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ARACHNIDS

Arachnids include the following groups:

i.                    spiders,

ii.                  scorpions

iii.                ticks,

iv.                mites etc

Description and Features

The typical arachnid body is divided into a cephalothorax and an abdomen with the exception of ticks and mites, where they are fused together. The abdomen is distinctly different from the cephalothorax. The cephalothorax bears six pairs of uniramous appendages (only one extension from the basal protopodite).

The first two pairs of appendages are ambulatory ie modified for feeding, and the last four for locomotion. This means that arachnids have four pairs of legs.

They are primarily terrestrial, and respiration is by the use of lung books and trachea. Excretion is carried out through gills or coxal glands. They possess a dorsal bilobed brain having a ventrally situated ganglion with nerves.

They only have simple eyes.

The sexes are separate.

They do not undergo true metamorphosis but are oviparous. They undergo direct development.

SPIDERS

Spiders belong to the Class Arachnida, and Order Araneae, which is large and highly successful. About 34,000 different species of spiders have been identified and described.

External Features

The body is divided into two parts namely the prosoma and the opisthosoma. The two are linked by a characteristic waist called the pedicel.

The Prosoma - Ambulatory (feeding) appendages are borne on the prosoma namely:

i.                    A pair of chelicerae which terminate in fangs through which ducts from the poison glands open.

ii.                  A pair of pedipalps which have basal parts with which they chew.

The prosoma also bear four pairs of walking legs. All the legs end in claws. In males the pedipalps are modified for use in the transmission of sperm during copulation.

The Oposthosoma - Borne on the opisthosoma are the trachea and book lungs which they use to carry out respiration.

Also Spinnerets are present on the 5^th and 6^th segments of the opisthosoma

Figure: External Morphology of spider (Ventral view)

 Figure: Internal Anatomy of Spider.

Feeding and Digestion

All spiders are predacious, and most of them are entomophagous (feed on insects). They exhibit notable behaviours in accomplishing the feeding act, and behaviours shown varies widely as the different types of spiders there are. Some stalk their prey, some lay a drag line while some other (e.g. jumping spiders) jump on the prey. In web spinners the prey is ensnared in the web spinned.  

When the prey is captured, it is seized with the chelicerae and pediplaps and then injected with poison from the poison gland, resulting in and eventual death. The prey is then squeezed and chewed by the gnathobases of the pedipalp.

The digestive system is located in the posterior half of the prosoma and most of the opisthosoma. It comprises the mouth, which leads into a slender oesophagus, a sucking stomach with five pairs of ceaca (one dorsal and four in the walking legs). There is also the intestine, which is embedded in the digestive gland. The rectum bears a large cloaca. The gut of a spider terminates in an anus.

Digestion begins outside the spider’s body when it first pours saliva on the prey. The food is first liquefied, and it is then sucked leaving the empty exoskeleton of the insect behind.

Respiration

Respiration in spiders is through the use of the lung books or the trachea, or a combination of both. Each book lung has 15 to 20 leaf-like vascularized plates which are connected to the heart by pulmonary veins. The trachea connects with the exterior through spiracles which open on the body wall. In achieving respiration the functioning of the trachea and the book lungs are coordinated. While oxygen is taken into the blood by the book lungs and distributed to the tissues, it is distributed directly to these tissues by the trachea. The tissues therefore get a rich supply of oxygen.

Excretion

Excretion is achieved through the Malpighian tubules working in conjunction with the rectal glands. Also, there are two coxal glands in the prosoma. These glands are modified nephridia. They open by ducts at the coxa (base) of the third walking legs. Guanine is the main excretory product.

Reproduction

There is marked sexual dimorphism in spiders; the males being smaller in size than females. There is often a long courtship before copulation. The sperm are first deposited and stored in the seminal receptacle of the pedipalp and then the male goes looking for the female. It often circles her with some food before the sperm can be transferred into the vagina (spermatheca) with the pedipalp.

The eggs are laid in a special egg case called cocoon. the number of eggs in each cocoon varies. While some species have just two eggs there may be about 1,000 in others. Eggs hatch into miniature adults (spiderlings) in two weeks. Spiderlings moult many times to become mature. Spiders show special care for their brood before they become mature.

Economic Importance of Spiders

(i)                 They serve as biological control agents for some noxious insects

(ii)                Web spinners trap dust particles.

(iii)             Spider bites are not fatal to humans but may result in serious consequencies.

(iv)             There may be life-threathening allergic reactions to spider bites as a result of the venom injected into the body system of the victim.