The one million species of insects make up the largest group in the Animal Kingdom. Insects differ from other Arthropods in having three pairs of legs, usually two pairs of wings and special insect mouthparts. Mandibles, derived from ancestral legs, replace the chelicerae of the chelicerate arthropods, as the first pair of mouth parts. Insects occur in all possible terrestrial habitats including freshwater, deserts and Polar Regions. The sea is the only habitat, which they have not invaded.
The insect body is divided into three distinct regions: the head, thorax and abdomen. The segments of the head are completely fused. The head bears a pair of antennae, various sensory receptors, and three pairs of mouthparts: a pair of mandibles adapted for tearing or cutting food; a pair of maxillae and a single labium, formed from the fusion of the second pair of maxillae, are used to handle food. The thorax is composed of three segments each bearing a pair of legs. The last two segments may also bear a pair of wings. The abdomen is composed of varying numbers of segments; it bears no legs but may possess appendages, e.g. claspers used in mating. Gas exchange occurs through a system of small tubes known as tracheae, which run throughout the body. Air enters the system through spiracles, openings in the side of the thorax and abdomen. This type of respiratory system is the major factor in restricting insect size.
Young insects metamorphose or develop into adults in one of three ways. The young of simple insects, like silverfish, resemble the adult when they hatch although they are not sexually mature. The first type of development in winged insects is termed hemimetabolous (incomplete) development. In slightly more advanced insects, grasshoppers and cockroaches, the young gradually develop wings and reproductive organs with successive moults. The immature stages up to the final moult are termed nymphs or naiads, if aquatic.
The second type of development in winged insects, as seen in butterflies and beetles, is termed holometabolous (complete) development. It has three distinct stages: an initial feeding larval stage at the end of which the larva ceases to feed and transforms into a second quiescent, pupal stage. During the pupal stage the adult structures develop until the mature imago or adult emerges from the pupal skin. The great advantage of this type of development is that it allows larvae and adults to exploit different habitats and food sources.
The twenty six orders of insects are divided initially into two subclasses: the more primitive wingless insects are assigned to the Subclass Apytergota (3 orders); the winged insects to the Subclass Pterygota (23 orders). The wingless condition of some members of this subclass is considered to be a secondary feature due to a loss of wings. The winged insects are in turn divided into hemimetabolous insects (13 orders) or holometabolous insects (10 orders).
Subclass Aptyergota (wingless insects): Order Protura; Order Thysanura; Order Collembolla.
Subclass Pterygota (winged insects):
- Hemimetabolous insects: Order Ephemeroptera (may flies);Order Odonata (dragonflies); Order Orthoptera (grasshoppers, crickets); Order Hemiptera (assassin bugs, water scorpions); Order Homoptera (cicadas, aphids); Order Dermaptera (earwigs); Order Plecoptera (stoneflies); Order Embioptera (web-spinning insects); Order Psocoptera (book and bark lice); Order Zoraptera (termite-like insects); Order Thysanoptera (thrips); Order Mallophaga (chewing lice); Order Anoplura (sucking lice).
- Holometabolous insects: Order Isoptera (termites); Order Hymenoptera (ants, bees, wasps); Order Lepidoptera (moths, butterflies); Order Diptera (flies); Order Coeloptera (beetles); Order Neuroptera (lacewings, antlions); Order Mecoptera (scorpion flies); Order Trichoptera (caddis flies); Order Strepsiptera; Order Siphonaptera (fleas).
Our insects are arranged in two sets of cases. The cases shown above are part of the original sequence of cases that illustrated all invertebrates including the arthropods. A more recent display includes representative insects from all the orders and illustrates current opinions of insect relationships.
Click to visit the phylogenetic display of the class insecta.
Two additional sets of pages illustrate the British Butterflies and the Order Coleoptera.