These are very successful insects which are widespread throughout the world. Individual aphids are small and different species may feed on the stems, leaves or roots of plants. Several generations may be produced each year but they have a complex life cycle which enables them to survive under varied environmental conditions. They feed by inserting a complex mouthpart or stylet into plant tissues and extracting from the phloem tissues organic materials which have been produced by the plant. Some females do not require fertilisation to produce young and these are said to be parthenogenetic. The offspring of these asexual females are called nymphs and are born alive and fully formed; these parthenogenetic females are therefore also described as being viviparous. A further complication is that these asexual females may have wings or they may be wingless. There are also normal females, with or without wings, which require normal fertilisation by the male before they can produce offspring. These sexual females lay eggs which rapidly hatch to produce nymphs, usually of the winged form. The winged form of both asexual and sexual aphids are often developed in response to adverse weather conditions or a lack of food, this assists in their movement to areas which are more favourable.

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The life-cycle of most weevils is not complicated since there are only four stages of development: the egg, the larva or small grub which hatches from the egg, the pupal stage and the adult weevil. The eggs are laid in grains of cereals such as maize, often in the field, in cavities which are bored into the grain by the adult female weevil. After they have been laid, the cavity which was made in the outer layer of the seed is sealed by the adult female. The larvae which hatch from the eggs feed for some time on the endosperm of the seed and then pass into the pupal stage which is the intermediate stage of development between the larva and the adult. The adults emerge from the grain about 4-5 weeks after the eggs were laid and the whole cycle begins again when the weevil has developed fully. Most weevils have wings, sometimes these are only rudimentary and not adapted for flying long distances.


Adults lay their eggs in sheltered places, often on the under surfaces of leaves. After a few weeks, their eggs hatch into nymphs which are similar in appearance to the adult grasshopper except that the head is much larger than the body. The wings and reproductive organs are also undeveloped at this stage. The nymphs feed actively and grow rapidly, passing through about 5 moults, or skin shedding phases, before reaching the adult stage by which time the outer skin thickens and becomes fairly resistant to insecticides. Grasshoppers are usually fairly gregarious individuals remaining close to each other and moving and feeding in groups but they may pass into the migratory  stage when fully developed. They then move rapidly in swarms, damaging the crops on which they feed to a considerable extent.


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