Arthropods are characterised by having the following features:
- a hard external skeleton (called a exoskeleton)
- a segmented body
- at least three pairs of jointed legs
Modern insect classification divides the Insecta into 29 orders, many of which have common names. Some of the more common orders are:
- Mantodea - praying mantids
- Blattodea - cockroaches
- Isoptera - termites
- Siphonaptera - fleas
- Odonata - dragonflies and damselflies
- Dermaptera - earwigs
- Diptera - flies
- Lepidoptera - butterflies and moths
- Orthoptera - grasshoppers, katydids, crickets
- Coleoptera - beetles
- Hymenoptera - wasps, bees, ants, sawflies
Scientists have described and given scientific names to about 920,000 species of insects in the world, which represents almost 85% of all known animal species. For comparison, only about 4,000 of the known animal species are mammals, man being one of these. There are more species of dragonflies than mammals, and almost as many species of cockroaches (3,500 species). There are about 9,000 species of birds, but almost twice as many species of butterflies.
Most insects have never been given scientific names. It is estimated that there are 20-30 million species of insects on the earth at present. In a good year, taxonomists throughout the world describe and name about 2,000 species of insects. At this rate of naming new species, it will take 10,000 years to describe and name 20 million species. Unfortunately, many of these 20-30 million species of insects will become extinct before they are named because of habitat loss and other environmental problems.
More than one-third of the named species of insects are in one order, the Coleoptera (beetles), which includes about 350,000 species. The next largest orders are Lepidoptera (butterflies and moths), with 165,000 species; and Hymenoptera (wasps and bees), with 103,000 species. The order Diptera (flies) includes about 120,000 species. These four orders, Coleoptera, Lepidoptera, Hymenoptera, and Diptera, include more than 80 per cent of the named species of insects.
Insect collecting is historically a widespread, essential predecessor of scientific entomology and at the same time the formerly popular (until the mid-20th century) educational hobby of collecting insects.
Insects are common in almost every part of world, and may be easily found by looking closely at plant leaves or flowers, under rocks and boards, in water, and so forth. At night, many types can be caught flying around lights. A butterfly net is quite useful for capturing flying insects. Insect traps, some of which are baited with small bits of sweet foods (such as honey), are also quite effective.
Once collected, a killing jar is often used to dispatch insects before they damage themselves trying to escape. The jar is usually charged with ethyl accetate which is very effective in killing the insect, but still keeps it soft enough to be mounted properly in a collection.