23.10.2010., subota
SYSTEM OF ARACHNIDS
 Spiders, Acari & Co. Arachnids belong to the group of chelicerates. The characteristics oft his group are jaws or chelicerae, specifically designed mouth appendages, with hollow pointed endings which are movable and from which the venom is injected into the prey. Spiders are not insects! The body of a spider consists of two parts, the front called prosoma, and the back called opisthosoma. In contrast, the insects have a three-part body, consisting of head (caput), thorax and abdomen. Spiders have 8 legs, while insects have only 6. Additinal pedipalps in spiders are transformed second pair of walking legs. On the contrary, the insects have antennae. On spider´s abdomen are lumps (spinnerets) through which spider silk (spiderweb) is excreted from silk glands. Spiders have six or eight individual eyes, which can be divided into main and secondary ones. The main eyes are used for recognizing images, while the secondary eyes serve for perceiving movements. Insects have reticular or compound eyes, consisting of many induvidual eyes. Spider poisons Almost all spiders have venoms which they use to catch prey. We talk of ˝venomous/poisonous spiders˝ when their venom can harm people. Out of 1000 spider species in Germany only a small number is ˝venomous˝. However, their bite is not life-threatening to healthy people. |
