SPIDERS IN GENERAL
SPIDERS IN GENERAL
Spiders (Araneae) are the most famous and largest group (about 40 000 species) of spider-like animals which have special cobweb glands. Their body is constructed of two parts and these two parts are connected to the narrow part called the stem (pedicel).
They are mostly terrestrial animals (in some cases they can survive in water) and they can grow to the size of 0.4 mm and even to the size of 10 cm. They live almost everywhere where there are insects, especially in the tropics. Most spiders are carnivorous and they feed on various insects which they catch in nets woven of cobwebs.
Appearance – a typical spider has eight eyes and a body divided into two parts:
A spider’s mouth is surrounded by a pair of poisonous claws and jaws that look like legs.They have a sensory function, and the males are used to carry seeds.
All spiders are predators and inject venom into their prey, while others use the cobwebs. However, some cobwebs serve to protect the spider’s eggs, and some use the cobwebs to move through the air or to fly in the breeze.
There are 37 766 spider species known today, and they are located into 109 families. All spiders, except for those of the Uloboridae family, have poisonous glands and can carry into their victim from 0.3 to 0.5g of venom with a single bite.
However, the venom of most spiders is not that important medically because their clamps are too weak to penetrate human skin.
Spider venom poisoning is called araneism. Poisonous spiders are much less common in Europe than in South America or Australia, for example.
THE DIGESTIVE SYSTEM
The digestive system of a spider begins with the mouth, which are adapted to the intake of liquid food and this means that the digestion has to start from the outside. A spider injects its venom into the prey in order to paralyze it and then it discharges secret from its salivary glands, thus turning the prey into a half-liquid mass.
The suction part is of specific structure, specially constructed for the extraction of liquid food. The oral cavity and the pharynx make a pipe that serves to transport liquid food to the esophagus, which is also tubular and extends to the stomach.
There are hairs that prevent the esophagus to reach solid food particles at the top of the throat. The foregut is covered with a cuticle (this indicates the ectodermal origin), which is especially thickened in the stomach. The stomach is covered with strong muscles whose contractions imitate pumps and it sucks in food along with similar pump movements of the oral cavity and pharynx.
The stomach is then followed by a bowel which does not have a cuticle because of its endodermal origin.
The secondary bowel is spread into multiple diverticulums in which the main digestion of food is performed. The rectum, also lined with cuticle, has a well developed layer of muscles.
The place where the middle and the lower intestine merge is extended into an expansion from which a short tube is separated (the rectum), which leads to the vent.
HOW DOES A SPIDER MAKE ITS WEB?
First the spider releases a sticky thread that is carried with the wind. If the breeze carries the silken line to a spot where it sticks itself the first bridge is there. Then the spider carefully reinforces the line to make it stronger. After the first bridge is made, the spider makes a second Y-shaped line. These are the first three lines of the web.
The spider makes non sticky constructions at first and the distance between the lines is very wide so the spider can span it with its legs. Then the spider woves the sticky thread between the circular thread and while the spider is attaching this sticky thread to the lines it removes the construction thread.
The web is completed with sticky circular threads and the spider rests in the centre of the web, with its head facing downward. After the spider hunted, the web is worn out, so the spider removes the silk by eating it and then it constructs a new web. If the web was not damaged it can be used again after the spider had reinforced it.
There are many variations of spider webs. Spiders can leave out sectors or make only one sector, leave a hole in the centre or make a single line web.