Page 185 - Biology_F5

Biology for Advanced Level Secondary Schools

Class Arachnida and abdomen (Figure 3.49). Most
The arachnids represent the second external appendages are attached to the
largest group of terrestrial arthropods cephalothorax. The appendages are legs,
after the insects. The class includes the chelicerae, mouthparts, and pedipalps. On
animals whose bodies are organised into the ventral part of the abdomen are two
two tagmata called cephalothorax or hardened plates covering the book lungs.
FOR ONLINE READING ONLY
prosoma (fused head with thorax) and The spinnerets, which produce silk, are also
abdomen (opisthosoma). Most of them located in the abdomen.
are carnivores, except the mites, which are
herbivores. Members of this class include
spiders, mites, ticks and scorpions.

Pedipalp
Distinctive features of class Arachnida Chelicerae
Arachnids can be distinguished from other Walking Carapace
members of the phylum Arthropoda by the legs Pedicel
Abdomen
following features: Spinneret
(a) Their bodies are divided into two
regions; prosoma and opisthosoma, Figure 3.49: Structure of a spider
separated by a narrow waist-like
constriction. Adaptations of spider to its mode of
(b) They lack antennae; instead they use life
pedipalps to detect external stimuli. Spiders live in almost every habitat. They
(c) They lack compound eyes, they only are adapted to different environments as
have simple eyes. follows:
(d) They lack true mouthparts; instead, (a) They have silk gland which produce
they have two pairs of appendages silk for capturing preys.
born from prosoma. One pair, the
chelicerae, is used for killing preys (b) They have pedipalps for sensation.
or defending themselves; the other (c) They have four pairs of legs for
pair called pedipalps holds the prey in locomotion.
place when the animal injects venom. (d) They have a pair of chericerae for prey

(e) Usually they have four pairs of walking capture, defence and venom injection.
limbs or legs.
(f) Their respiratory structures are book Class Chilopoda
lungs or book gills or trachea. This class consists of organisms found in
terrestrial environment. They are terrestrial
animals abundant in moist areas, such as
Structure of a spider leaf litters, under logs or rocks. An example

The spider has two main body parts: a fused of chilopods are centipedes.
head and thorax, called cephalothorax

Form Five Student’s Book
178