Arachnida, in the subphylum Chelicerata, is a genus of joint-legged invertebrate animals (arthropods). Arachnida involves orders that include scorpions, spiders, mites, ticks, solifugae, and harvestmen. Horseshoe crabs were also positioned in Arachnida in a molecular phylogenetic analysis in 2019.
Almost every adult arachnid includes eight legs, though other species have adapted the front pair of legs to a sensory feature, whereas in other species, separate appendages may develop big enough to take on the existence of additional pairs of legs.
Almost every existing arachnid, living mostly on soil, are terrestrial. Some occupy freshwater habitats, however, and even marine environments, with the exception of the pelagic zone. About 100,000 named species are included.
Unlike adult insects, some of which have six legs, nearly all adult arachnids have eight legs. Arachnids, however, often have two more appendage pairs that have been modified for defence, feeding, and sensory perception. Chelicerae, the first pair helps in serving the mechanism of feeding and defence. For feeding, locomotion, and/or reproductive functions, the next pair of appendages, the pedipalps, were adapted.
The mites and Ricinulei larvae have just six legs; a fourth pair normally appears when nymphs are moulded. Mites, though, are variable: as well as eight, there are six or even four-legged adult mites.
The fact that they have no antennae or wings further separates arachnids from insects. Two tagmata called the prosoma or cephalothorax, and the opisthosoma, or abdomen, are grouped into their bodies.
A single and unsegmented carapace normally covers the prosoma. With the more primitive types, the abdomen is observed to be segmented, but in certain classes, different degrees of integration between the segments occur. Usually, it is classified into pre-abdomen and post-abdomen, but this is only fairly evident in scorpions, as well as the abdominal parts are fully fused in certain orders, including the Acari.
In scorpions, where that has been altered to a stinger, a telson is present, and in the Schizomida, whip scorpions and Palpigradi.
Arachnids have such an exoskeleton, like other arthropods, and that they also have an interior cartilage-like tissue structure, called the endosternite, to which some muscle groups are connected. The endosternite in some Opiliones is found to be calcified.
There are two body divisions in spiders, the cephalothorax and the abdomen. As per Jo-Anne Nina Sewlal, an arachnologist at the University of the West Indies in Trinidad, the first, or front portion, is the cephalothorax that is created by the fusion of the head and thorax. The mouth, eyes, and legs are found in the cephalothorax.
The mouth is made of many pieces. According to University of Kentucky entomologists, the spider's jaws, called the chelicerae, are tipped with fangs.
Such appendages are used to retain prey when venom is injected by the spider. The labium and labrum have been behind the jaws, which act together to direct food into the mouth of the spider.
Diet and Digestive System
Arachnids, relying on the pre-digested bodies of insects as well as other small animals, are mainly carnivorous. Intake of solid food particles and therefore vulnerability to internal parasites occurs only in harvesters and among mites, including the house dust mite, however, it is not uncommon for spiders to consume their own silk. To kill prey or foes, some classes secrete venom from specialized glands.
In their stomachs, arachnids create digestive juices and also utilize their pedipalps and chelicerae to spill them over their dead prey. The digestive fluids easily transform the prey into a nutrient broth, which is sucked by the arachnid into some kind of pre-buccal cavity situated right in front of the mouth.
The stomach is tubular in form, spreading across the body with several diverticula. Both the stomach and its diverticula create digestive enzymes and the food absorbs nutrients. It spreads across much of the body and, in the posterior side of the stomach, leads to a small sclerotised intestine and anus.
Two kinds of eyes are available to arachnids: the lateral and median ocelli. From compound eyes, the lateral ocelli developed and therefore has a tapetum, that increases the ability to gather light. Excluding scorpions, that might have up to five lateral ocelli pairs, more than three pairs aren't ever found. A transverse bend of the ectoderm develops into the median ocelli.
The descendants of the original arachnids possibly had both kinds, but one type or the other is often missing in modern ones. The eye's cornea often serves as a lens and seems to be continuous with the body's cuticles. A translucent vitreous body is under this, and afterwards the retina and, if visible, the tapetum.
Most arachnids include two main types of sensory organs, in contrast to the eyes. The fine and thin sensory hairs which surround the body and give the animal the sense of touch are the most essential to most arachnids. These may be relatively basic, but perhaps more complex structures, named trichobothria, also occur in many arachnids.
Finally, the slit-like pits surrounded by a thin layer are slit sense organs. A little hair contacts beneath the membrane within the pit and senses its motion. It is suspected that split sense organs are included in proprioception, and possibly even hearing.
Arachnids, that are found in the abdomen, can have either one or two gonads. Mostly underneath the second abdominal segment, the vaginal entrance is normally located. The male passes sperm to the female in a packet, or spermatophore, in many other species. In certain arachnids, complicated courtship rituals have evolved to guarantee the sperm's smooth return to the female.
Sexual dimorphism is demonstrated by members of several orders.
Yolky eggs that hatch into young ones that mimic adults are commonly laid by arachnids. Even so, based on the species, scorpions seem to be either ovoviviparous or viviparous and live young. In many other arachnids, even females have parental care, with some of those exceptions being harvestmen.
Below given are the spider characteristics:-
1. Capturing Prey:
Sticky webs are the greatest method of prey capture. Different web placements allow different spider species to trap various insects in the same location, such as flat horizontal webs that trap insects flying up from the vegetation below, while flat vertical webs trap insects in horizontal flight. Web-building spiders have low vision but are highly vibration-sensitive.
Argyroneta Aquatica water spider women construct submerged "diving bell" webs which they fill through the air and further use that for prey digestion, moulting, breeding and growing offspring. They tend to live inside the bells almost entirely, flitting out to capture prey animals hitting the bell or the strings anchoring it.
2. Non-predatory Feeding:
While spiders are typically cited as predatory, as part of a mutually beneficial relationship with an ant species, the jumping spider Bagheera kiplingi gets over 90 percent of its food from fairly solid plant material provided by acacias. In the Anyphaenidae, Clubionidae, Corinnidae, Thomisidae and Salticidae families, juveniles of some spiders feed on plant nectar.
Laboratory experiments suggest that they do so purposefully and over long periods, and clean themselves regularly while eating. These spiders often prefer plain water to sugar solutions, which means that they are looking for nutrients.
In addition to sugar, nectar includes amino acids, lipids, vitamins and minerals, and studies have shown that when nectar is available, other spider species live longer. The risk of battle with prey and the expense of developing venom and digestive enzymes are prevented by feeding on nectar.
There is clear evidence that the colouration of spiders is camouflage which allows them to escape their main predators, birds and parasitic wasps, both with great colour sight.
In order to blend with their most natural backgrounds, numerous spider species are coloured, and most have destructive colouration, streaks and blemishes that break up the outlines.
In large colonies, several spider species which construct webs live together and display social activity, while not as complex as in social insects.
Anelosimus eximius may produce colonies of up to 50,000 individuals (in the family Theridiidae).
There is a definite inclination towards sociality in the genus Anelosimus: many known American species become social, and species are at least somewhat social in Madagascar.
Spiders' life cycles vary substantially from species to species. Although several popular house spiders survive for many years, some of them can survive for up to 7 years. But on the other hand, Tarantulas will survive into their 20s. A 28-year-old tarantula discovered in Mexico was the arachnid with the longest recorded spider lifespan until Number 16.
The list of spider facts have been given below:-
Spiders species can live under the water: The diving bell spider, which could be located in ponds in areas of Asia and Europe, lives submerged for the whole of its life. As these spiders breathe oxygen, they do the same by holding the air in bubbles that only an underwater web keeps in place. Just after primitive bell-shaped tools that individuals once used to explore the oceans, these systems were called diving bell webs.
Spiders tend to fly over hundreds of miles: Spiders don't really have wings, however by throwing threads of silk further into the wind, most spiders (small ones for that matter) may float across hundreds of miles. Throughout the wind currents, the silk gets swept up, and voila, the spider has become free to "fly" back to distant locations.
Spiders can dance:- From every continent apart from Antarctica, jumping spiders are identified. In comparison to capturing prey on websites, they have excellent vision and actively search for prey. They often participate in very complex dances of courtship.