Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Arthropods
Subphylum Chelicerata - Class Arachnida

General characteristics

Chelicerates include around 40,000 species, predominantly terrestrial Arthropods. The ancestral chelicerates were aquatic animals. Their body consists of a cephalothorax and an abdomen. In some forms (such as scorpions), the abdomen is subdivided into a preabdomen and a postabdomen. The cephalothorax bears 6 pairs of uniramous appendages, while the limbs of the abdominal segments are either modified or absent.

A key feature of chelicerates is the reduction of antennae. The first pair of cephalothoracic appendages is modified into chelicerae, which serve for crushing and grinding food. The second pair, the pedipalps, differs less from the walking legs that follow them. Pedipalps perform sensory and frequently grasping Functions.

The Subphylum Chelicerata comprises 3 classes, of which we will examine only the most numerous Class, Arachnida.

The class Arachnida encompasses over 36,000 species of animals with quite diverse outward appearances, living exclusively on land: in fields, forests, gardens, vegetable patches, human dwellings, etc. A considerable number of them are parasitic forms. A characteristic feature of arachnids is the absence of antennae and compound eyes.

The body of the vast majority of arachnids consists of 2 tagmata: a fused cephalothorax, bearing 6 pairs of appendages and several pairs of simple eyes, and an abdomen. The first pair of appendages is located in front of the Mouth opening and is designated as the chelicerae (Fig. 51). They consist of 2–3 segments and terminate in a claw, hook, or stylet. The second pair of appendages is situated behind the mouth and is referred to as pedipalps (or palps). They consist of several segments and may resemble a pincer or walking legs. The remaining 4 pairs of cephalothoracic appendages are walking legs, consisting of 6–7 segments and terminating in claws.

Fig. 51 Chelicerae and pedipalps of arachnids:

A — ventral view of the cephalothorax of a female cross spider: 1 — claw-like segment of the chelicera; 2 — basal segment of the chelicera; 3 — lobe of the pedipalp; 4 — lower lip (labium); 5 — bases of the walking legs; 6 — pedipalp.

B — ventral view of a scorpion's cephalothorax: 1 — 1st segment of the chelicera; 2 — 2nd segment of the chelicera; 3 — terminal segment of the chelicera (movable "finger"); 4 — fixed "finger" of the 2nd cheliceral segment; 5 — basal segment of the pedipalp; 6 — pedipalp (pincer); 7 — lobe of the 2nd leg; 8 — 1st leg; 9 — 2nd leg; 10 — mouth.

The abdomen in arachnids is not always segmented. For example, in spiders, the abdominal segments are fused, but the cephalothorax is clearly demarcated from it (see Fig. 52). In scorpions, the abdomen is distinctly segmented. In many mites and ticks, all abdominal segments fuse, and the abdomen itself merges with the cephalothorax, forming a single unsegmented body. There are no appendages on the abdomen of arachnids, but in spiders, some of them have evolved into spinnerets. The Digestive System of arachnids consists of foregut, midgut, and hindgut. As a rule, they feed on semi-liquid food. A spider, for instance, punctures the prey's integument, injects saliva into the wound—which dissolves the victim's Tissues—and then sucks up the semi-liquid, partially digested food. The foregut includes the mouth, the Pharynx with the opening ducts of the Salivary Glands, the Esophagus, and the sucking Stomach.

The midgut of arachnids forms 5 pairs of lateral outgrowths (diverticula) that increase its absorptive surface area. The ducts of a well-developed Liver open into the midgut. At the border between the Midgut and Hindgut, the ducts of excretory Organs—most commonly a pair of branching Malpighian tubules—empty into the alimentary canal. The hindgut terminates in the anus.

Fig. 52 General appearance of arachnids:

1 — scorpion; 2 — spider; 3 — harvestman; 4 — mite/tick.

The respiratory organs of arachnids are diverse: book Lungs (e.g., in scorpions), tracheae (e.g., in mites and ticks)—a system of branching, tapering tubes reaching various organs—or both lungs and tracheae combined (e.g., in most spiders). Both lungs and tracheae communicate with the external environment through special openings called spiracles.

The Development of the Circulatory system in arachnids depends on body size and The complexity of the Respiratory system. In small mites, The Heart is very small or entirely absent. In larger spiders and scorpions, the heart is tubular, with Blood Vessels extending from it. Blood is poured from these vessels into the body cavity (hemocoel).

The primary excretory organs of arachnids are the Malpighian tubules. Excretory glands, which are typically poorly developed in adults, also participate in eliminating Metabolic waste products of complex Organic compounds.

The Nervous system of arachnids consists of a supraesophageal ganglion connected to a ventral nerve cord. Characteristic features include the concentration and fusion of ventral nerve ganglia into a single ganglionic mass or a small number of ganglia.

Arachnids are dioecious (gonochoric). In many species, Sexual Dimorphism is quite pronounced. For instance, male spiders are significantly smaller than females, and their pedipalps are modified into a copulatory apparatus. Some scorpions are viviparous. Newborn scorpions do not leave the female immediately, and she carries them on her back for some time. Development in most arachnids is direct.

The class Arachnida comprises over 10 orders, of which we will examine 4 of the most widely distributed.