In all organisms gas exchange, the supply of oxygen to and removal of carbon dioxide from cells, depends ultimately on the rate at which these gases diffuse in the dissolved state. The diffusion rate is proportional to (1) the surface area over which diffusion is occurring and (2) the diffusion gradient (concentration difference of the diffusing material between the two points under consideration divided by the distance between the two points). Diffusion alone, therefore, as a means of obtaining oxygen or excreting carbon dioxide can be employed only by small organisms whose surface area/volume ratio is high (i.e., where all cells are relatively close to the surface of the body) and organisms whose metabolic rate is low. Organisms that are larger and/or have a high metabolic rate must increase the rate at which gases move between the environment and the body tissues by improving (1) and/or (2) above. In other words, specialized respiratory structures with large surface areas and/or transport systems that bring large quantities of the gas closer to the site of use or disposal (thereby improving the diffusion gradient) have been developed. For most terrestrial animals prevention of desiccation is another important problem, and this has had a major influence on the development of their respiratory surfaces through which considerable loss of water might occur. Typically, respiratory surfaces of terrestrial animals are formed as invaginated structures within the body so that evaporative water loss is greatly reduced.

In insects the tracheal system, a series of gas-filled tubes derived from the integument, has evolved to cope with gas exchange. Terminally the tubes are much branched, forming tracheoles that provide an enormous surface area over which diffusion can occur. Furthermore, tracheoles are so numerous that gaseous oxygen readily reaches most parts of the body, and, equally, carbon dioxide easily diffuses out of the tissues. Thus, in most insects, in contrast to many other animals, the circulatory system is unimportant in gas transport. Because they are in the gaseous state within the tracheal system, oxygen and carbon dioxide diffuse rapidly between the tissues and site of uptake or release, respectively, on the body surface. Oxygen, for example, diffuses 3 million times faster in air than in water (Mill, 1972). Again, because the system is gas-filled, much larger quantities of oxygen can reach the tissues in a given time. (Air has about 25 times more oxygen per unit volume than water.)

The eminent suitability of the tracheal system for gas exchange is illustrated by the fact that, for most small insects and many large insects at rest, simple diffusion of gases in/out of the tracheal system entirely satisfies their requirements (but see Section 3.3). In large, active insects the gradient over which diffusion occurs is increased by means of ventilation; that is, air is actively pumped through the tracheal system.

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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