Among the 46,700 arachnids, spiders are of the largest order and rank 7th in the entire species diversity of organisms. Often we come across a common question, ‘Are spiders insects?’ Well, the answer is no, a spider is not an insect. Spiders are as different from insects as fishes and birds are. Unlike insects, spiders have 8 legs. The spider scientific name is Araneae. The body of a spider is divided into two parts. The appearance and behaviour of spiders are immensely diversified. Besides Antarctica, spiders are found everywhere in the world. The most commonly found species are Wolf Spider, American House Spider, Brown Recluse, Jumping Spider, Grass Spiders, Hobo Spiders, and Black Widow. However, the spiders outside Japan, North America, and Europe have not been collected or studied minutely.
As per the spider classifications, most spiders feed on other insects, that is, they are predators. Some spiders chase their prey and overpower them while some spiders weave a web and trap their prey. The spiders that chase and overpower their prey are active hunters. They are gifted with an advanced sense of sight and touch. The cob-webs are so woven that flying insects easily get trapped in them. Some species of spiders first kill their prey by injecting poison into their blood and then feed on them. Also, some other species of spiders immobilize their prey by wrapping them with silk traps and then feed on them.
The size of a spider’s body may range from 0.5 mm to 90 mm, that is, 0.02 inches to 3.5 inches. The spiders having the smallest body length were collected and studied for the first time in the 1980s. There are many species of these small spiders and these are most commonly found in tropical areas. Another spider information: the tarantulas or hairy mygalomorphs are the species of the largest spiders. In general, they are found in slightly warmer locations, such as the Americas.
The female spiders are way larger than the male spiders, due to sexual dimorphism in animals.
For example, the female orb weavers belonging to the Araneidae family are almost double the size that of the males belonging to the same species.
The size dimorphism in spiders favours bridging in male spiders and fecundity in female spiders.
Bridging refers to the phenomenon wherein a male spider makes a silk thread that is carried by the wind to the nearest object, thereby making a bridge. Hence, light-weight, small spiders are apt for building and travelling on silk bridges. Heavier male spiders may not be able to traverse the silk bridges as fast as the light-weight spiders.
Hence the small size of the male spiders is favoured.
Spider Classification and Distribution
Spiders are more commonly found in warm areas, that is, more species of spiders are found in the tropical regions than in the temperate regions. A few species are collected or studied in the Antarctic or Himalayan regions. Yes, spiders are also found at altitudes of 5000 meters. Most spiders are found on the land, and a couple of species are found in the fresh waters or salt waters’ surface. Like Tarantulas are commonly found in the Americas, there is a Eurasian species of spider that is found in slow-moving freshwaters.
Most small-sized spiders are spread across the globe with the help of ballooning. They secrete silk threads, which get them attached to the wind and get carried to newer regions. This phenomenon is called ballooning and has helped a lot in the distribution of spiders. There are several species of spiders that are distributed across the world with the help of the northern jet stream. Certain spiders are able to move through the air at elevations of 3 meters to 800 meters, with the help of ballooning.
Spiders are the most common predators due to their abundance and distribution all around the world. They are utilized as a pest-controlling species in the rice fields in South America, and China, and in the apple orchards in some parts of Israel. In fact, we can say that spiders have become a part of the modern strategies of pest management in these continents.
Yet again, the venom-secreting spiders are highly dangerous for humans.
Black Widow (Latrodectus): The poison secreted by the black widow is detrimental to humans, and is highly painful. It can also be referred to as a nerve poison.
Brown Recluse (Loxosceles): The poison released by the bite of a brown recluse leads to the death of local tissue.
Other species of spiders that are common in North America include Cheiracanthium mildei and Cheiracanthium inclusum. These species are mostly found entering the houses in the fall.
There are certain species of tarantulas that are highly poisonous. These include the Australian Atrax and the baboon spiders belonging to the African Theraphosidae family. The bite of these spiders can cause the death of tissues at the site of the bite.
Tarantulas are found abundantly in the North and South Americas. Certain species of tarantulas shed their abdominal hairs, as a defense mechanism against the higher-order predators. These abdominal hairs are capable of penetrating the mucous membrane on the skin and can cause allergic reactions.
Even though spiders are small-sized predators, yet, certain tarantulas are observed to be hunting on Vesper bats or Sheath-tailed bats. The spiders belonging to the families of Araneidae, Sparassidae, and Nephilidae are observed as potential bat-hunters. As a matter of fact, flying vertebrates (even birds) make a good source of prey for spiders.
Spiders have an exoskeleton and an endosternite within the cephalothorax. The muscles of the legs and jaw are attached to the endosternite. The body of any spider can be studied in two parts, namely, the abdomen and the cephalothorax. The brain and the stomach of a spider are enclosed in the cephalothorax that is also referred to as the prosoma. The legs of a spider are connected to this part of its body. The cephalothorax is covered by a carapace that provides protection to the enclosed organs. The underside of the cephalothorax is covered by the sternum and labium. The abdomen of the spider or the opisthosoma encloses the other organs of the spider’s body such as the heart, gut, silk glands, and reproductive organs. The abdomen of spiders is not segmented externally. It is attached to the cephalothorax by a pedicel. This pedicel is a slim stalk and for certain species of spiders, the blood vessels, gut, nerve cord, and other respiratory tubules pass through it. This facilitates the movement of the body parts of the spiders when they construct the web.
There are 6 pairs of appendages in the body of a spider. The first pair is in the jaw and is called chelicerae. There are poison glands in each fang of a spider. Also, the poison ducts pass through this pair of appendages.
The next pair of appendages are the pedipalps. In female spiders and young spiders, the pedipalps act as sense organs and cater to carry food. These pedipalps resemble the legs of a spider. These appendages are modified in the full-grown male spiders to carry sperm. The segment enclosing the pedipalps in the body of a spider is modified into an endite that caters to feeding.
The pedipalps are succeeded by the four pairs of legs. There are 8 segments in each of these 8 legs. Interestingly, spiders are capable of amputating their legs, during molting. The segments contained in each of the legs are as follows.
Spider Characteristics: Nervous System
The cephalothorax of a spider contains the entire nervous system. As per the latest research, the brain of a spider can be studied to understand the behaviours. It has been observed that web-building spiders happen to have a larger posterior part of the brain than those with a hunting instinct. The nervous tissue masses are fused with other ganglia beneath the esophagus and below and behind the brain.
The fine hair-like structures on the legs of a spider also act as sense organs. These leg-hairs are quite sensitive to the vibrations and air-currents. The other sense organs include the minute slits and parallel slits that are found near leg-joints and sometimes found all over the body of a spider. The slit is covered by thin membranes on either side. On the inner side, the slit is likely to be penetrated by nerves. These slit sense organs are quite sensitive to any stress on the cuticle. Spiders also have hearing and vibration receptors that sense the organs. The receptor organs have information on the position and movement of spiders. Spiders have their olfactory organs in the form of hollow hairs. These olfactory organs are located at the pedipalps’ tips.
The digestive system of spiders is quite simple. They ingest the food through the mouth, which leads to the pharynx. The ingested food passes from the mouth to the pharynx to the stomach. The midgut of a spider’s body is followed by several blind extensions leading to the first pair of leg appendages. A digestive gland is located at the front of the abdomen. A cecum, located at the end of the gut, leads to the anus.
Spiders have a preoral pattern of digestion, that is, they digest their food outside their mouths. Some spiders cover up their prey with digestive enzymes as they chew it. There are certain species of spiders that inject digestive enzymes into their prey when they bite it and then ingest the liquefied tissues of the prey.
The excretory system of spiders contains nephrocytes, which are large cells. These cells are located in the cephalothorax. These cells accumulate nitrogen-containing wastes. The other parts of the excretory system include Malpighian tubules, coxal glands, hypodermis (a pigment-storing layer). The abdominal gut ends are filled with excretory pigment. This pigment called guanine is white in colour. Mostly the excreted wastes are composed of guanine, uric acid, adenine, and hypoxanthine.
The respiratory system of spiders contains tracheae and book lungs. It is located in the abdomen area. The lungs lead to the atria that open up to the outside through slits. These slits are called spiracles. Air is passed through the tracheae to all the tissues in the body of a spider. These tracheae are located at the back of the abdomen whereas the pair of book lungs are located at the front of the abdomen. However, in small-sized spiders, there are two pairs of tracheae at either end of the abdomen. Also, in the species like tarantulas, there are two pairs of book lungs at both ends of the abdomen. In all spiders, the respiratory organs are enclosed within hard plates.
Spiders have a tubular heart located in the abdomen. It has several openings along the sides. These openings are referred to as Ostia. There is a pair of arteries to carry blood forward and backward during the contractions of the heart. When the heart contracts the Ostia closes. The blood enters the heart through Ostia. The blood flowing through the body of a spider contains a pigment called haemocyanin. During molting, there are changes in the blood pressure that helps the spider to break the skin and stretch its legs.
The circulatory system of spiders is variably developed. Those species of spiders that have book lungs also happen to have a highly developed circulatory system. However, spiders that have only tracheae as parts of the respiratory system, happen to have a less-developed circulatory system.
The reproductive organs of spiders are located in their abdomen. The sperms are kept in the seminal receptacles of the female spiders after mating. On passing through the oviduct the eggs of a female spider are fertilized. The fertilized eggs then develop into zygotes. Now, coming to the question, are spiders insects? The fertilized eggs are highly rich in yolk similar to any other arthropod eggs. So, spiders are not insects.
Specialized Spider Facts
Besides Uloboridae, most spiders have venom glands. These glands can be concluded as accessory digestive glands of spiders that facilitate the digestion of the prey pre orally. The secretion of spitting spiders acts like glue and fixes the prey to the surface. In the spitting spiders, these venom glands are located in the large dome-shaped carapace. Some spiders secrete mostly digestive enzymes in the venom, whereas the secretion of certain spiders is highly venomous to subdue even the vertebrates. In most spiders, the venom glands are located either under the carapace or in the chelicerae. The ducts carrying venom, stretch through the chelicerae and open up in the fangs.
The silk secreted by spiders is quite similar to that of insects. It is composed of a protein known as fibroin. The glands secreting silk are located in the abdomen of spiders. The silk is secreted when abdominal pressure is exerted on the glands. There are muscular valves in the ducts that regulate the secretion of silk from these glands. The most primitive species of spiders have two variants of silk glands. The orb-weaver spiders happen to have a minimum of seven types of silk glands. Each gland secretes a different type of silk.
Some other interesting facts about spiders are as follows.
About 40,000 species of spiders secrete silk.
7 types of silk can be secreted by a spider.
Unlike the common types of spiders, there is a species in Central America that does not feed on insects. This species is herbivorous and feeds on protein-rich buds.
Certain species of spiders happen to be near-sighted.
The brown recluse species of spiders have six eyes whereas the other species have eight eyes.
Spiders are one of the most abundantly found arthropods in this world. Apart from a few species, air and water colonizations of spiders have not yet been collected and studied. All the features of spiders along with the basic overview of the organ systems have been discussed above.