Physical characteristics

There are at least 10 features of arachnids that are often used to describe the group, including:

• carapace may be uniform or in part segmented

• pedicel may be absent or present

• sternum may be uniform or segmented

• opisthosoma may be uniform or segmented

• chelicerae may contain two or three segments (podomeres)

• pedipalpi may be pincer-like or leg-like

• coxae of legs or pedipalpi may or may not contain gnathobases (plate-like anterior expansions)

• first leg may be used as a leg or like an antenna

• legs may be of seven segments (podomeres) or may be sub-segmented anywhere heart gut cecum ostium intestine excretory tubules stercoral pocket heart pocket

spinnerets

Arachnid anatomy. (Illustration by Patricia Ferrer)

abdomen tarsal claw legs spinnerets

Arachnid anatomy. (Illustration by Patricia Ferrer)

• coxae may meet and hide the sternum or may be separated

Anatomical features such as two pairs of limbs, the pedi-palps and chelicerae, are distinctively present but greatly modified for different uses in various arachnid species.

The 18-segment arachnid body is often protected by ster-nites below and tergites above, connected by a soft pleural membrane, and is divided into two tagmata: anterior and posterior. The anterior (front) part, called the cephalothorax (or prosoma), contains sense organs, mouthparts, and limbs or appendages in pairs. The cephalothorax is composed of an anterior, unsegmented region called the acron, and six true segments (each bearing a pair of appendages). It accommodates both the head and limbs. A carapace-like shield completely or partially covers the cephalothorax of arachnids. The first pair of limbs (chelicerae) attached in front of the mouth may form pincers or poison fangs, and the second pair (pedipalps) behind the chelicerae may serve as pincers, feelers, or additional legs. The other limb pairs are used for walking. The 12-segment posterior (rear) part of the body, the abdomen (or opisthosoma), contains the genital opening and other structures. The abdomen may be segmented (as in scorpions) or unsegmented (as in most ticks and spiders). Abdominal appendages are either lacking, or modified into special organs such as the spinnerets of spiders and the pectines of scorpions.

Arachnids breathe by means of tracheae (windpipes), book lungs (modified gills), or both. The mouth of arachnids is not readily noticeable from the external surface. They do not possess jaws (mandibles), but instead have cutting or piercing appendages called chelicerae. The open circulatory system distributes blood from the heart to an enlarged blood space by the use of arteries. The heart is a tubular organ located dorsal to the mid-gut, containing various openings so that blood can be returned to the heart. The central nervous system consists of two cerebral ganglia connected to a pair of sub-esophageal ganglia by means of a circum-esophageal linkage (commisure). Arachnids possess a number of sense organs, many related with the outer body covering (cuticle). The most common of these sense organs is the hair-like setae that are sensitive to various stimuli; they are generally located throughout the surface of the body.

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