Essay on Spider

In this essay we will discuss about Spider:- 1. Habit and Habitat of Spider 2. External System of Spider 3. Digestive System 4. Respiratory System 5. Circulatory System 6. Excretory System 7. Nervous System 8. Reproductive System 9. Breeding and Life History.

Contents:

1. Essay on Habit and Habitat of Spider
2. Essay on External System of Spider
3. Essay on Digestive System of Spider
4. Essay on Respiratory System of Spider
5. Essay on Circulatory System of Spider
6. Essay on Excretory System of Spider
7. Essay on Nervous System of Spider
8. Essay on Reproductive System of Spider
9. Essay on Breeding and Life History of Spider

Essay on the Habit and Habitat of Spider:

The spiders are of various types (30,000 species) and are seen to be adapted to the various environmental conditions. Broadly speaking, the spiders may be grouped into— wanderers (do not spin) and spinners (web some form of net).

Most of them capture insects, and some forms are known to cap­ture even birds. The examples of common spiders are Nephila (garden spider), Lycosa, Salticus (both are hunting spiders) and Latrodectus (Black widow). The largest spider Theraphosa leblondi of Guyana may grow up to 9 cm in length.

Essay on the External System of Spider:

The body is divided into two regions— cephalothorax or prosoma and abdomen or opisthosoma (Fig. 18.115). The outer part is covered by a hard exoskeleton which exhibits diverse striations and colourations in differ­ent types.

Cephalothorax:

The cephalothorax is covered dorsally by a uniform carapace and ventrally by a broad, strongly built un-segmented sterman. The cephalic or head part contains eyes, append­ages and mouth.

Eyes:

The eyes are simple but distinctly developed. Eyes are never more than eight and are placed in two transverse rows. Each row consists of two median eyes, and two lateral eyes are placed one on each side of the median eyes.

Appendages:

Two pairs of cephalic ap­pendages are known as chelicerae and pedipalpi. The first pair, i.e., chelicerae is placed anteriorly on the dorsal side of the mouth (Fig. 18.116A, B). Each is made up of two parts—broad basal segment and termi­nal clawed part. Within each chelicera and often extended within the cephalothorax, lies a cylindrical poison gland. The poison duct opens at the convex side of the claw.

The chelicerae may be modified in different groups according to the way of life. The pedipalpi may be leg-like, antenna-like or reduced. In some, together with chelicerae it forms a pre-oral chamber.

The pedipalp con­sists of six segments; the basal one is called coxa and this is followed by trochanter, femur, patella, tibia and tarsus. The tarsus consists of a single segment and metatarsus is absent. An intromittent organ or palpal organ is present at the base of the distal article of each pedipalp (Fig. 18.116C, D).

Mouth:

Mouth is bounded by two lips— upper lip and lower lip. The upper lip, also known as rostrum, is present between che­licerae and pedipalpi and its inner side consti­tutes the upper part of the epipharynx. The lower lip or labium forms the ventral part of the mouth and help in sucking liquid food.

Both the tergum and sternum of the tho­rax have grooves along which the body muscles remain attached. Four pairs of walk­ing legs are present in the thoracic region. Each leg is made up of the following seven segments—coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The tarsus is provided with a pair of comb-like claws.

Abdomen:

It is connected to the cephalothorax by a thin pedicel. The pedicel remains covered under the abdomen. The anterior part of the abdomen is concave and is called epigastrum. It is separated from the rest by an epigastric furrow.

The abdomen contains the following structures:

1. Stigma or Spiracle:

Four in number and communicate with the respiratory or­gans. The position of the spiracle varies. In some, it may open posteriorly on the ventral side of the segments VIII and IX. In others, posterior spiracles may be at the posterior end of the abdominal segments.

2. Genital apertures:

In both the sexes, the reproductive openings are present at the side of the book-lung and in the middle of the epigastric furrow. In the male the open­ing is simple but in the female it is associated with a complex structure epigynum.

Silk gland and spinnerets:

The silk glands are fairly large and present in the 10th to 11th segments. Each gland is lined by a single layer of cells. These cells produce silk, which pass through spinning tubes. The silk glands vary in different spe­cies. In orb spiders, there are six kinds of silk glands, each liberating a special kind of silk.

The silk of spider is highly elastic and has the maximum tensile strength. The silk is com­posed of fibroin, a kind of protein. Each thread is either flat or cylindrical and its diameter varies. The spinnerets are specialised structures for spinning silk (Fig. 18.117). The numbers of spinnerets vary in different spiders. These are present on the posterior part of the abdomen.

These are called fore, mid and hind spinnerets. Each spinnerest has a stout base, but the tip is membranous and covered by barbs and hairs. Over the surface of the spinning field there are many spinning tubes which are of different shapes. Special structures, called the cribellum (ac­cessory spinning organ) and calamistrum (brush-like hairs on the metatarsus of the 4th leg) are often present to work with the spin­nerets.

Essay on the Digestive System of Spider:

The alimentary canal is divisible into:

(a) Foregut,

(b) Midgut and

(c) Hindgut (Fig. 18.118A).

The tore- and hindguts are lined by cuticle and the mid gut has epithelial lining.

The foregut consists of following parts:

(i) Mouth,

(ii) Pharynx,

(iii) Oesophagus and

(iv) Stomach.

The lateral walls of the mouth parts are provided with rows of minute hairs which filter out small particles. The pharynx is broad and flat and leads to a narrow oesophagus.

The oesophagus has a small di­verticulum which bears chemoreceptors to determine the taste of the food. The stomach has folded wall with thick chitinous lining. The stomach possesses two sets of powerful muscles, one set is responsible for contrac­tion of the stomach and another set dilates it.

The midgut sends diverticula or caeca within the cephalothorax and it sends branches towards each leg. The abdomen is also filled up with several diverticula from midgut. These are called liver or digestive diverticula.

Each gland or diverticulum contains two types of cells:

(i) Secretory and

(ii) Absorptive.

An albuminous substance given out from the secretory cells mixes up with the food. These are then taken by specialised cells, within which the food is broken down into simpler forms. The absorptive cells collect indiffusable and insoluble products. Such filled up cells are thrown out during egestion.

The hindgut is short and receives Malpighian tubules. Acloacal diverticulum or stercoral pocket is present posteriorly, where faeces are stored temporarily before being ejected through the anus. The opening and closure of the anus are controlled by sphinc­ter muscles.

All spiders are carnivorous. The food ranges from flies to small birds. Most spiders spin webs for the capture of prey. The cap­tured prey is first paralysed by the secretion of poison gland. The salivary glands present at the underside of the lip produce proteo­lytic enzymes, which are either poured or injected within the body of the prey.

When the prey is softened, the spider sucks out the juice by the contractile action of the pharynx and stomach. Large particles are not al­lowed to enter into the gut by the setae and spines of the labrum.

The transversely ar­ranged ribs on the anterior wall of the phar­ynx also act as filter to remove particles. Further digestion and absorption take place within the midgut diverticula. Faeces pass into the hindgut and before final egestion remain temporarily stored within the sterc­oral pocket.

Essay on the Respiratory System of Spider:

Two types of respiratory organs are seen— book-lung and tube trachea. Two pairs of book-lungs are usually present in spider. The structure of the book-lung is same as that of scorpion. Inside each book-lung, sclerotised lamellae are arranged like the pages of a book. The book lungs open through stig­mata or spiracles, one on each side of the genital pore and on the anterior and ventral surface of the abdomen.

The tracheal system may be of varied types. In some (e.g., Dysderidae), a pair of large tracheal trunks, called sieve tracheae, give rise to several small tracheae. In all spiders, the tracheal system is well-developed in young stage. In early stage of development the tracheal spiracles are placed posteriorly which subse­quently shift upwards.

Essay on the Circulatory System of Spider:

The heart is abdominal (Fig. 18.118B) and placed within distinct pericardium. In forms having less developed tracheal system, the heart is large, and arterial network is well- developed. Two to five pairs of ostia open within it.

The heart sends:

(i) An anterior artery,

(ii) Paired lateral arteries depending upon the size of the heart and

(iii) A posterior artery as extension of the heart.

6. Excretory System of Spider:

The excretion in spider is carried out by:

(a) Absorptive cells,

(b) Guanine cells,

(c) Nephrocytes,

(d) Malpighian tubules and

(e) Coxal glands.

(a) Absorptive cells:

Absorptive cells are present in the intestinal wall. Some of these cells collect egestable substances. These cells either pour their contents inside the lumen of the gut or the entire cell drops into lumen or are then removed from the body.

(b) Guanine cells:

Guanine cells are also distributed near the inner layer of intestine. These cells collect excretory products which are called guanates and release them within the lumen of the gut in liquid form. After coming to the cloacal pocket, these liquid guanates mix up with some acids produced by the hindgut and become crystalline.

(c) Nephrocytes:

These specialised cells are present in the cephalothorax and are supposed to have some excretory functions.

(d) Malpighian tubules:

These are the chief excretory organs and are represented by two sets of pseudo-ciliated dichotomously branched tubules which arise from the cloacal pocket. A rythm of muscular contraction which begins from the tip of the tubule drives the excretory products downwards. Each tubule is a syncitium and produces a slightly acid secretion. Malpighian tubules remain restricted to the abdomen.

(e) Coxal glands:

These glands are either one or two pairs and are confined to the prosoma. Each gland is a sac-like structure and a short excretory duct from this sac nans up to the posterior end of walking legs. Their openings are placed at the posterior end of first and third walking legs. Where one pair of coxal glands present, it opens through the first walking legs.

Essay on the Nervous System of Spider:

The nervous system exhibits highest de­gree of condensation. In the cephalothorax, the brain is represented by supraoesophageal ganglion. It is connected with a large ventral suboesophageal ganglion, which is formed by the union of all the posterior ganglia. The sense organs include sensory setae, trichobothria, slit sense organs and eyes.

The sensory setae are hair-like structures and remain scattered all over the surface, specially on legs. The elongated trichobothria, resembling those of scorpion, are present on the pedipalpi. The slit sense organs, like those of scorpion, remain distributed over the integument.

These are responsible for detecting vibration and sensing mechanical stresses. Eyes are simple but well developed. They are never more than eight and are arranged in two transverse rows. Each row consists of a pair of median eyes and a pair of lateral eyes on the sides. In median eyes, the rhabdomes face the lens but in other eyes the rhabdomes are moved inwards.

Essay on the Reproductive System of Spider:

The sexual dimorphism is the character­istic of the spiders. The males are much smaller than the females.

In males, a pair of testes is present. Each testis includes several tubes which are ar­ranged in parallel rows along the ventral margin of the abdomen. From each testis arises a genital duct which opens within a common median atrium.

The male genital apertures are present in the abdomen near the stigmata. The pedipalp carries a palpal organ or intromittent organ (a structure used to transfer the sperm within female body).

In females, the reproductive organs are paired ovaries (Fig. 18.118B). The ovaries unite posteriorly to form a common oviduct which enlarges as an atrium or uterus. A pair of seminal receptacles opens within the atrium. It opens to the exterior through an aperture, called vagina, which is located at the epigas­tric furrow.

The female genitalis is covered in most cases by a complex structure, called epigynum. It possesses a guiding groove and functions to guide the passage of male intro­mittent organ. On the side of each oviduct a spermatheca is present and is connected with epigynum groove.

Essay on the Breeding and Life History of Spider:

The reproductive behavior of spider is interesting and it varies widely. Usually a mature male spins a kind of web, called sperm web. It deposits a drop of sperm on it. After a few minutes, the male immerses its intromittent organ repeatedly in that drop of sperm. Such dipping continues for one and half hour.

When the organ is filled up, the sperm web is destroyed. The male then looks for a mature female. The female waits in her web and the male detects her with the help of its chemoreceptors. Various types of court­ship behaviours follow in different spiders. Finally, the male introduces either one or both the palpi within the female gonopore.

The sperms are deposited in the seminal receptacles of the female. The fertilization takes place shortly before the laying of eggs. Some species of spider exhibit care of the eggs and young’s.

A newly hatched spider moults 5-12 times before becoming an adult. Immediately before moulting the thickness of the exoskeleton reduces to one-third, due to prior enzymatic digestion of the endocuticular part. The spider takes no food immediately before or immediately after moulting.