Physical Aspects

Growth in insects and other arthropods differs from that of mammals in various respects. In insects growth is almost entirely restricted to the larval instars, though in some species there is a short period of somatic growth in newly enclosed adults when additional cuticle may be deposited, and growth of flight muscles and the alimentary canal may occur. As a consequence, the length of the juvenile stage is considerably longer than that of the adult. An extreme example of this is seen in some mayfly species whose aquatic juvenile stage may require 2 or 3 years for completion, yet give rise to an adult that lives for only a few hours or days. Growth in many animals is discontinuous or cyclic; that is, periods of active growth are separated by periods when little or no growth occurs. Nowhere is discontinuous growth better seen than in arthropods, which must periodically molt their generally inextensible cuticle in order to significantly increase their size (volume). It should be appreciated, however, that, though increases in volume may be discontinuous, increases in weight are not (Figure 21.1). As an insect feeds during each stadium, reserves are deposited in the fat body, whose weight and volume increase. In a hard-bodied insect this increase in volume may be compensated for by a decrease in the volume occupied by the tracheal system or by extension of the abdomen as a result of the unfolding of intersegmental membranes. In many endopterygote larvae, of course, the entire body is largely covered with extensible cuticle, and body size increases almost continuously (but see below).

For many insects grown under standard conditions the amount of growth that occurs is predictable from one instar to the next; that is, it obeys certain "growth laws." For example, Dyar's law, based on measurements of the change in width of the head capsule which occurs at each molt, states that growth follows a geometric progression; that is, the proportionate increase in size for a given structure is constant from one instar to the next. Mathematically expressed, the law states x/y = constant (value usually 1.2-1.4), where x = size in a given instar and y = size in previous instar (Figure 21.1). Thus, when the size of a structure is plotted logarithmically against instar number, a straight line is obtained, whose gradient is constant for a given species (Figure 21.2). In those insects where it applies Dyar's law can be used to determine how many instars there are in the life history. However, so many factors

FIGURE 21.1. Change in head width with time to illustrate Dyar's law.

FIGURE 21.2. Head width plotted logarithmically against instar number in various species. [After V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author.]

FIGURE 21.2. Head width plotted logarithmically against instar number in various species. [After V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author.]

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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