ECOLOGY. As a class, insects are represented in practically' all parts of the world. While flourishing more abundantly in the tropics, they are found in countless numbers in the temperate regions, and are also very numerous in boreal regions. They abound inside the Arctic Circle. and in the short Arctic summer many species of nearly all orders may be collected. As a rule, as with other classes of animals, the forms occurring in tropical regions are larger in size and more brilliant in color. :Many groups arc confined to the tropics; others have a wide geo graphic distribution. The value of the class Insecta in the study of the geographic distribu of life is very great with certain groups, while others have comparatively slight faunistie value. Civilization exerts a direct and destruc tive influence upon the insect faunas of large regions. Insi.els being largely dependent. directly or indirectly', upon vegetation, the destruction of the wild flora and sylva and the introduction of cultivated crops almost immediately change com pletely the characteristics of a given insect fauna. The rapid development of methods of transportation, and especially intercommunica tion between remote regions, by means of the constantly increasing number and speed of ves sels, has resulted in the accidental introduction and aCel1111:1117.11I ion of Many hundreds of spe cies of insects into regions remote from their original homes, many of which succeed in estab lishing themselves. The facility of acclimatiza tion varies greatly with different groups, and this facility is dependent upon the degree of sim plicity of the life of the insect and upon the degree of simplicity of its natural environment. co great has become the danger of the introduc tion of injurious speeies from one eountry to another during late years. that many nations seek to protect themselves by special quarantine From what has preceded. it is evident that insects. in spite of their small size and their consequent slight strength, have been remarkably successful in the so-called struggle for existence. They have a long geological history, and the in sects of Tertiary rocks are in sonic instances almost generically related to living forms. Rapidity of growth and power of multiplication have been prime factors in this persistenee, while the relations which exist between circulation and respiration have been almost equally significant. In many instances the growth from the egg to the individual occupies only a few days, while in some species it single female will lay several thousand:4 of eggs. The funetions of circulation
and respiration are so related that nutrition can be carried on very rapidly and very efficiently. By the phenomenon of metinorphosis, growth and development are isolated from one another, allowing growth to go on unchecked and plicated by development.
The social life of insects is very remarkable; organized societies are formed with many species, the Ilymenoptera (bees. wasps, and ants) and the Isopterft (white ants or Termiti the). In these societies great nintibers of indi viduals live together and are greatly modified in structure in accordance with the different func tions which they perform in the community. See ; ; and IaSrf'tS, REertonuclIox. Nearly gill inserts undergo in the course of their lives remarkable ehanges in form. A few forms are ovoviviparous—that is, bring forth living young, hut the great majority lay eggs. (See Eqo.) With some forms there is a development without metamorphosis, and in the young insect just hatched from the egg is of the same form as the adult inseet. With others there is what is termed an ineomplete metamorphosis—that is to say. where, although the young greatly resemble the adult. there is still a striking change of form during life. With others still there is what has been termed a complete metamorphosis (q.v.). in which the young just hatched is strikingly different from its subsequent stages. With the butter flies, flies. beetles. and other insects, the form which hatches from flu• and which is known a- the 'larva,' is a 'grub' or a 'caterpillar.' This, after reaching full growth, passes into another form, which is known as the pupa, and in which in the majority of eases the insect is quiescent, while from this stage there eventually emerges the perfect insect. Larva' grow by molting; the skin is more or less had and is composed of chitin; a new skin is formed beneath the old skin, which eventually bursts and permits the larva to emerge. The number of molts differs with the larva. of ditTerent groups and ranges from two or three to ns many as twelve or more. The phenomenon known as hypermetamorphosis sometimes occurs. With certain of the blister beetles, for example, the first larvae which are born possess legs, by the aid of which they can eling to a bee and be en rr led to its nest, where they will live on the food stored by the bee; after a molt they lose their legs and become almost organless, floating about in the honey. Later still, another form of larva is found. Sec Al ETA ORPIIONIS tin Animals); LARVA ; PCPA.