Insects have a distinct set of characteristics which all must have if they are insects. They must have three pairs of legs (even though sometimes they may appear different as larvae, such as caterpillars), a set of mouth parts, and a head, thorax, and abdomen.
Arachnids have eight legs, and examples are spiders, ticks, and mites.
Crustaceans have hard body shells and are usually aquatic, although some, such as woodlice, live on land.
Arachnids, crustaceans and insects are three different types of animals within the animal kingdom. Arachnids include species like spiders, ticks, mites and scorpions. Crustaceans include species like barnacle, crayfish, crabs, lobster and shrimp. Insects include species like ants, bees, butterflies, grasshoppers, mosquitoes and wasps.
The defining characteristics of these three different types of animals are described below.
- Body structure: Crustaceans have no skeleton. They have a tough outer shell called exoskeleton that gives it its distinctive shape and protects internal organs. Insects also have exoskeleton.
- Main body parts Arachnids have two main body parts - abdomen and cephalothorax. Crustaceans have three main body parts - head, thorax and abdomen. Insects body is divided in three main parts.
- Legs: Arachnids contain four pair of legs Crustaceans have 6 to 14 pairs of jointed legs Insects have three pairs of jointed legs.
- Antennae Arachnids have no antennae. Crustaceans have two pairs of antennae. Insects have a pair of antennae.
- Wings: Arachnids and crustaceans have no wings while insects have wings.
- Breathing: Some arachnids breathe through tubes, while others have lung like organs called book lungs. Most crustaceans breathe through gills. However smaller ones breathe through their skin. Insects breathe through holes on their sides.
- Blood circulation: In larger crustaceans blood circulation is by means of heart, In smaller ones the blood circulation is by means of body movement. In insects the blood circulation is not by blood vessels. Rather the blood fill the whole body cavity and bathes all organs and muscles. The heart for circulation of the blood is in the form of a tube.
- Eyes: Arachnids have one to six pairs of simple eyes. Some arachnids have no eyes. Crustaceans have a pair of compound eyes, but some have a pairs of simple eyes. Insects have a pair of compound eyes.
Ex.: Spider, cancer, millipede,ant,scorpio, crawfish, lobster
General character of arthropods:
- Segmented body covered with an exo skeleton.
- Three or four pairs of Appendix united among themselves. - Muscles well developed, the link with exo skeleton. - The body is divided into two (three at insect) main regions: cefalo thorax(insect - head and thorax) and abdomen. -Well developed Mouth.
- Specialized senses, highly developed.
- Circulatory system, open type.
- Articulated limbs.
- Breathing through gills, trachea and pulmo-trachea.
Insects are the arthropods most valued and most numerous (approximately one million species). General characters of insects:
- The two pairs of wings.
- Oral apparatus.
- Metamorphosis is complete.