Monday, April 22, 2013

Wonders of the world: Spiders

Tree spider


When I was a child like my other friends, I was scared of spiders. Even now when I see a spider in my room, certainly I do not scream, but yet, I avoid them and try to chase them out from my room. When I grew up little by little, I wanted to get rid of this fear and crazy imaginations about spiders, so I started reading about spiders in order to get familiarize with these little fascinating creatures.
We all know that spiders are disliked by many people due to their lack of knowledge about spiders. According to the taxonomy of spiders, they belong to the class Arachnida Phylum Arthropoda. These spiders are carnivores which mean they feed on small insects or flesh. They do it by trapping their pray using their web or by hunting on ground. In addition, these spiders can be seen all over the world except in air since they do not have wings to fly and in ocean since they are air-breathing animals. Yes, there are some spiders who can live under water such as Diving Bell Spider ( Argyroneta Aquatica) but they also need air. In this case they trap air in a bubble held by hairs on its abdomen and legs.
Another reason is their size and the appearance which give an expression that they should not be petted or loved, they are venomous and dangerous. When we consider about their size, most spiders are relatively small (2-10 mm) but there are exceptions like Tarantula whose body length will extend up to 80-90 mm. Even though size varies from one spider species to another, their body consists of two main parts. First part is the “Cephalothoraxe” (Prosoma) which is the anterior part and the other part which is the posterior part, known as “Abdomen” (Opisthosoma)
Parts of a spider (www.biological-diversity.info)
All six pairs of extremites which control the movement, food uptake and nerve interactions of the spider are located in prosoma. This anterior part, Prosoma Consists of two plates, upper plate or the dorsal plate is known as “Carapace”. Carapace is formed by six fused segments which bear the dorsal mussels and most importantly eyes of the spider. Ventral side of the cephalothoraxe is called “Sternum”. Usually, sternum is derived from four fused sternites. Both “sternum” and “carapace” are connected by soft pliable tissues called “Pleurae” and form prosoma exoskeleton.
Anterior median (AM) and Anterior lateral (AL) eyes of Maevia inclemens
(www.arthropodawordpress.com)
As I mentioned earlier, the most important organ located in carapace is the eyes of the spider. These eyes are important because they not only give a visionary aid but also they are very useful in spider classification. Usually, a spider has eight eyes which are located either in  the anterior row or the posterior row. According to the position of the eyes they can be categorized in to four groups. They are, ALE, AME, PLE and PME (i.e. A= Anterior, P= Posterior, L= Lateral  M= Median, E= Eyes). Although spiders have eight eyes, sometimes the number of eyes can be reduced to six, four and even two. Some spiders like Hawaiian cave wolf spider (Adelocosa anops) don’t have eyes at all.
Six pairs of extremities consist, “Chelicerae”, “Pediphals” and four walking legs. These “Chelicerae” have two parts, a movable articulated fang and a stout basal part which act as resting place for the fang. When a spider bites, the fang moves out from the basal segment and penetrates the pray. From the small opening at the fang, venom is injected to the pray. “Pediphals” and the mouth parts are the second pairs of appendages. These pediphals look similar to legs in appearance but they are only used for pray catching activities. Besides that, male spiders use their pediphals as copulating organ. This special feature of male spider is unique to them whereas pediphals are used only for pray catching by other Arthropods. Last extremites that are connected to the pleura is four pairs of walking legs. Each leg has seven segments which are, “ Coxa”, “Trochanter”, “ Femur”, knee like “Petella”, ”Tibia”, “Mettarsus” and finally “Tarsal” with two claws, which are known as “tarsal claws” which are serrated like a comb. In some spider species another additional middle claw is present, whereas this third claw is used to catch and hold the silk threads of their web. But in some hunting spiders beside these “tarsal claws” there is a dense cushion of hair on their feet which is known as “scopulae”. The spider walks with the help of these scopulae hairs, whereas these scopulae hairs split in to thousands of fine cuticular extensions called “end feet” which act as suction cups. According to the Homon, Roscoe and Walke (1991), these microscopic end feet structures use to achieve physical adhesion which is enhanced by the capillary force of the extremely thin water film on the substrate so that spider has a “surefooted” grip while walking on vertical walls, window planes and other smooth surfaces. Other than walking these legs act as sensory organs of the spider since sensory hairs are located in legs. These sensory hairs are mechano sensitive “sensilla” which is used to sense the touch and the vibrations. Besides mechano sensitive sensilla, chemo sensitive hairs are present which nerve fibers are exposed directly to the environment to sense the changes in chemical compositions in the environment. Except mechanical and chemical sensitive sensilla, another group of sensory hairs are present to sense the humidity level of the environment.
Spider Scopular (www.howstuffworks.com)
Another interesting fact about spiders is the way of food intake; mode of food intake differs from spider species to species. But the common way is the pre-digested method which happens in the following way. The spider releases digestive fluid on to the pray which is wrapped with silk or pray that subdued with a venomous bite. And then it sucks the pre digested liquid. This method is used by the spiders that don’t have cheliceral teeth whereas spiders with cheliceral teeth mash up the pray.
Blackwidowspider (animalsnationalgeographic.com)
For a spider, venom is the most important tool in their survival. Spider venom is used for immobilization of the pray. Even though it immobilizes the pray, as a secondary action of the venom it can cause death to the pray. Spider venom is produced in venom glands of the spider which is located in prosoma and it contains large amount of neurotoxic peptides which paralyze the pray and biogenic amines and amino acids which may have a potential threat to human.  But out of more than 40000 spiders around 200 species are poisonous to human and they belong to Atrax, Latrodectus, Loxosceles and Phoneutria genera whereas these genera include, Black Widow, Brown recluse spider and Certain ctenid. But well known Tarantulas are not lethal to human but they are lethal to small animals such as frogs. This is because their venom is designed to paralyze their pray and as a defensive bite against large animals.
Spider web
Spider web is another beautiful thing about the spiders where spiders use proteinaceous silk which is produced in their spinning glands to build the web. This proteinaceous fibrion Spider silk scientists believe that water soluble, and when the change of orientation of polypeptide chain from an alpha configuration to beta configuration silk solidify make the web. Change of configuration of polypeptide chain simply by pulling it while weaving the web. Spider silk proteins contain different amino acids such as “Alanine”, “Glycine”, “Serine”, “Glutamic”, and “Proline”. The presence of these amino acids makes the spider web serve as a wound dressing in folk medicine. In modern medicine, spider silk is tested as a natural material for regeneration of nerve fibers. Spider cocoon threads are used in cartilage regeneration, and new material called silk bone is now developed to regenerate bon tissues.
Daddy spider (www.lucianabartolini.net)
Although we notice spider webs everywhere, not all spiders are capable of weaving a web. Only web spiders can weave a web. Depending on the web spider, they develop different webs. Funnel web spiders and sheet web weavers weave horizontal sheets, and cob web and daddy long legs spiders build irregular meshes. Spiral orb webs are usually built by family Araneidae, Tetragnathidae and Uloboridae. Spiders belonging to the family Theridiidae weave tangle webs or cobwebs. Funnel webs are usually weaved by the families of Agelenidae, and Hexathelidae.