INSECTS AS FRIENDS OR FOES OF MAN

MANY insects act as scavengers. Others seek out dead animals and lay their eggs upon them—both larva? and adults feeding on them. Some beetles excavate the earth

below a dead rat, bury it, and proceed to lay eggs upon it. The larva: complete the process by consuming it. Ants will pick a skeleton clean. Other insects pollinate our flowers, and an interesting example is the Yucca moth which lays her eggs in the ovaries of the Yucca flower.

Many flowers have structural devices for ensuring cross pollination by insects; others pollinate themselves, but the Yucca flower does neither. As the caterpillar of the Yucca moth requires the pod in which to feed, it is essential from the moth’s point of view that the ovary should be fertilised, and this it sets out to do by means of special mouth-parts; the moth collects a mass of pollen and actually places it on the stigma of the flower. This is beneficial to the flower, to the insect, and indirectly to man.

We have already seen how some insects are useful to man by parasitising harmful ones. Some are also useful in destroying weeds—a two-winged fly has destroyed the Canadian thistle in Indiana.

Then there are the harmful insects of medical, agricultural and horticultural importance. Lice transmit the germs of typhus from one person to another. Mosquitoes are responsible for the transmission of malarial and yellow fevers. The tsetse fly is responsible for the spread of sleeping sickness. The Colorado beetle has created havoc amongst the potatoes of the United States of America. The Hessian fly is a serious grain pest. The depredations of Aphis on the fruit trees, roses and beans of our gardens are only too familiar.

INSECTS THAT ARE EXPERT MIMICS

CERTAIN insects, which make good eating for birds, mimic other insects which have a nasty taste. Palatable insects try to look like unpalatable ones to warn off the birds. The Stick insects, both in their colour and attitudes, closely resemble the plants on which they live. Some unarmed insects simulate the colours of wasps, and, in this connection, we might note that insects armed with stings are often very brightly coloured, e.g. the warning colours of yellow and black. Anyone who does any work at all in the field is bound to come across examples of mimicry, and there is abundant evidence of its quite wide occurrence.

We must now leave the insects and pass on to the remaining two groups of the Arthropoda—the Myriapoda (centipedes and millipedes) and the Arachnida (spiders, scorpions &c).

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