The oldest of the arachnids that live on Earth are scorpions. Millions of years have passed, and they have not changed much. Today, science knows more than 700 of their species.
Scorpions are inhabitants of not only desert areas, as is commonly believed. They are also found in humid and hot tropics and subtropics, some species can be found in cooler places - in the mountains at high
altitudes. But ticks are a relatively young
group of arachnid arthropods.
But in my article I will give preference not to ticks and scorpions, namely to spiders, although I especially do not spare them. It is interesting to trace how in the process of evolution there was a decrease and simplification of the external structure of the spider, caused by the merger of the segments and distinguishing it from the scorpion. Here I will answer the eternal controversial question of how many legs a spider has, and I will also tell how its web is obtained. Go!
What does he look like
The external structure of the spider also fundamentally distinguishes it from insects. If their body clearly consists of three departments: head, chest and abdominal, then these arachnids have only two: the cephalothorax, formed by the fusion of the head and chest and consisting of strong chitin, and the soft abdomen. Eight eyes are located on the cephalothorax! Do not be surprised, but in general the minimum number of spider eyes is six, the maximum is twelve.
The cephalothorax is connected to the soft abdomen by a very thin and small tube. At the end of the non-articulated abdomen are the so-called spider web warts. I will tell you about their purpose below.
How many legs does a spider have?
Many of us still cannot correctly answer the question of how many legs a spider has and probably think that there are six of them, like all insects. But it was not there! This is the main difference - the presence of exactly eight legs, while insects, in fact, have only three pairs of legs. The first pair of spider legs are chelicerae. These are poisonous head extremities that serve to capture and grind food. Next are three pairs of legs designed for walking.
Here we are and we know how many legs a spider has, what departments it consists of, how many pairs of eyes it has and, perhaps most importantly, that it is not an insect! These are its main external signs. We pass to the internal "spider review".
The internal structure of the spider
It is a nervous system controlled by a brain located in the cephalothorax. The spider also has a heart, which is located in the upper front part of the soft abdomen (red in the picture). The heartbeat rate is 30-70 beats per minute, but if for some reason the spider is tense or exhausted, then this frequency increases to ... 200 beats per minute! At the end of the abdomen are the so-called spinning organs. They produce silk and are associated with glands that produce various proteins. As a result of mixing and polymerization of proteins, liquid silk is obtained, passing through special spinning organs and turning into a thread. Here it is - the web!
HabitatSpiders are spread all over the world - from Antarctica and highlands to deep gorges and caves. Many of them live on land, and some - in the seas, in fresh springs and in underground waters.