Dlv

588 [pushing the existing sperm to the rear of the storage organ (Section 4.3.1)], sperm loading

(injecting a high number of sperm relative to the number already there), and sperm incapacitation (killing or inhibiting the activity of sperm already there by components of the seminal fluid) (Simmons, 2001). It should be stressed that there is currently little evidence for the occurrence of the second and third mechanisms. Other strategies, for which the evidence is more compelling, include sperm removal prior to ejaculation (Section 4.3.1) and sperm flushing whereby previously deposited sperm are washed out by the most recent ejaculate.

Females may also exert selection over which male's sperm they use to fertilize their eggs. For example, they may discard sperm soon after ejaculation has occurred (i.e., before it reaches the storage organ). Failure to transport the sperm to storage or fertilization sites has also been reported for some species. Rapid remating with a "better" male and reduction in the number of eggs deposited are other mechanisms that females may use to control which male's sperm they use (Eberhard, 1996).

6.2. Sperm Entry into the Eggs

In almost all insects sperm enter the eggs as the latter pass through the common oviduct during oviposition. However, in Cimicoidea and some scale insects sperm enter the eggs in the ovary, and in Strepsiptera sperm entry occurs as the eggs float within the hemocoel. Two problems associated with the entry of sperm are (1) release of sperm from the spermatheca in synchrony with movement of an egg through the oviduct and (2) location by sperm of the micropyles. The solution to the first problem remains unclear, though in some species the presence of muscle in the wall of the spermatheca and its duct indicates that sperm are squeezed toward the oviduct rather than moving of their own accord. In Periplaneta, it has been suggested that a nervous pathway might exist between sensory hairs found within the oviduct and the spermathecal muscle that could synchronize these events. Other insects have spermathecae with rigid walls, and in these a chemotactic stimulus for induction of sperm movement seems more likely.

Various mechanisms ensure that sperm can locate and enter the micropyles. In D. melanogaster, whose egg has but a single micropyle, the egg is precisely oriented as it moves along the oviduct so that the micropylar region directly faces the opening of the ventral receptacle, which contains the sperm. Furthermore, movement of an egg along the oviduct may stop briefly at this point. Where an egg's orientation is less precise, there may be a large number of micropyles. For example, in Periplaneta up to 100 of these funnel-shaped structures occur in a cluster at the cephalic end of the egg. In Rhodnius the micropyles lie within a groove that encircles the anterior end of the egg. As an egg moves along the oviduct, the groove comes to lie opposite the openings of the paired spermathecae (Davey, 1965). Actual entry of sperm into the micropyles, whose diameter at the inner end may be only a fraction of a micrometer, may result from release of a chemical attractant by the oocyte, though there is little evidence to support this suggestion.

Polyspermy (the entry of two or more sperm into an egg) is common in insects, though only one sperm normally undergoes subsequent transformation into a pronucleus, and the remainder degenerate.

6.3. Fertilization

Fertilization, that is, the fusion of male and female pronuclei, does not occur until after oviposition. Indeed, completion of the meiotic divisions of the oocyte nucleus, which give rise to three polar body nuclei and the female pronucleus, is inhibited until after an egg is 589

laid. Whether or not entry of sperm is responsible for removal of this inhibition is uncertain.

Pronuclear fusion may occur at a relatively fixed site to which both pronuclei migrate or may occur randomly within an oocyte, depending on the rate and direction of movement of the pronuclei. The polar body nuclei normally migrate to the periphery of the oocyte and eventually degenerate.

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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