An invertebrate is an animal without a backbone. More than 90% of all animals are invertebrates. Insects form the largest group of invertebrates. Like millipedes and centipedes, crustaceans and spiders, they are arthropods, with jointed bodies and an outer protective casing. There are also many soft-bodied creatures, such as worms and jellyfish, often living in water or damp areas where sun and air will not dry out their bodies. Molluscs are also soft-bodied, but many of them are protected by an outer shell.
All living things are placed into groups depending on common characteristics. The animal kingdom is informally divided into two groups, the vertebrates and invertebrates. Invertebrates are a group of animals that have no backbone, unlike animals such as reptiles, amphibians, fish, birds and mammals who all have a backbone.
Over 95% of all animals on the earth are invertebrates of one form or another. Invertebrates are found just about everywhere in both terrestrial and aquatic habitats, and include animals ranging from sponges, corals and seastars to insects, crabs and worms, just to name a few. For information on collecting aquatic invertebrates in freshwater environments see the Water-watch site. Over 80% of all invertebrates are grouped into the single phylum Arthropoda that includes spiders, crustaceans, centipedes, millipedes and insects.
Insects are different from most other invertebrates. They are the largest Class of organisms and account for over 75% of all animal species. Insects can be separated from other invertebrates as they generally have 6 legs and conform to a common body plan. This body plan comprises of 3 parts, the head, thorax and abdomen although some parts may be more distinct than others. Particular insect orders may have some structures absent, reduced or greatly modified and some young stages can appear very different from their mature adult form.
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