Introduction to the Arachnides (spiders, scorpions, mites)
Dr. Rajan Sankaran
Dr. Meghna Shah
The arachnida are the most familiar of the arthropods, outside of the ever-present insects. With over 80,000 named species, they are united by the possession of eight legs, chelicerae and pedipalps, and the lack of biting and chewing mouthparts.
The class arachnida is extremely diverse in form and in lifestyles, and little more can be said that includes them all. This diversity is reflected in the classification scheme shown below.
Arachnida are divided into 13 subclasses/orders of which the Araneae (spiders) and the Scorpiones (scorpions) are the best known.
- Order: Scorpiones (Scorpions)
- Order: Pseudoscorpiones (Pseudoscorpions)
- Order: Solifugae (Sun Spiders or Wind Scorpions)
- Order: Palpigradi (Miniature Whip Scorpions)
- Order: Uropygi (Whip Scorpions)
- Order: Schizomida
- Order: Amblypygi (Whip Spiders)
- Order: Araneae (Spiders)
- Order: Ricinulei
- Order: Opiliones (Harvestmen)
- Order: Acari (Mites)
Most arachnids are terrestrial (live on land).
Most arachnids eat live prey. Most are predacious and carnivorous by nature as they feed on insects. Some are scavengers, and a few mites are parasitic.
The arachnids evolved in the sea, but now they are almost entirely terrestrial and have developed several important features to help them survive on the land. These include: a waterproof or waxy exocuticle, internal fertilization, malpighian tubules as a metabolic excretory system and internal organs for breathing and gaseous exchange.
An arachnid’s body is divided into two parts: head and thorax. This is fused to form a cephalothorax, which is joined to the abdomen, in some species by a narrow stalk.
The cephalothorax bears six pairs of appendages.
- First pair; known as chelicerae*, are pincer-like* or piercing fang-like These are used mainly for feeding.
- Second pair; the pedipalps*; leg-like or enlarged with claws at the end. These are used as hands and in some male spiders are modified as copulatory organs.
- Four pairs of walking legs with two or three enlarged claws. The two-clawed spiders are hunters (e.g., jumping spiders, huntsman spiders, ground spiders). Most have thick hairbrushes (scopulae or claw tufts) on the ends of the legs. These improve traction on smooth or sloping surfaces like leaves or tree trunks. Many of the three-clawed spiders are web builders, often with claws and hairs modified for silk handling (e.g., orb-weavers, gum-footed web spiders, lace-web spiders).
In all arachnids the mouth is small and narrow, and they lack jaws for biting and chewing. Unlike insects, they all suck up their food in a liquid or semi-liquid form. Food is ingested in most cases as a liquid. Often the food is given some help to become liquid by the secretion of, or regurgitating onto or into the food, digestive enzymes from the stomach. However, this food is often torn apart by the chelicerates before ingestion.
The production of silk and poison is characteristic of some orders, but the methods of production and their origins are varied.
- Silk is produced from abdominal glands in spiders and from the chelicerae in pseudoscorpions.
- Poison is produced from the chelicerae of spiders, the tails of scorpions, and the pedipalps of pseudoscorpions.
The number of eyes varies, with up to 12 in some scorpions, but generally vision is poor. Many species are nocturnal and equipped instead with sensory hairs to detect prey. The few exceptions are the Salticidae, Thomisidae and Lycosidae, which have exceedingly good vision. Most spiders can detect polarized light, which helps them to know where they are in the world. Most spiders have eight eyes, while a few families have six (Dysderidae, Sicariidae, Oonopidae). The spiders’ eyes are arranged around their heads in a set pattern that is often helpful in field identification.
Arachnids lack wings and antennae, in contrast to insects.
Life cycle and reproduction
All arachnids are dioecious*. Male arachnids pass a packet of sperm into the female with the help of pedipalps, chelicerae, or legs. The female releases the egg, which hatches immediately in some scorpions. In some species the young develop and hatch inside the female’s body. Parental care of the young is common to all. This is seen in the form of egg guarding.
Method of attack and defense
Larger arachnids rely on strength to subdue their prey, while a few like spiders, pseudoscorpions, and scorpions can also inject venom.
The English word spider comes from the old English Spithra for ‘spinner’.
- Size ranges from 0.5 to about 90 mm (0.02–3.5 inches).
- The body is divided into 2 parts, the cephalothorax (prosoma) and the abdomen (opisthosoma).
- 8 legs.
- The exoskeleton of spiders is composed of a tough material called cuticle.
Difference between snakes and spiders
Patient expressions seen in both Snakes and Spiders
Possible human spider expressions
- Practical jokes
- Stand up comics
Patients expressions of Spider qualities-
|Behavior||Sexuality||Impulse||Attack /Defense mechanism|
|Intense pace, speed, hyperactivity.Busy
Quick and constant movement.
Patterned, rhythmic movement.
Short life span
Sensitive to temperature changes
|Male female ratioMales attract females
Female kills male after mating
|Sudden urgesShort bursts
Sudden fear of death
|Caught and trappedBurrowing
Deceit, feign, pretence, disguise
Kill and hide
|Ambush attack from burrows or retreats or behind trapdoorsLurking behind trapdoors
Not commonly seen
Longer live spans
Non sticky webs
More confined, less mobile
Chelicerae: pointing downward
|Trap in sticky webs or hunt by rapid chaseLie in wait
Seen very commonly
Shorter life spans
Not very aggressive
Large, sticky webs
Constant movement, busy
Chelicerae: Pinching action
To briefly summarize Spiders we can note the following points–
S-small, speedy, sudden.
P-pursued, paralyse, punish.
I-irritable, impulsive, insane.
D-deceit, destructive, death.
E-excite, ensnare, escape.
R-restless, rhythm, rage
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