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618 above minimum required X duration of exposure to this temperature) is constant for a given species. This heat requirement is typically measured in degree-days. Outside these developmental limits, yet within the limits of viability, an egg may survive but does not develop. Under these conditions, it is said to be quiescent and in this state may survive for a considerable length of time. In a quiescent state, an egg is always ready to take advantage of favorable conditions, even if only temporary, to continue its development. However, quiescence is a relatively sensitive developmental state; that is, outside certain temperature limits, an egg will be killed. For many species, therefore, which exist in habitats exposed to climatic extremes, especially of temperature but also of precipitation, a more resistant state of developmental arrest, diapause, has evolved to permit their survival. Diapause is discussed at greater length in Chapter 22 (Section 3.2.3), though in the present context it is worth noting that diapause may occur at different stages of embryonic development and with varied strength in different species. In all instances, however, it is characterized by a cessation of morphogenesis and a considerable lowering of the metabolic rate. Also, the water content of an egg is often low at this time. In Bombyx, diapause, which begins in overwintering eggs almost as soon as they are laid, is extremely strong; that is, even when eggs are experimentally maintained at 15°C to 20°C from the time of laying they will not develop. Development begins only after they have been exposed to a temperature of about 0°C for several months. In eggs of the damselfly Lestes congener, diapause is also strong but does not commence until after anatrepsis. Diapause in eggs of the grasshopper Melanoplus differentials also occurs after anatrepsis but is weak. Should the temperature to which eggs are exposed be maintained at summer levels (around 25° C), some of the eggs will develop directly, though more slowly than those that have undergone chilling. In eggs of some insects, for example, certain mosquitoes (Aedes spp.) and the damselfly Lestes disjunctus, embryonic development is almost completed before diapause is initiated.

Through an effect on parthenogenesis, temperature may also affect the sex ratio of the offspring. For example, in Hymenoptera higher temperatures often favor production of haploid males. In some bisexual species, extreme high or low temperatures may disrupt the normal sex chromosome distribution that occurs during meiosis, so that a preponderance of males or females results.

Water is another important requirement and in eggs of many species must be acquired from the external environment before embryonic development can begin. When it is available to an egg in insufficient quantity, the embryo becomes quiescent or remains in diapause (though this was not induced by the lack of moisture). Some species can obtain sufficient water from moisture in the air. For example, eggs of the beetle Sitona, when kept at 20°C and 100% relative humidity, hatch in 10.5 days; at the same temperature but only 62% relative humidity, development takes twice as long. In other species contact of the egg with liquid water is necessary for continued development. Such is the case in the damselfly eggs mentioned above which pass the winter in snow-covered, dried-out Scirpus stems and do not continue their development until the stems become waterlogged following the spring thaw.

Though the presence of ecdysonelike molecules and juvenile hormone was first reported some 40 years ago, their sources and roles in embryonic development are only slowly being clarified. As noted above, the source of these hormonal factors, at least until after katatrepsis, is maternal; that is the compounds are deposited as conjugates within the egg prior to oviposition. For species such as locusts and grasshoppers that produce several embryonic cuticles, there is a clear correlation between peaks of free ecdysone and bouts of cuticle synthesis. In other species the onset and termination of embryonic diapause is associated with changes in free ecdysone levels. Juvenile hormone seems to be involved in embryonic development in a manner similar to that in larvae, namely, the qualitative expression of cuticle structure; however, little experimental evidence supports this conjecture. Some studies have identified juvenile hormone in quite early stages of embryogenesis (i.e., well before cuticulogenesis) suggesting that it has other roles. One such role may be the regulation of ectodermal growth leading to dorsal closure. Though neurosecretory cells and corpora cardiaca have been identified in descriptions of embryonic development, and specific products have been assayed in a few species, their roles in embryogenesis remain unknown.

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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