What are Arachnids?

Arachnids are a Class (Arachnida) of joint-legged Invertebrate Animals in the subphylum Chelicerata. All Arachnids have multiple legs and look like Insects, the only difference is that Insects have six leg and Arachnids have 8 legs. They are largely terrestrial and solitary Animal, gathering only for mating. Fossils suggest that Arachnids were among the first animals to live on land, perhaps in the early Devonian Period, nearly 400 million years ago. Some of the common Arachnids are Spiders, Scorpions, Ticks and Mites.

It is estimated that a total of 98,000 Arachnid Species have been described, and that there may be up to 600,000 in total, including undescribed Species. Majority of the Arachnids are Spiders. According to the '2010 IUCN Red List' 18 Arachnid Species are at risk of extinction, their status is - Endangered =1, Vulnerable = 9, Lower Risk/Near Threatened =1,  and Data Deficient = 7. Arachnids play an important role, they keep Insect population under control. The bites of some Spiders, such as the Black Widow Spider and the Brown Recluse Spider and the stings of a few Species of Scorpions are dangerously poisonous to humans.

Scientific Classification

Kingdom - Animalia.
Phylum - Arthropoda.
Subphylum - Chelicerata.
Class - Arachnida.


Arachnids Characteristics

  • Presence of 8 Legs - Almost all Adult Arachnids have 8 legs. Arachnids also have two further 'Pairs of Appendages' that have become adapted for feeding, defence, and sensory perception. The first pair, the 'Chelicerae', serve in feeding and defence. The next pair of appendages, the 'Pedipalps' have been adapted for feeding, locomotion, and/or reproductive functions. In Solifugae, the palps are quite leg-like, so that these Animals appear to have ten legs. The Larvae of Mites and Ricinulei have only six legs; the fourth pair appears when they moult into nymphs. However, there are also adult mites with six, or even four legs.
  • Segmented Body - The chief external feature of Arachnids is the division of the body into two parts called the 'Prosoma' and the 'Opisthosoma'. The 'Prosoma' is composed of a united head and Thorax. The Prosoma anchors the Chelicerae (pincers), Pedipalps (mouth parts) and four pairs of legs. The 'Opisthosoma' may bear various sensory, genital opening  or silk-spinning appendages. 'Opisthosoma' is also known as the Abdomen. The abdomen is usually marked off by a distinct constriction or waist. In a few cases the abdomen is merged with the Thorax. Most Arachnids have 'Fine Sensory Hairs' that cover the body and give the Animal its sense of touch. Many Arachnids also possess more complex structures, called 'Trichobothria'.
  • Absence of Antennae - Though Arachnids look like Insects but there is one distinct features which separates it from Insects and that is the Absence of Antennae in any Arachnid.
  • Absence of Wings - Arachnids lack wings unlike Insects.
  • Exoskeleton - Just like Insects all Arachnids have an external skeleton. The hard Exoskeleton is made out of protein  and a tough 'Polysaccharide' called 'Chitin'. Periodically, an Arachnid must shed this covering when it moults.
  • Fluid Feeders - Arachnids are generally Carnivorous, feeding on the body fluids of their prey or secreting enzymes to digest prey externally. Arachnids pour digestive juices produced in their stomachs over their prey after killing it with their pedipalps and chelicerae. The digestive juices rapidly turn the prey into a broth of nutrients which the Arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, sclerotised pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some Arachnids, the oesophagus also acts as an additional pump. One of the features of the feeding of the Arachnida is their extreme voracity. Prolonged fasts are common amongst them and therefore they seem concerned to extract the last drop of moisture from each prey.
  • Cold Blooded - Arachnids are Cold-Blooded. Their body temperature depends on the temperature of their environment. If it's cold weather they will tend to be sluggish and if the weather is sunny they will be active.
  • Respiratory System - Arachnids breathe air through 'Tracheae' (windpipes) or 'Book-Lungs' which are gill-like structures. Some may utilise both.
  • Open Circulatory System - 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.
  • Central Nervous System - The Central Nervous System consists of two cerebral ganglia connected to a pair of sub-oesophageal ganglia by means of a circum-oesophageal linkage (commissure).
  • Presence of Haemocyanin - Instead of Haemoglobin the blood of Arachnids consists of Haemocyanin. The respiratory function of Hemocyanin is similar to that of Haemoglobin. Haemocyanin is blue coloured and contains copper. Haemocyanin has a higher affinity to oxygen and therefore it is more useful for oxygen storage than for rapid release at the tissues. Arachnids as such often show extreme bursts of activity but have remarkably little stamina.
  • Nocturnal Animals - Arachnids are usually nocturnal, lying at rest during the day in their nests or holes or crouching under stones, logs or fallen leaves. The chief exceptions to this are the Scorpions, which are often active in daytime, a few Solpugida and a number of Spiders also hunt by day.
  • Protective Mechanism - One of the most characteristic methods of protection shown by nearly all the Arachnida is their ability to cast a limb and so to escape, purchasing life and freedom at the price of a leg. This widespread action has been closely studied by Dr. F. D. Wood. According to her study - This Mechanism is simply the inability of the skeletal and muscular components of the leg to resist more than a certain force. The leg, when pulled, parts at its weakest point. Consequently all Spiders cast their legs at the 'Coxa-Trochanter Articulation', all Opiliones at the 'Trochanter-Femur Articulation' and all Pedipalpi at the 'Patella-Tibia Articulation'. Limbs which are sacrificed are not in general permanently lost, but reappear during moultings of the Exoskeleton.

 Arachnid Identification


Colour -  Majority of Arachnids are soberly coloured, to achieve invisibility against a sandy or stony background. Conspicuous or bright colours are rare, although they are found among tropical spiders and some others that live among flowers and foliage.


Weight - The 'Largest Arachnids' body weighs 85 g (3 oz) and the 'Smallest Arachnid' weighs few milligrams.

Size - The 'Largest Arachnid' is 16-28 cm in length and the 'Smallest Arachnid' is about 1,3 cm in length.

Diet - Most of the Arachnids are Carnivorous. Some of them, like the Mites feed on plants while others like Ticks are parasites living on the blood and tissue fluid of other Animals. They eat Insects, other Arachnids and a variety of Invertebrates.

Predators - Almost all the Classes of Animals eat one or the other Species of Arachnids.

Arachinds Distribution (geographical range & habitat)

Arachnids are found throughout the World from Equatorial to Polar Regions. They are most abundant in numbers and diversities in very warm to hot, arid and tropical/subtropical regions. Arachnids are essentially 'Terrestrial Animals' that are found in nearly every Habitat around the World.

Types of Arachnids

Arachnida Orders and Suborders

1.  Acarina (Mites and Ticks) - There are about 30,000 Species.


  • Acariformes - There are about 32,000 described Species.
  • Opilioacariformes - There are about 20 Species found in North America.
  • Parastiformes - There are about 11,000 Species.

Araneae (Spiders) - There are about 20,000 Species of Spiders.


  • Mesothelae - Very rare, basal spiders, with abdomen segmented and spinnerets median.
  • Opisthothelae - Spiders with abdomen unsegmented and spinnerets located posteriorly.

Opisthothelae Infra-order:

a)  Araneomorphae - Most common Spiders.
b)  Mygalomorphae - Tarantulas and Tarantula-like Spiders.

2.  Amblypygi (Whip Spiders) - There are about 50 Species similar to Whip Scorpions.

3.  Opiliones (Harvestmen) - There are about 2400 Species of Harvestmen or Daddy Long-Legs.

4.  Palpigradi (Miniature Whip Scorpions) - There are about 21 Species of Miniature Whip Scorpions.

5.  Pseudoscorpionida (Pseudoscorpions) - There are about 110 Species of Small Arachnids with Venomous Pincers.

6.  Ricinulei - There are about 15 Species of Ricinuleids, small, short-legged Arachnids from Africa.

7.  Schizomida - Micro Whip Scorpions. There are about 110 Species.

8.  Scorpiones (Scorpions) - There are about 600 Species of Scorpions.

9.  Solifugae (Sun Spiders or Wind Scorpions) - There are about 570 Species of Camel or Sun Spiders.

10.  Thelyphonida  (vinegarroons or whip scorpions) (formerly Uropygida) - There are about 100 Species.

11.  Trigonotarbida - It includes extinct Species.

12.  Phalangiotarbida - It includes extinct Species.

History and Evolution of Arachnids

The earliest forms recognizable as Arachnids include a Scorpion that dates from the Silurian Period (about 443.7 to 416 million years ago) and an Acarid from the Devonian Period (416 to 359.2 million years ago). Spiders with segmented abdomens and presumably four pairs of spinnerets are known to have existed 345 million years ago during the early Carboniferous Period. Micro Whip Scorpions have been described only as 190-million-year-old fossils from the Jurassic Period in Europe, and the Schizomids are known from about 7 million years ago, during the late Cenozoic Era, in Arizona. The Mesozoic Era (about 251 to 65.5 million years ago) is poor in Arachnid fossils, but the Cenozoic Era (from about 65.5 million years ago to the present) is rich in them. The stem group of the Chelicerates (either of the first pair of fang-like appendages near the mouth of an arachnid) is believed to be among the members of the trilobite-like Olenellinae. These date from the Cambrian Period (542 to 488.3 million years ago). During Paleozoic times the 'Eurypterids', large aquatic Animals resembling modern Scorpions, were abundant, and both groups can be traced to a common ancestor. The transition to land habitats probably started in moist environments, such as under leaf-litter-like material. Many changes in anatomy and reproductive behaviour had to occur before the Arthropods were successful in their transfer to terrestrial life.

Arachnid Reproduction

Arachnida have a common prevalence of 'Pre-Nuptial Activities' when the sexes meet. The courtship of the Scorpions is features a kind of dance, known as 'A Promenade À Deux' in which the Scorpions move sideways and backward in a dancelike motion, their tails are raised and entwined, until the Male grasps the Female and leads her away to a sheltered spot where he digs a burrow for her reception. The reproductive organs of Arachnids are generally contained in the abdomen and open ventrally on the second abdominal somite. Male sex organs may consist of one diffuse testis or one or two compact testes. The spermatozoa produced are conveyed to a median gonopore through one or two excretory ducts (vasa deferentia). Insemination into the Female may come from the Male gonopore in a liquid medium (as in spiders) or may be contained in packages called 'Spermatophores' (as in ticks and scorpions). An intermittent organ or penis may or may not be present to direct the spermatozoa into the Female during mating. Females possess a single or paired ovary, which may be either compact or diffuse and one or two oviducts may lead to the median gonopore. Eggs may be laid underground, in the shelter of a stone, under tree bark, enclosed in a cocoon, or other variations of these methods and structures. Females usually guard eggs or young, which are often born live and as miniatures of the adult with regard to appearance. Eggs may number from one to more than 1,000 in a single brood. Young Arachnids, hatch from eggs in all the Orders except the Scorpions, which are Viviparous. Young Scorpions, like young Wolf-Spiders, are carried for a time on their mother's back.

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