Eco Bites

The Insect World

The animal kingdom has 36 groups called phyla; one of these, the phylum Arthropoda, is of interest to us. Arthropoda – the word means “jointed legs” – all have exterior skeletons and bodies consisting of a head and a varying number of segments behind the head, some or all of these segments have a pair of appendages.

The Arthropoda rose from primitive segmented worms known as annelids when a pair of appendages developed on each body segment. In the more advanced arthropods the rear segments lost their appendages, only a few pairs on the anterior segments remain. In these advanced arthropods some of the segments are often fused to form a single body unit.

A living remnant of the ancestral arthropod is Peripatus, a primitive animal found mostly in forest debris.

The insect family tree

Looking at the family tree of the original proto-annelid (below), one can see how the Arthropoda branched off. The class Insecta contains some of the most advanced arthropods, they are characterised by having 3 pairs of legs, and many have developed wings enabling them to fly, the only invertebrate that has the ability of true flight. Interesting to note on the family tree is that insects are much closer related to the Crustacea (crabs, shrimps, etc) than the Chelicerata (spiders, ticks, etc).

Insect bodies are typically divided into a head containing sensory organs (compound eyes & antennae) and mouthparts; a thorax of 3 segments (fused in more advanced insects) with the three pairs of legs and two pairs of wings (in adults only if present); and an abdomen with a varying number of segments and, in the adults, the reproductive organs at the end.

The word insect comes from the Latin “insectum” meaning “with a notched or divided body”; the word was first used in the early 1600’s. Originally the class Insecta was named Hexapoda (meaning six-legged), but Hexapoda is now the subphylum containing the Insecta and 3 six-legged wingless groups originally placed with the insects in one class.

The history of insect development

Insects are a very old group of animals on earth, probably developing about 450 million years ago. The first trilobites, a primitive arthropod now extinct, appear in the fossil record about 500 million years ago; and by 300 million years ago insects were already well-developed. By the time the dinosaurs appeared some 225 million years ago, most of the insect groups we know today were already present. An important exception are the butterflies, moths, bees, wasps, etc – all those dependent on flowering plants – they only developed after flowering plants appeared 130 million years ago. The fact is that for the last 100 million years, since long before mammals became the dominant species, insects have remained relatively unchanged (below left).

If one considers the number of animal and plant species on earth (above right), insects are without doubt the dominant group. Of the total approximately 1,8 million described non-bacterial species, insects number over 1 million, or more than 56%. Other invertebrates and plants total just over 20% each; the remaining 3.3% include all the other groups.

Insects have thus been hugely successful in colonising the earth, and are found in just about every single available habitat. They have also proved extremely adaptable, adapting to changing conditions much faster than other organisms; thus enabling themselves to flourish under conditions that have caused the extinction of many other animals or plants.

Classifying insects

The class Insecta is divided into Orders. These orders are subdivided into three groups – from the most primitive to the most advanced.

Apterygota (Wingless insects with no true metamorphosis) Thysanura Silverfish, fish-moths *Diplura 2-pronged bristletails *Protura Proturans, coneheads *Collembola Springtails *Some taxonomists now place these orders in classes of their own.

The Apterygota are wingless insects with no true metamorphosis - after hatching each instar will moult into a larger one until it finally moults into a sexually mature adult. There could be up to 30 or 40 instars.

Exopterygota (Undergoing partial metamorphosis) Ephemeroptera May-flies Odonata Dragonflies & damselflies Plecoptera Stone-flies Notoptera Rock crawlers Orthoptera Grasshoppers Phasmida Stick insects Dermaptera Earwigs Embioptera Web spinners Dictyoptera Roaches and mantids Isoptera Termites Zoraptera Zorapterans, angel insects Psocoptera Book lice & bark lice Mallophaga Biting lice Siphunculata Sucking lice Hemiptera True bugs Thysanoptera Thrips

The Exopterygota undergo partial metamorphosis - wings develop in those that have flight, sexual organs develop and in man species there are also a change in the mouthparts if the diet of the adults is different to that of the nymphs. The nymphs hatch out of the eggs and moult regularly into a larger nymph - in some species there could be more than 30 such moults before the final moult into the adult.

Endopterygota (With complete metamorphosis) Neuroptera Lacewings Mecoptera Scorpion flies Siphonaptera Fleas Trichoptera Caddis-flies Strepsiptera Stylops Coleoptera Beetles Lepidoptera Butterflies & moths Hymenoptera Ants, bees & wasps Diptera True flies

The Endopterygota undergo complete metamorphosis, so much so that in many cases adults and larvae were first described as separate species. A larva, usually wormlike, will develop from the egg; the larva will go through several moults before turning into a pupa - during the pupal stage metamorphosis from the larval form to the adult form takes place. A complete adult will emerge from the pupa.

Of the 1 million insect species, 79.5% are in the last four orders in the table – the Coleoptera, Lepidoptera, Hymenoptera and Diptera. The Coleoptera has the most species, about 400,000 – it is the single largest order of animals or plants on earth.

Insects are the most successful animal class on earth - in every conceivable habitat one or more species flourish; and they have been doing so for millions of years. Insects are also very adaptable and have easily adapted to changes in their environment caused by man; due to rapid generational cycles, their adaptation has been much faster than other animals or plants.

» Click here to read more about insects found in wetlands.

Compiled by Niel van Wyk from various sources