WAITING FOR A MEAL TO WALK PAST

THE ticks are of the greatest interest to man as they are all parasitic and live on the blood of their hosts. In the East they occur on many different kinds of small animals like squirrels. Ticks may also be the carriers of disease. They lay thousands of eggs in the soil of fields, and these hatch into six-legged larva? which climb up the grass and sit tight until a suitable animal passes their way. When such an animal appears they become excited and should it pass near enough to them they will leave their grasses and cling to its hair. Having secured a foothold, they push their beaks into the host’s flesh and gorge themselves with blood. This meal has to last them a long time, for when their hunger is appeased they drop off and burrow into the ground and digest it at their leisure. Later, the larval skin is shed and they enter upon a nymphal stage with eight legs. Once more they climb up the grass and repeat the performance of waiting for a suitable host, finding it, feeding, and again dropping to the ground and moulting. This moult produces the adult males and females which search for a third animal on which to feed and pair, after which the female once more seeks the ground and lays her eggs.

This is an extraordinarily complex life-history for an arachnid. The number of risks run by each batch of young ticks is enormous, and so we find that these risks are met to some extent by the large numbers of eggs laid by one female so that the chances of a few young finding a host on which to

feed are much increased. The period of waiting for a host and thus for a meal may be long, so we find they have a capacity for undergoing prolonged fasts without incurring any damage.

The itch mites include that species parasitic on man. The human itch mite in former times caused terrible trouble amongst armies in the field. The itching which they cause is said to be intolerable. The fertilised females burrow into the skins of their hosts and line their burrows with dozens of eggs. In this way they block up their only exit to the outer world, and so they die at the ends of their tunnels. As only a month is taken from the hatching of the egg to the fully developed adult it will be realised how quickly infection spreads by contact with infected persons.

The familiar cheese-mites are related to the itch mites. One of this group of mites lives in the breathing tubes of bees and causes a virulent disease amongst them. Some members of this group are responsible for mange in mammals, and others are vegetarians which cause galls on plants.

So ends our review of the vast group of the Arthropoda with its infinite number and great diversity of forms.

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