat and immovably attached to the cephalothorax. The opisthosoma is divisible into two parts—mesosoma and metasoma. The seven segmented mesosoma is broad at the anterior end and narrow posteriorly.

The metasoma is narrow, elongated and made up of five segments which appear as jointed rings. The metasoma remains bent over the body and is movable. A soft skin separates the terga and sterna of mesosoma, but such skin is absent in metasoma.

Following structures are seen in the abdomen or opisthosoma:

1. Genital operculum:

It is present as a movable, bifid flap on the mid-ventral re­gion of first mesosoma. It covers the repro­ductive opening.

2. Pectines:

These peculiar paired struc­tures are present on the ventral side of sec­ond mesosomatic segment. Each pectine is formed of a three-segmented shaft carrying at the free posterior end a row of 4-36 movable processes like the teeth of a comb. Pectines are tactile sense organs and are probably olfactory too. Pectines are larger in males than in females.

3. Stigmata:

These are slit-like openings communicating with the inner respiratory organs, called the book-lungs. Each segment from third to sixth bears on the ventro-lateral border a pair of such stigmata.

4. Poison gland:

The terminal segment of metasoma is bulb-like and contains a slightly curved sharply pointed poison sting. The bulb contains a pair of poison glands which open on each side below the tip. The glands are operated by a special muscle inside the bulb. The venom is used to kill the prey. It contains a paralysing neurotoxin which may affect respiratory movement specially in children and elderly people.

3. Integumentary System of Scorpion:

The integumentary system forms the exoskeletal covering. As in other arthropods, here also it consists of three parts—cuticle, hypodermis and basement membrane.

(i) Cuticle:

The cuticle is divisible into a very thin (0.004 mm) outermost epicuticle and a thick basal procuticle. The epicuticle does not contain chitin and is imperviou, to water.

The procuticle contains chitin and a polysaccharide related to cellulose and is permeable to water. The procuticle may again be divided into an outer exocuticle and inner endocuticle. The cuticle is traversed by nu­merous canals which open through minute pores on the outer surface.

(ii) Hypodermis:

It is composed of a mono­layer of cubical epithelial cells. Some cells are peculiarly modified as tactile hairs and such hairs project out of the cuticle. The cuticle is formed by a product secreted by this hypodermal layer. Various colouration of scorpion is due to the pigment content of the hypodermal cells.

(iii) Basement membrane:

It is a thin non- cellular layer between hypodermis and the lower muscle layers.

In various parts of the body the integu­ment may project either entirely as a hollow process (spine, claw) or by the protrusion of cuticle alone (denticles or chela).

4. Endoskeleton of Scorpion:

The coxae of the walking legs are inserted within the body and form internal skeletal structures for the attachment of muscles. These inner protrustions of coxae are called apodemes. In addition to apodemes, a large triangular endosternite and a transversely placed diaphragm, work as endoskeletal framework.

The endosternite is present ob­liquely between prosoma and mesosoma and has three pairs of processes—anterior, poste­rior and horizontal for the attachment of muscles. The diaphragm is attached ventrally with the coxae of the fourth legs and dorsally to the inter-segmental membrane between prosoma and the tergum of eighth segment.

5. Digestive System of Scorpion:

It consists of alimentary canal and digestive glands (Fig. 18.112A).

(i) Alimentary canal:

In addition to the usual three divisions—foregut, midgut and hindgut, there is a distinct pre-oral cavity (Fig. 18.109) in front of the foregut.

The pre-oral cavity is bounded dorsally by two chelicerae, ventrally by the gnathobases of the first two pairs of legs and laterally by the coxae of the pedipalpi. A club-shaped labrum or rostrum hangs within the pre-oral cavity. Fig. 18.109 shows the transverse sec­tion of the pre-oral cavity of scorpion.

The sclerotised foregut begins with a small opening, called mouth (see Fig. 18.112A). It is placed inside the pre-oral cavity and near the base of the labrum. The mouth leads into a well-developed suctorial pharynx. The wall of pharynx is provided with numerous muscles.

The pharynx acts as a pumping organ to suck food through mouth. The pha­rynx continues into a narrow cuticularised oesophagus which runs posteriorly between tritocerebrum of brain and sub-oesophageal ganglion to open into the midgut.

The epitheliated midgut begins as stom­ach, which is small and sac-like. The stomach continues up to diaphragm and is followed by the intestine. The intestine is a straight tube and is distinctly divisible into a broad mesosomal part and a narrow metasomal part. The division is marked by the presence of two to four Malpighian tubules.

From the mesosomal part of the intestine, five to six pairs of lateral hepatic ducts or caeca are given off to the digestive gland (liver). The metasomal intestine lying in the last metasomal segment passes into the small cuticularised hindgut. The hindgut opens to the exterior through a ventral opening, called anus, which is located in between the last segment and poison bulb.

(ii) Digestive glands:

In scorpions, two sets of digestive glands are seen. They are salivary glands or stomach glands and gastric glands or liver.

a. Salivary glands:

These are paired glands present within the prosoma on either side of oesophagus. From each gland a duct opens within the oesophagus. The juice is known as saliva.

b. Gastric glands or liver:

This is a pair of massive, much branched glands occupying major part of the pro- and mesosomatic regions. The lateral ducts from the intestine are branched within the gastric glands to collect the juice.

Mechanism of nutrition:

The scorpion is carnivorous and predator but it lives on liquid diet. The food includes various insects and spiders. As it hunts in darkness, the food is found only when it comes in contact with the pedipalps and legs. It rarely chases the prey. The chela of the pedipalps strongly grabs the prey, which is then repeatedly stung.

The prey is then brought beneath the pre- oral cavity and with the action of chelicerae is torn into fragments. These small frag­ments are taken within the preoral cavity. Here the food comes in contact with various enzymes like amylases, lipases and proteases.

Only the digested part of the food is sucked inside the pharynx. Undigested part and the exoskeletal part of the prey are discarded from pre-oral cavity. From intestine, food enters within the lateral ducts, where it comes in contact with juices from the liver for complete digestion. Residual matters are finally ejected through the anus.

6. Respiratory System of Scorpion:

Respiratory organs of scorpions are known as book-lungs.

Number:

There are four pairs of book- lungs, one pair in each of the third, fourth, fifth and sixth mesosomatic or preabdominal segments.

Location:

The organs are found on the ventrolateral side of each segment and are housed within special chambers, called pul­monary sacs.

Origin:

In embryos, the book-lungs origi­nate externally from the ectoderm but in adult they are tucked in and are finally lodged within the inner pulmonary sacs. It is believed that book-lungs have evolved from the book-gills that are found in the aquatic horse-shoe crabs (e.g., Limulus).

Structure:

Each book-lung is a com­pressed sac-like structure and cavity of the book-lung is lined by thin cuticle which is formed of nearly 140 folded delicate leaves or lamellae (Fig. 18.110) with chitinous lin­ing. These are the respiratory surfaces. The lamellae are arranged vertically one after the other giving the appearance of the leaves of a book.

Each book-lung opens to the exterior through an oblique opening, called stigma or spiracle. The stigma leads into a proximal small chamber, called atrial chamber or atrium. It opens to the distal spacious cham­ber, called pulmonary chamber (Fig. 18.110) which contains the lamellae.

The outer side of the lamellae bears fine bristles which keep the adjacent lamellae separate, and the space between the two lamellae, called inter-lamellar space, is filled with air. The edges of the lamellae remain attached with the wall of the palmonary sac by one end, while the other end remains free. Each lamella contains a hollow cavity through which blood flows.

Blood supply:

The venous blood from ventral sinus is sent to each book-lung. The blood is aerated in the lamellae of each book- lung and returns through pulmonary vein which opens into the pericardial sinus.

Mechanism of respiration:

The working of special set of muscles, called dorsoventral muscles and atrial muscles, cause the contraction and relaxation of book-lungs. The contraction of dorsoventral muscles causes the compression of pulmonary cham­ber and the air from the lamellae is forced from the pulmonary chamber to the atrial chamber and then is expelled through the stigma by the contraction of the atrial mus­cles.

When the atrial muscles and dorsoventral muscles relax, the book-lungs resume their normal shape by rushing the air inside through stigma to the atrial chamber and inter-lamellar spaces. So the gaseous exchange takes place through thin walls of the lamel­lae and blood within lamellae becomes aer­ated and carbon dioxide diffuses out into the air. (Fig. 18.111).

7. Circulatory System of Scorpion:

(i) Blood:

Like that of prawn, blood is bluish in colour due to the presence of a copper containing respiratory pigment, called haemocyanin, dissolved in the plasma. In this liquid are suspended large and oval leucocyte-like corpuscles.

(ii) Heart:

Elongated and tubular heart is present in mid-dorsal region and extends between eighth and fourteenth segments. The heart consists of seven chambers—one in each of these segments (Fig. 18.112B). Heart is placed within a haemocoelomic space called pericardium. Each chamber is in communication with the pericardial cavity by means of a pair of valve-like openings, ostia.

(iii) Blood vessels:

From each chamber of the heart arises a pair of lateral arteries in each segment. At the anterior end, heart sends a truncus arteriosus or anterior aorta, which sends a pair of branches to the alimentary canal and then bifurcates to form two vessels to supply different parts of the prosoma. The two branches towards the alimentary canal encircle the digestive tube and unite on its ventral side.

After union, it runs posteriorly above the ventral nerve cord and becomes supraneural artery. From the posterior end of the heart originates a branch known as dor­sal aorta or posterior aorta which runs along the mid-dorsal line to the posterior-most tip of the body. It sends branches to the different parts of the posterior region.

Five large sinuses—the pericardial, the dorsal, the ventral and the two laterals are present in scorpion for collecting blood. The dorsal sinus is placed above the pericardial sinus. The pericardial sinus receives oxygenated blood from the book-lungs by several pairs of pulmonary veins—one pair from each seg­ment.

The upper wall of the ventral sinus is connected with the floor of the dorsal sinus by means of seven pairs of veno-pericardial mus­cles. The contraction of heart drives blood to the different parts of the body through the arteries. The arteries finally open into the haemocoelomic spaces at the different parts of the body.

The deoxygenated blood from different parts of the body is collected first within lacunae and then into dorsal and lateral sinuses. Finally, the deoxygenated blood comes to the ventral sinus.

From ven­tral sinus, blood is sent to the book lungs for oxidation. The oxygenated blood from each book lung returns to the pericardium through a pulmonary vein. When the heart expands, the blood from pericardial space enters within the cavity of the heart through ostia.

8. Excretory System of Scorpion:

Several sets of excretory organs are seen in scorpion:

(a) Malpighian tubules,

(b) Coxal glands,

(c) Large nephrocytes and

(d) Lymph tissue organ.

(a) Malpighian tubules:

Two to four Malpighian tubules are present near the beginning of the metasomatic part of the intestine. One end of these thread-like tubes opens within the intestine.

The other end is blind and removes excretory material as guanin and uric acid. Malpighian tubules in scorpion have different embryonic origin and thus are not homologous to those of insects. One Malpighian tubule enters within the salivary gland and the others penetrate within the lobes of the liver.

(b) Coxal gland:

These paired glands are present in the prosomatic region, one on each side and near the base of third walking leg. Internally, each gland has three parts— end sac, labyrinth and bladder. From blad­der, a slender duct arises and opens through a minute aperture, present at the base of third walking leg. The coxal glands usually eliminate uric acid.

(c) Large nephrocytes and (d) Lymph tissue organ:

These specialised structures are present in the wall of the mesosoma and are believed to be both phagocytic and excre­tory in functions. The lymph tissue organ is absent in the family Buthidae.

9. Nervous System of Scorpion:

The nervous system (Fig. 18.113) is built up on typical arthropod plan and consists of central nervous system, peripheral nervous sys­tem and sense organs.

(i) Central nervous system:

The central nervous system does not ex­hibit extreme condensation which is the peculiarity of other arachnids. It consists of a pair of small supra-oesophageal ganglia or brain situated in the prosoma and in between the median eyes (Figs. 18.112C & 18.113). In each side, the brain sends a circumoesophageal connective.

It encircles the oesophagus and unites with a ventromedian suboesophageal ganglion. The suboesophageal ganglion is formed by the fusion of ganglia of second to eleventh segments. A double ventral nerve cord arises from the suboesophageal ganglion and runs posteriorly up to the fourth segment of the metasoma.

In the mesosomal part, the nerve cord is narrow and round in cross section but in the metasomal part it is tape­-like and flat. The nerve cord bears three ganglia in the mesosomal part (Pre abdominal ganglia) and four in the metasoma (Postabdominal ganglia). The last metasomal ganglion is formed by the fusion of ganglia belonging to eighteenth and nineteenth segments.

(ii) Peripheral nervous system:

From brain arises a pair of optic nerves to supply median and lateral eyes. Numerous slender nerves also originate from brain to innervate the pre-oral cavity, pharynx and oesophagus.

From sub-oesophageal ganglion six pairs of lateral nerves arise to supply the prosomal appendages and two to four pairs of vagus nerves to supply the pectines, genital operculum and first two pairs of book-lungs in the mesosomal part of the abdomen.

From each ganglion on the ventral nerve cord, paired nerves are given out to innervate various structures in the corresponding seg­ments. The two pairs of posterior or metasomal book-lungs are supplied by nerves from the first two metasomal ganglia.

(iii) Sense Organs:

The sense organs of scorpion include sen­sory setae, trichobothria, slit sense organs, pectines and eyes.

a. Sensory setae:

These are fine hair-like structures with supply of nerve fibres. Such hairs are sparsed all over the cuticle.

b. Trichobothria:

Each trichobothrium is composed of a seta which is inserted within a cuticularised flask-shaped bothrium through the middle of a circular outer mem­brane. Within the bothrium the seta bends and gradually tapers within a fluid-filled cylinder to become a helmet-like structure.

This helmet-like tip of the seta is supplied by branches of nerve fibres. Each trichobothrium is capable of moving the seta in one direction only and is sensitive to air current. There are about 66-68 trichobothria on each chela and are arranged in different planes.

c. Slit sense organs:

These sense organs are distributed all over the body, but are usually crowded over the appendages. Each slit sense organ consists of an outer slit with an epicuticular membranous covering. Each slit measures nearly 0.005-0.16 mm in length and 0.002-0.003 mm in breadth.

The slit leads into a crevice which penetrates within the pro-cuticle as a membrane-lined tube. Numerous fine nerve fibres supply the inner wall of the tube. The sense organ chiefly reacts to the movement of the joint and also works as vibration receptor.

d. Pectines:

The structure of pectine has already been described. These are tactile sense organs and play important role in food capture. The males use it for the detec­tion of suitable surface to deposit spermatophores and the females use it for collecting spermatophores.

e. Eyes:

Both the median and lateral eyes are simple and are provided with lens. The median eyes are diplostichus type; here the hypodermal cells in the lower part of the lens are transformed into a vitreous body and the rhabdome of the retinulae is made up of five rhabdomeres. The lateral eyes are monostichus type.

Here vitreous body is absent and the structure of rhabdome is irregular. The function of both the median and lateral eyes is not understood. Being a nocturnal animal it depends very little on eyes. It shows negative phototropism but is often attracted by strong glare of light, i.e., campfire, lantern, etc.

10. Reproductive System of Scorpion:

Scorpions have separate sexes but the sexual differences are not distinct. The males are marked with narrower abdomen and powerful pedipalpi than females. The pec­tines of male contain more teeth than that of females.

(i) Male reproductive system:

The male reproductive organs or testes are paired, lat­erally placed tubular organs, present in the third to sixth segments of the pre-abdomen. These are connected to each other by trans­verse tubes (Fig. 18.114A). Thread-like sperm cells are collected from the testes by a pair of vasa deferentia.

Each vas deferens begins from the outer border of the testis and runs anteriorly along the lateral wall of the body cavity. Each vas deferens opens within a genital chamber. Immediately before opening into the genital chamber, the vas deferens receives accessory gland and a broad seminal vesicle.

Posteriorly, the genital chamber is drawn into a bag-like paraxial organ, which contains a spiny and grooved chitinous rod, called flagellum.

The two genital chambers unite to form a common genital chamber, which opens to the exterior through a genital pore beneath the genital operculum in the first mesosomal segment. The sperms remain stored in the seminal vesicle. The paraxial organ produces a structure, called spermatophore.

During copulation, the spermatophore enters the genital chamber and receives the sperm cells from the seminal vesicle. The accessory glands produce a kind of cement­ing material to constitute the adhesive stalk of the spermatophore and also for sealing the two halves of the spermatophore.

A sper­matophore is 5 mm in length and is depo­sited on a suitable substratum at 45° angle (Fig. 18.114B). Each spermatophore consists of a lower stem and a base for attachment and the stem bears a sperm container and an ejection apparatus. With the ejecting appara­tus is attached a dagger-shaped lever.

(ii) Female reproductive system:

The female reproductive organ or ovary is single and it includes three elongated tubes or ovarioles, with numerous tubular interconnections (Fig. 18.114C). It is present in the same position of pre-abdomen as that of testes in the male. Within the inner lining of the tubular ovariole, the eggs or ova are produced but are not released before fertilization.

Within the ovary the eggs grow on bud-like processes, called follicles, which may be of varied shapes in different scorpions. Two oviducts, one from each lateral ovarioles, run anteriorly and unite to form a common oviduct or genital chamber. The chamber opens to the exterior through small opening beneath the genital operculum.

Mechanism of reproduction:

The scor­pion is noted for its peculiar courtship dances (Fig. 18.114 a-c). The male, while approach­ing a female flickers the post-abdomen ver­tically. When the female responds by doing similar movement, the male grabs the female’s pedipalps with its own and moves backward in a straight line. This movement may continue for 1/2 hour- 2 hours.

At this time, the male, with the help of its anterior pairs of legs, touches the pectines and genital opercula of the female. The pectines of male frequently touch the surface and its posterior legs perform a scratching movement on the surface.’

Formerly, it was thought that the nuptial dance of scorpion is to excite female. But now its correct meaning is understood. This dance is intended only to find a suitable surface for depositing the spermatophore.

When the spermatophore is deposited, the male drags the female over it. When the genital operculum of female touches the le­ver of spermatophore, the ejecting apparatus shoots the sperm within the female genital tract.

The pectines of male play important role in finding the suitable surface. It has been shown experimentally that a male scor­pion with excised pectines performs routine nuptial dance with the female but it never deposits the spermatophores.

11. Development of Scorpion:

The scorpions may be viviparous or ovoviviparous, depending upon the content of yolk in the egg. In the viviparous forms the yolk content is poor. Such fertilized eggs grow on the follicles of the ovary. The follicle acts as ‘placenta’ and provides nutrition to the embryo. In ovovi­viparous forms the yolk content is high and in them the fertilized eggs drop within the tube of the ovary and develop there.

In the embryo, seven pairs of appendages appear in the mesosoma but the appendages in the fourth to seventh segments are tucked in to form the book-lungs.

Newly born- scorpion resembles its mother and immediately after birth climbs on her back. It remains there for 40-63 days and undergoes there moults. It then starts independent life but maturity takes 1-5 years to complete and during that time the scorpion moults seven times.