Overview

Phylum Anthropoda: Joint Footed Animals

CLASS INSECTA

Characteristics
Over 800,000 different species of insects have been described, some 80% of all animals. Although these vary greatly in size and structure they all possess certain fundamental characters which distinguish them from other animals.

Insects are invertebrates so do not possess backbones and, being jointed in limbs and body, they qualify for the phylum Arthropoda. This includes animals such as shrimps, centipedes and spiders but the insects themselves belong to the Class Insecta.

The earliest known fossil insects date back to the Middle Devonian period some 300 million years ago, whilst winged insects appeared quite suddenly in Carboniferous rocks which are about 250 million years old.

The fundamental characters displayed by all insects to a greater or lesser extent are:-

EXOSKELETON
A hard chitinous exoskeleton provides support and protection. It is arranged as a series of segments which may total 20 but are usually grouped together and specialised in some way so there appear to be far less.

THREE PART BODY
The insect's body is divided into three distinct parts: the head, the thorax and the abdomen.

The head is composed of six segments fused together and bears mouthparts and some sensory organs. The thorax is composed of three segments each bearing a pair of legs and the second and third often bearing a pair of wings. This is the basic pattern but may be modified in some way eg, flies possess only one pair of wings whilst beetles have one pair of functional wings (the hind wings) and have modified forewings which usually cover both the thorax and abdomen and provide protection. The abdomen typically consists of 11 segments but these are often telescoped.

The immature forms of insects, the larvae and pupae, exhibit the same basic characteristics although often in a much simplified form. Thus the eyes and limbs are very simple structures and the wings only exist as buds.

---

Classification
The existence of an exoskeleton imposes a restriction to growth necessitating moulting if the insect is to grow any bigger. Moulting is therefore a major process in the development of the adult insect. It also enables the insects to be classified according to their life cycle and this can be important when considering control measures. This classification is:-

Sub-class Apterygota - wingless with no metamorphosis. Moulting will still occur when the adult stage is reached.

Orders containing insects of public hygiene significance:

Thysanura	Silverfish and Bristletails
Collembora	Springtails

Sub-class Pterygota - 'winged' insects with adults distinct from young and not subject to moulting. There are two types:

Division Exopterygota, Wings develop externally; they exhibit incomplete metamorphosis with nymphs resembling adults.

Division Endopterygota, Wings develop internally; they exhibit completed metamorphosis ie there is a dramatic change from egg to adult involving distinct larval and pupal stages.

Orders containing insects of public hygiene significance:

Diptera	True Flies
Siphonaptera	Fleas
Lepidoptera	Moths
Coleoptera	Beetles
Hymenoptera	Wasps and ants

Acquisition of pest status
Insects are the most abundant of all land animals and have inhabited every environment except the sea. Their success can be attributed to:

• Exoskeleton, for protection and efficient water conservation
• Mobility through crawling and especially flying which helps in the efficient location of food, shelter, breeding sites etc.
• Complete metamorphosis with larvae specialised for feeding and growing and adults for mating and dispersal. This also enables the forms to occupy different environments which provides greater scope for survival and development.
• Rapid reproduction enabling rapid exploitation of resources when conditions are favourable.
• Small size - typically between 0.5-40mm long - facilitating ready exploitation of food and shelter.
• Adaptable, exhibiting a wide variety of forms suited to the environments in which they live.

Pests can be defined as troublesome or destructive animals. Only about 0.5% of insect species are pests and most of these infest plants and plant materials. Suitable conditions for insect pests are provided wherever crops have been concentrated in the field or store. Man and his environment, including domesticated animals, can also be infested when conditions are right. Insects in the non-agricultural sector are pests because they cause a:

Loss of commodities in store.
Reduction in the quality of stored commodities through:
Direct feeding damage
Tainting
Physical contamination
Secondary effects eg heating of grain
Nuisance
Health hazard eg:
Non-specific vectors of disease
Specific vectors of disease

In addition insects may be responsible for allergic reactions, bites or stings whilst an imagined infestation with ectoparasites may lead to the psychological disorder, 'delusory parasitosis'.