Insects

Insects are a group of extremely successful animals because they have been able to adapt to exploit a wide range of environments and food sources. The adaptations often involve changes in body form and behaviour. About 700,000 species have been identified, but many more are still to be described, especially in tropical forests.

Additionally, there are countless individuals of many species. Social insects, e.g. bees, wasps, ants, termites, are especially impressive in view of what can be achieved through the co-operation of many fairly weak individuals.
Perhaps it is human nature to separate insects into ones which are of use to Man and those which may be harmful, but the syllabus expects you to know about some insects from each category. These include bees, flies. mosquitoes, and various pests of protected cultivation.

From a classification point of view. insects are actually a class within the phylum Arthropoda, which are characterised by jointed limbs. There are many orders and families of insects. All members of this group have exoskeletons, so the body is effectively a hollow box, and the fairly light yet stiff outer covering (containing a compound called chitin) also serves as a barrier to water loss, which is a major problem in small land animals.

Body plan

In insects, the body is pinched in and divided into 3 main sections:

Head
bearing eyes (usually compound), antennae and mouthparts modified to suit particular feeding methods.

Thorax
(3 segments) with legs - How many? > 6 (3 pairs)
and usually wings attached How many? > 4(2 pairs) or > 2 (1 pair) - in the Diptera (true flies)

Abdomen
containing parts of digestive and other systems, e.g. reproductive

The insect body plan is incapable of being scaled up to be larger than a few centimetres, but insects can make up for this by producing larger numbers of individuals, and reproducing rapidly in suitable conditions.

The main problem caused by the exoskeleton is growth. This necessitates periodic moulting of the outer layer or cuticle, and relatively large increases in size whilst the soft and vulnerable body beneath is capable of expansion.

Life cycle

adult (imago) ---> egg -->->->-> larva ( ---> pupa ) ---> adult
etc
Insects thus usually have several larval stages (instars) which are often specialised to feed on a different food than the adult, and perhaps to live in different environment. This has the advantage of reducing competition between different stages in the life cycle.

Not all insects possess a pupal stage, which is a (usually apparently fairly inactive) resting stage during which the insect body is re-modelled for a major change - complete metamorphosis.

In others, there are a number of larval stages (nymphs) which gradually grow larger, following the usual moulting process, to resemble the adult stage, which is the only stage able to reproduce.

The adult stage - also called the imago - is often capable of moving quite long distances in order to colonise new areas, and insects may perform special routines in order to find or attract a mate. After mating and (internal) fertilisation, eggs are laid, usually near to a potential source of food.

Explain the following - it is not enough to give a single technical term.

Why do you think insects perform internal fertilisation ?
> because the egg has a waterproof covering - vital if laid on land
What would be the consequence if adult insects ate the same food as larvae?
> competition - the food source could be used up so that the adult would starve the next generation of larvae
What is the common name of the larva of a butterfly?
> caterpillar
What is the main food source of the adult cabbage white butterfly?
> nectar - sugary liquid from flowers
What adaptation to the mouthparts does that necessitate?
> proboscis forming a tube through which nectar is sucked up
What is the main food source of the larva of the cabbage white butterfly?
> Cabbage leaves
What adaptation to the mouthparts does that necessitate?
> strong mandibles (working from side to side)
What is the main food source of the adult mosquito?
> blood (or plant juices)
What adaptation to the mouthparts does that necessitate?
> sharp stylets for biting and sucking
In what way is the behaviour of the adult mosquito modified to suit its lifestyle?
> flies at night when its hosts are resting
What environment does the mosquito larva inhabit?
> still water - in ponds, ditches etc. and small bodies of water - even in tyre dumps!
What is the main food source of the larva of the mosquito?
> algae
What adaptation to the mouthparts does that necessitate?
> "mouthbrush" to filter water
What adaptation is shown by the larva of the mosquito to breathe air?
> snorkel-like tube at tail end to reach though surface film of water
What is the main food source of the adult housefly?
> any food? - solid or liquid
What adaptation to the mouthparts does that necessitate?
> pad and sucking tubes
What is the common name of the larva of a housefly?
> maggot/gentle
What is the main food source of the larva of the housefly?
> rotting bodies/dung
What adaptation to the mouthparts does that necessitate?
> flexible sideways chompers
In what way is the body shape of the housefly larva modified to suit its lifestyle?
> tapered for burrowing - no major projections