The Abiotic

but, in their virgin condition, have daily rhythms of calling (secretion of male-attracting ENVIRONMENT

pheromones) that enable males to locate them.

3.1.1. Circadian Rhythms

In a few species daily rhythms of activity are triggered by environmental cues and are therefore of exogenous origin. For example, the activity of the stick insect Carausius morosus is directly provoked by daily changes in light intensity. However, in most species these rhythms are not simply a response to the onset of daylight or darkness; that is, dawn or dusk do not act as a trigger that switches the activity on or off. Rather, the rhythms are endogenous (originate within the organism itself) but are subject to modification (regulation) by photoperiod and other environmental factors. That the rhythm originates internally may be demonstrated by placing the organism in constant light or darkness. The organism continues to begin its activity at approximately the same time of the 24-hour cycle, as it did when subject to alternating periods of light and darkness. Because the rhythm has an approximately 24-hour cycle, it is described as a circadian rhythm. When the rhythm is not influenced by the environment, that is, when environmental conditions are kept constant, the rhythm is described as "free-running." When environmental conditions vary regularly in each 24-hour cycle, and the beginning of the activity occurs at precisely the same time in the cycle, the rhythm is "entrained." For example, if a cockroach begins its locomotor activity

2 hours after darkness, this activity is said to be photoperiodically entrained. The role of photoperiod is therefore to adjust (phase set) the endogenous rhythm so that the activity occurs each day at the same time in relation to the onset of daylight or darkness. Though photoperiod is probably the most important regulator of circadian rhythms in insects, other environmental factors such as temperature, humidity, and light intensity, as well as physiological variables such as age, reproductive state, and degree of desiccation or starvation may modify behavior patterns. Photoperiodically entrained daily rhythms are known to occur in relation to locomotor activity, feeding, mating behavior (including swarming), oviposition, and eclosion, examples of which are given below.

Many examples are known of insects that actively run, swim, or fly during a characteristic period of the 24-hour cycle, this activity usually occurring in relation to some other rhythm such as feeding or mate location. In Periplaneta and other cockroaches activity begins shortly before the anticipated onset of darkness, reaches a peak some 2-3 hours after dark, and declines to a low level for the remaining period of darkness and during most of the light period (Figure 22.2A). Drosophila robusta (Figure 22.2B) flies actively during the last

3 hours of the light phase but is virtually inactive for the rest of the 24-hour period. Male ants of the species Camponotus clarithorax are most active during the first few hours of the light period but show little activity at other times (Figure 22.2C). The above examples show a well-defined single peak (unimodal rhythm) of activity. Other species, however, have bimodal or trimodal rhythms. For example, females of the silver-spotted tiger moth, Halisidota argentata, show two peaks of flight activity during darkness, the first shortly after darkness begins, the second about midway through the dark period (Figure 22.3A). Males of this species, in contrast, have a trimodal rhythm of flight activity (Figure 22.3B) (Beck, 1980).

Rhythmic feeding activity is apparent in larvae of some Lepidoptera, for example, H. argentata, which feed almost exclusively during darkness. Female mosquitoes, too,

FIGURE 22.2. Locomotor activity rhythms in insects, illustrating photoperiodic entrainment. (A) Periplaneta; (B) Drosophila; and (C) Camponotus. [From S. D. Beck, 1968, Insect Photoperiodism. By permission of Academic Press, Inc., and the author.]

FIGURE 22.3. Photoperiodically entrained flight activity in Halisidota argentata (Lepidoptera). [From D. K. Edwards, 1962, Laboratory determinations of the daily flight times of separate sexes of some moths in naturally changing light, Can. J. Zool. 40:511-530. By permission of the National Research Council of Canada.]

FIGURE 22.3. Photoperiodically entrained flight activity in Halisidota argentata (Lepidoptera). [From D. K. Edwards, 1962, Laboratory determinations of the daily flight times of separate sexes of some moths in naturally changing light, Can. J. Zool. 40:511-530. By permission of the National Research Council of Canada.]

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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