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450 of the legs. However, these features considerably reduce the frictional force between the legs and water surface which is necessary for locomotion. This problem is overcome in many species by having certain parts of the tarsus, particularly the claws, penetrate the surface film and/or by having special structures, for example, an expandable fan that opens when the leg is pushed backward. In the Gerridae, however, the backward push of the legs is sufficiently strong that a wave of water is produced that acts as a "starting block" against which the tarsi can push (Nachtigall, 1974).

The functional morphology and mechanics of movement have been examined in detail in Gerris (Brinkhurst, 1959; Darnhofer-Demar, 1969). This insect has greatly elongated middle legs through which most of the power for movement is supplied. Some power is derived from the hindlegs, though these function primarily as direction stabilizers. The articulation of the coxa with the pleuron is such that the power derived from contraction of the large trochanteral retractor muscles is used exclusively to move the legs in the horizontal plane, that is, to effect the rowing motion. Equally, the coxal muscles serve only to lift the legs from the water surface during protraction. At the beginning of a stroke, the forelegs are lifted off the surface. The middle legs are rapidly retracted so that a wave of water forms behind the tarsi. As the legs are accelerated backward, the tarsi then push against this wave, causing the insect to move forward. After each acceleration stroke, the insect glides over the surface for distances up to 15 cm. The power developed in each leg of a segment is identical and the insect glides, therefore, in a straight line. Turning can occur only between strokes and is achieved by the independent backward or forward movement of the middle legs over the water surface.

Waterstriders of the genus Velia and the staphylinid beetle Stenus use an ingenious means of skimming across the water surface. They release a surface tension-reducing secretion behind themselves and are thus pulled rapidly forward, reaching speeds of 45-70 cm/sec (Stenus).

3.2.2. Swimming by Means of Legs

Both larval and adult aquatic Coleoptera (Dytiscidae, Hydrophilidae, Gyrinidae, Hali-plidae) and Hemiptera (Corixidae, Belostomatidae, Nepidae, Notonectidae) swim by means of their legs. Normally, only the hindlegs or the mid- and hindlegs are used and these are variously modified so that their surface area can be increased during the propulsive stroke and reduced when the limbs are moving anteriorly. Modifications include (1) an increase in the relative length and a flattening of the tarsus; (2) arrangement of the leg articulation, so that during the active stroke the flattened surface is presented perpendicularly to the direction of the movement, whereas during recovery the limb is pulled with the flattened surface parallel to the direction of movement; the leg is also flexed and drawn back close to the body during recovery; (3) development of articulated hairs on the tarsus and tibia that spread perpendicularly to the direction of movement during the power stroke, yet lie flat against the leg during recovery; such hairs may increase the effective area by up to five times; and (4) in Gyrinus, development of swimming blades on the tibia and tarsus (Figure 3.24A). These are articulated plates that normally lie flattened against each other. During the power stroke, the water resistance causes them to rotate so that their edges overlap and their flattened surface is perpendicular to the direction of movement of the leg.

In addition to the surface area presented, the speed at which the leg moves is proportional to the force developed. Thus, it is important for the propulsive stroke to be rapid, whereas

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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