Diffusion

The absence of obvious breathing movements led many 19th century scientists to assume that insects obtained oxygen by simple diffusion. It was not, however, until 1920 that Krogh (cited in Miller, 1974) calculated, on the basis of (1) measurements of the average tracheal length and diameter, (2) measurements of oxygen consumption, and (3) the permeability constant for oxygen, that for Tenebrio and Cossus (goat moth) larvae at rest with the spiracles open, the oxygen concentration difference between the spiracles and tissues is only about 2% and easily maintainable by diffusion.

Even in large active insects that ventilate (Section 3.3), diffusion is a significant process, because the ventilation movements serve only to move the air in the larger tracheae. For example, in the dragonfly Aeshna, oxygen reaches the flight muscles by diffusion between the primary (ventilated) air tubes and tracheoles, a distance of up to 1 mm. Even in flight, when the oxygen consumption of the muscle reaches 1.8 m1/g/min and the difference in oxygen concentration between the primary tube and tracheoles is 5-13%, diffusion is quite adequate (Weis-Fogh, 1964).

Diffusion is also important in moving gases between the tracheoles and mitochondria of the tissue cells. Because diffusion of dissolved gases is relatively slow, the distance over which it can function satisfactorily (in structural terms, half the distance between adjacent tracheoles) is directly related to the metabolic activity of the tissue. In highly active flight muscles of Diptera and Hymenoptera, for example, it has been calculated that the maximum theoretical distance between tracheoles is 6-8 |^m. In practice, tracheoles, which indent the muscle cells, are within 2-3 |~im of each other, allowing a significant "safety margin" (Weis-Fogh, 1964).

In many insects, distal parts of tracheoles are not filled with air but liquid under normal resting conditions. During activity, however, the tracheoles become completely air-filled; that is, fluid is withdrawn from them only to return when activity ceases. Wigglesworth (1953) suggested that the level of fluid in tracheoles depends on the relative strengths of the capillary force drawing fluid along the tube and the osmotic pressure of the hemolymph. During metabolic activity, the osmotic pressure increases as organic respiratory substrates are degraded to smaller metabolites, causing fluid to be withdrawn from the tracheoles (perhaps via the pores mentioned earlier) and, therefore, bringing gaseous oxygen closer to the tissue cells (Figure 15.5). As the metabolites are fully oxidized and removed, the osmotic pressure will fall, and once again the capillary force will draw fluid along the tracheoles.

Though carbon dioxide is more soluble, and has a greater permeability constant, than oxygen in water and could conceivably move by diffusion through the hemolymph to leave the body via the integument, this route does not normally eliminate a significant quantity of the gas (e.g., 2-10% of the total in some dipteran larvae with thin cuticles). The great majority of carbon dioxide leaves by gaseous diffusion via the tracheal system.

FIGURE 15.5. Changes in level of tracheolar fluid as a result of muscular activity. (A) Resting muscle; and (B) active muscle. [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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