Intraspecific responses to the presence of sibling insects are of apparent ecological significance and often involve directional movement and/or migration. Changes in internal organs or metabolism elicited by population density, being independent of integumental constraints, are not restricted to the molting period, but the temporal difference between internal and external responses is not of fundamental significance. Because of their exoskeletons, which are persistent for the duration of each instar and endure throughout adult life, overt changes in morphology or coloration are restricted to the molting period and shortly afterward, when cuticular hardening and pigmentation are expressed. The arithmetic density of a population is the total number of people in proportion to the area of land (land size) The physiological density of a population is the total population in proportion. A common feature of both short-term and long-term effects is that they are graded, dependent not only on density but also on the duration and on phase history of the maternal generation. 3641, Table I-1 (Area and Demographic Data for the World's States) provides only the arithmetic density for each of the world's states (the ratio of its total population to total land area). This measure of population density refers to the number of people per unit area of arable land, a basic indicator of a country's food-producing capabilityand the human pressures placed upon it. Insects respond to crowding in a variety of ways that are usually exemplified by rapid changes in behavior and culminate in enduring long-term morphological and/or chromatic responses. The concept of physiologic density is discussed in several chapters.
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