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General chelicerate characters

Among chelicerates the body is divided into a cephalothorax and an abdomen.

The cephalothorax bears 6 pairs of uniramous appendages. The first pair are always preoral while the second pair, known as the pedipalps, are variously developed, but usually 6-jointed.

The function of the pedipalps varies considerably from order to order with some employing them in sensory or feeding functions and others, as reproductive organs.

The third to 6th pairs of appendages are typically 7-jointed and usually developed as legs although the third pair are modified as sensory organs in some orders and the 5th pair as reproductive organs in the order Ricinuleida.

Abdominal appendages are present in the adult only in scorpions, as a pair of combs and in spiders as spinnerets.

Arachnid characters

External Characters

The head is always fused with the 5 thorcic segments, forming a single unit or tagma. This is always distinct from the abdomen.

The cephalothorax is covered with a single shield or carapace known as the peltidium.


The eyes of all arachnids are comprised of a single a corneal lens, the outer surface of which may be either flat or covex. The eye itself comprises a glass body which is renewed with each instar (growth phase).

The number and orientation of the eyes vary among orders. One pair is often median in position and the others, of which there may be up to 5 pairs, are lateral. In many cases the eyes are elevated either on a common eye tubercle or on seperate tubercles. In other cases they are sessile, in other words, their base is on a level with the surface of the carapace.

The configuration and number of eyes is a usefull tool for taxonomic classification.

However, the orders Palpigradia, Anthracomartida and Ricinuleida have no eyes.


The shape and the proportions of the carapace are also important for the purposes of classification, but in many cases, the carapace of the male differes significantly from that of the female of the same species.

The ventral surface of the cephalothorax is occupied by 6 pairs of appendages and the thoracic sternites


The abdomen is always composed of 12 segments in the early embryological stages of the animal . This segmentation may be retained into adulthood or it will undergo various changes during development, masking or changing its appearance.


Cephalothoracic appendages

Chelicerae: mouth parts

Pedipalps: small anterior appendages which can be used for walking, however, these are modified in some orders for use as sensory or reproductive organs and in some cases they can be used for ingestion.

legs: four pairs of walking limbs, typically 7-jointed. Claws may be present at the terminus of walking limbs of certain orders.


Abdominal appendages

Abdominal appendages are present only in scorpions and spiders.

In scorpions these arise as a series of combs on the third sternite.

In spiders they occurr as spinnerets


 Wolf spider showing pedipalps, cephalothorax and opisthosoma


 Scorpion anatomy

Ecdysis (moulting)

During ecdysis, the entire chitnous exoskeleton is shed along with the chitinous lining of the book lungs, tracheal tubes, fore-gut and hind-gut.

Moulted parts of these components can be found exceptionally well preserved in amber


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Author: Ceri-Wyn Thomas

Last updated: 22.11.05
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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2005-6