The spider

There are two concentrations of ganglia in the prosoma (head-breast part or cephalothorax) which forms the brain. A number of nerves extends from the brain to the legs, eyes and the rest of the body. The brain takes up a volume of 20 - 30% of the prosoma.

Spiders have several sensory organs to get an impression of the surrounding in which they live.

A spider does not have ears. Mammals hear with help of hairs that are located in the ear where pressure waves in the air are converted to electric signals. These signals are then sent to the brain and interpreted as sound. A spider hears with very tiny hairs on her legs (thrichobotria). She is very capable of localizing the origin of a sound by interpreting the movement of the air produced by that sound.

Hearing hair of Amaurobius similis ^(bg)

The eyes of spider differ greatly between families. Spiders who hunt without a web like wolf spiders (Lycosidae), lynx spiders (Oxyopidae) and jumping spiders (Salticidae) have a well-developed eyesight. Jumping spiders can see nearly as well as humans. Experiments have shown that they are even capable of seeing colors. Cave spiders, which live in the dark, have no or hardly any eyesight. They depend completely on sound and feeling.
The structure of the eye is in basic similar to our eye; behind a single cuticular lens lays a cellular vitreous body and the visual cells. Together with pigment cell this forms the retina.
The spider has two types of eyes; the main eyes and the secondary eyes. The main eyes are always the middle largest ones of the eight eyes the most spiders possesses. There are a few families of spiders with six eyes
Sometimes the secondary eyes have a light reflecting layer (tapetum). This can be easily observed if one shines with a light in the eyes of a spider during the night.

Our well-known orb weaving spiders, like the Araneus diadematus, have very small eyes. They do not depend heavily on their eyesight to catch their prey. They have a good developed feeling mechanism that makes them capable of detecting the movements in her web.

Spiders detect smell with scent sensitive hairs located on their legs. A sense of taste in their mouth is missing. A spider feels her prey with chemo sensitive hairs on her legs and senses if the prey is consumable.

Beside the above-mentioned hairs on the legs, spiders have more features worth studying. The legs consist of seven segments.
Beginning from the body these are in the following order and called coxa,
trochanter, femur, patella, tibia, metatarsus and tarsus.

Who has never found a house spider crawling in the sink incapable of getting out? But jumping spiders are never found in the same sink.
There must be a difference in the structure of the legs between families.

Tiny hairs on the tarsi of Pardosa sp. ^(bg)

Claws of the orb web spider Larinioides sclopetarius

Legs ends of some spiders

Orb weaving spiders have claws on their tarsi. This is the reason they cannot climb on smooth surfaces but are very capable of hanging on threads. The spider grabs a thread with its middle claw and squeezes the middle claw against serrated bristles, situated opposite the claw. This gives a firm grip. To release the thread the claw is elevated and the bristles push back the thread. The elasticity of the thread also causes it to spring back out of the clasp of the claw. And why does the spider not stick to its own web? The answer is simple; the spider avoids walking on the sticky lines and when it touches a sticky line by accident the contact area is too small to stick permanently.

The movement of the legs of a spider is partly hydraulic, partly by muscles. The spider stretches its legs by raising the blood pressure in the legs. A jumping spider can create a force that makes her capable of jumping 25 times her length. This hydraulic system works excellently and can be seen if we compare the legs of a grasshopper to those of a spider. The grasshopper has two gigantic legs with a lot of muscles packed in it. Looking at the jumping spider one cannot detect these extra muscles.

The legs of the jumping spider, Philaeus chrysops in comparison with the legs of the grasshopper Miramella alpina