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452 Dragonfly larvae (Anisoptera) normally take in and expel water from the rectum during gas exchange (Chapter 15, Section 4.1). In emergencies this arrangement can be converted into a jet propulsion system for moving an insect forward at high speed (up to 50 cm/sec). Rapid contraction of longitudinal muscles causes the abdomen to shorten by up to 10%. Simultaneous contraction of the dorsoventral muscles leads to an increase in hemolymph pressure which forces water out of the rectum via the narrow anus at speeds approaching 250 cm/sec.

Female Hydrocampa nympheata (Lepidoptera) and adult Dacunsa (Hymenoptera) use their wings in addition to legs for swimming underwater. Other Hymenoptera (Polynema and Limnodites) swim solely by the use of their wings.

3.3. Flight

As noted in Chapter 2, Section 3.1, wing precursors originally had functions quite unrelated to flight. Subsequent evolution led to enlargement and perhaps articulation of these structures as they took on a new function, propulsion of an insect through the air, partly as a result of which insects were able to move into new environments to become the diverse group we know today. Despite this diversity, there is sufficient similarity of skeletal and neuromuscular structure and function to suggest that wings had a monophyletic origin (Pringle, 1974).

Examination of the form and mode of operation of the pterothorax reveals certain trends, all of which lead to an improvement in flying ability. Primitively, the power for wing movement was derived from various "direct" muscles, that is, those directly connected with the wing articulations. These muscles serve also to determine the nature of the wing beat. Even today, the direct muscles remain important power suppliers in the Odonata, Orthoptera, Dictyoptera (Blattodea), and Coleoptera. In other insect groups, efficiency is increased by separating the control of wing beat (by the direct muscles) from power production, which becomes the job of large "indirect" muscles located in the thorax.

There are important differences in the fine structure and neuromuscular physiology of the direct and indirect flight muscles. Generally, in insects that flap their wings relatively slowly (up to 100 beats/sec), each beat of the wings is initiated by a burst of impulses to the power-producing muscles, which are of the tubular or close-packed type (Figure 14.3B, C). This applies to all users of direct muscles for powering flight, plus Lepidoptera in which the indirect muscles are used. In contrast, in fliers that use indirect muscles and whose wing-beat frequency is high (up to 1000 beats/sec), muscle contraction is not in synchrony with the arrival of nerve impulses at the neuromuscular junction. Rather, the rhythm of contraction is generated within the muscles themselves, which are fibrillar (Figure 14.3D). Accordingly, the two forms of rhythm are described as synchronous (neurogenic) and asynchronous (myogenic), respectively. The use of asynchronous muscles to power flight has evolved several times within the Insecta. Its significance appears to be the facilitation of high wing-beat frequencies, thereby moving more air, so that even insects with small wings relative to their body size are efficient fliers.

3.3.1. Structural Basis

Each wing-bearing segment is essentially an elastic box whose shape can be changed by contractions of the muscles within, the changes in shape causing the wings to move up and down. The skeletal components of a generalized wing-bearing segment are shown

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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