Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/111

105 air is taken in through these spiracles and carried through the body to all parts. Breathing spiracles are never found on the head.

Skeleton.—An insect is interesting because of the fact that its skin is its skeleton. There is no bony framework upon which the muscles, tissues and organs are hung as in the case of the back-boned animals, but its place is supplied by this horny skin, which is known as an "Exoskeleton," or outside skeleton. There are no bones whatever in an insect's body.

Natural History.—With few exceptions, all insects are produced from eggs. Subsequent to the egg stage, all insects go through remarkable changes of form. This change is known as a metamorphosis. Three distinct periods are to be recognised, namely, larva, pupa, imago, or adult. The larva is the small organism that issues from the egg or is born alive. It is during the larval stage that insects generally do the greatest damage. The pupa is the stage which follows the full grown larva and is usually a resting period. During this stage we have the miracle of the transformation of an ugly caterpillar into a beautiful butterfly, or a maggot into a fly. Imago or adult—this stage follows the pupa and is the final. All insects with wings are full grown.

There are, however, some groups of insects which do not have a complete metamorphosis. The egg is laid as in the first group, but, instead of a larva, we have a nymph, which has a direct resemblance to its parent, with the exception of wings. It goes through a series of moults, finally appearing with wings. This is known as an incomplete metamorposis. Examples—Grasshoppers, bugs, etc.

Friendly Insects.—Before passing to the discussion of pests, which follows, it is, I think, as well to draw attention to the number of insects which act towards us in a beneficial manner by destroying destructive species. The number of insects which, directly or indirectly, benefit us is very large, but their work is often unappreciated because unnoticed. There is no doubt that, without the natural enemies of insects, man would be helpless in combating them, particularly over large areas of forest where, owing to the great growth and density of the trees, artificial measures would be very costly if at all practicable.

In conjunction with other insect destroying birds and animals, predaceous and parasitic insects are man's greatest ally. They work silently and without the necessity of any attention on the part of man. It is, therefore, important that we should study as much as possible the life histories, habits, and relationships of such of these insects which occur in our country, in the hope that we may learn to appreciate their value and be able to conserve these natural aids, and possibly artificially foster, encourage, increase, and transport, or extend the distribution of the most useful. A careful observer is soon able to separate the useful and injurious insects from each other, by noting their feeding habits, etc.

Beneficial insects belong primarily to the orders:—


 * Hymenoptera (Bees and Wasps), etc.


 * Diptera (Tachnid and Syrphidae flies), etc.


 * Coleoptera (Ladybird and other carnivorous beetles).


 * Neuroptera (Lace-winged flies, dragon flies, etc.).

There are a few others scattered amongst the remaining orders.

Speaking in common language insects are divided into four groups: Chewing, Sucking, Injurious, and Beneficial.

Beneficial insects are again divided into two groups—Predaceous (those that seize their victims and feed upon them) and Internal Parasites (those that lay their eggs upon their host or into the body, the larval and sometimes pupal stage being spent inside their victims).

Destructive Forest Insects.—The animal pests of forest trees are mostly included in the Hexapoda, or insects. The amount of loss caused by forest insects