The Abiotic Environment

show peaks of feeding activity either at dawn or dusk, or during both of these periods, 665

though there is some argument with regard to whether feeding activity is endogenous or simply a direct response to a particular light intensity.

Several good examples may be cited to illustrate the importance of photoperiod in entraining daily endogenous rhythms of mating behavior. Many virgin female Lepidoptera begin to secrete male-attracting pheromones shortly after the onset of darkness and are maximally receptive to males about midway through the dark period. Equally, males show maximum excitability to these pheromones in the early part of the dark period. The males of certain ant species undertake mating flights at characteristic times within the light period, typically near dawn or dusk. Mosquitoes and other Nematocera form all-male swarms that females enter for insemination. Formation of these swarms, which occurs both at dawn and at dusk, is an endogenous rhythm, entrained by photoperiod, though temperature and light intensity are also involved (Beck, 1980).

For some insect species, egg laying has been shown to be a photoperiodically entrained endogenous rhythm. In the mosquitoes Aedes aegypti and Taeniorhynchus fuscopennatus, for example, oviposition is concentrated in the period immediately after sunset and before dawn, respectively. In other mosquitoes, however, no oviposition rhythm exists and egg laying appears to be dependent on light intensity.

Many examples are known of insects that molt to adults during a characteristic period of the day. Many tropical Odonata exhibit mass eclosion during the early evening and are able to fly by the following morning. Corbet (1963) suggested that this minimizes the effects of predators such as birds and other dragonflies that hunt by sight. In temperate climates where nighttime temperatures are generally too low for emergence, there may be a switch to emergence during certain daylight hours. In more rigorous climates temperature appears to override photoperiod* as a factor regulating emergence, which occurs opportunistically at any time of the day provided the ambient temperature is suitable (Section 2.3). Species of Ephemeroptera and Diptera also have daily emergence patterns, which may be associated with immediate mating and oviposition. Though many insect species are known that have a daily emergence rhythm, for only a few of these, mainly Diptera, is experimental evidence available that proves the endogenous nature of the rhythm. In contrast to the previously described rhythmic processes, emergence occurs but once in the life of an insect and results in the appearance of a very different developmental stage, the adult. Nevertheless, this single event, like daily repeated processes, is an endogenous rhythm, entrained by environmental stimuli, especially photoperiod, that exert their effect in earlier developmental stages. For example, populations of many Drosophila species emerge at maximum rates 1-2 hours after dawn on the basis of photoperiodic entrainment either in the larval or pupal stage. Thus, if a culture of Drosophila larvae of variable ages is maintained in darkness from the egg stage except for one brief period of light (a flash lasting as little as 1/2000 of a second is sufficient) the adults will emerge at regular 24-hour intervals, based on the onset of the light period being equivalent to dawn; that is, the beginning of the light period serves as the reference point for entraining the insects' emergence rhythm (Beck, 1980).

Physiological and molecular studies of light-regulated circadian rhythms have revealed that, although there is a basic operating system, this may occur in a variety of forms (Saunders, 2002). The basic system comprises three components: an input pathway, which includes a photoreceptor; an oscillator ("clock" or pacemaker) that receives the entraining

* In the Arctic summer there are, of course, 24 hours of light per day, and photoperiod cannot serve as an entraining factor for diurnal rhythms.

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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