In some species larval growth rates are affected by photoperiod. For some species the abiotic growth is accelerated under long-day conditions (when there are 16 or more hours of light ENVIRONMENT in each 24-hour cycle) and inhibited in photoperiods that contain 12 or fewer hours of light; for other species, the converse is true. Often the effect of photoperiod on growth rate is correlated with the nature of diapause induction; that is, species that grow more slowly under short-day conditions tend also to enter diapause as a result of short days. However, it should be noted that the growth rate of many species that enter a photoperiodically controlled diapause is not affected by photoperiod.

Exposure to different photoperiods such as occur in different seasons may result in the development of distinct forms of a species, that is, polyphenism. The physiological (endocrine) basis of polyphenism has been outlined in Chapter 21 (Section 7), and the present discussion is restricted to a consideration of its induction by photoperiod. Sometimes the forms that develop are so strikingly different that they were described originally as separate species. Beck (1980) cited as an example the European butterfly Araschnia levana, described originally as two species, A. levana and A. prorsa, but which is now known to be a seasonally dimorphic species. Caterpillars reared under long-day conditions metamorphose into the non-diapausing, black-winged (prorsa) form; when they have developed at short day lengths the caterpillars emerge as red-winged (levana) adults that overwinter in diapause. This example shows a typical feature of most dimorphic Lepidoptera, namely, that one form is characteristically found in summer and is non-diapausing, whereas the alternate form is the diapausing, overwintering stage.

Photoperiodically influenced polyphenism is also seen in the seasonal occurrence of normal-winged, brachypterous, and/or apterous forms of species of Orthoptera and Hemiptera. But perhaps the best-known example of the effects of photoperiod on development is that of polyphenism in aphids. The life cycle of aphid species (see Figure 8.8) is complex and varied but shows beautifully how an insect takes full advantage of suitable conditions for growth and reproduction. A key feature of the life cycle is the occurrence within it of wingless, neotenic females that reproduce viviparously and parthenogenetically. In many species the offspring are entirely female. This combination of features enables aphids to reproduce rapidly and build up massive populations in the spring and summer when weather conditions are good and food is abundant. As a result of the crowding that results from this reproductive activity, winged migratory forms develop, and a part of the population moves on to alternate host plants. From these migratory forms several more generations of female aphids (alienicolae) are produced (again through viviparity and parthenogenesis), which may be winged or apterous. Eventually the alienicolae give rise to winged sexuparae (all female) that migrate back to the original host plant, and whose progeny may be either winged males or wingless females (oviparae). These reproduce sexually and lay eggs that pass the winter in diapause on the host plant. The following spring each egg gives rise to a female individual, the "stem mother" or fundatrix, normally wingless, that reproduces asexually, and from which several generations of neotenic females (fundatrigeniae) arise. There are many variants of this generalized life cycle, most often through its simplification; that is, one or more of the life stages is omitted as, for example, in species that do not alternate hosts when migrants and whose offspring do not appear as distinct forms. Indeed, in some species sexual forms have never been described and reproduction appears to be strictly parthenogenetic.

The development of these seasonally occurring aphid forms is influenced by a variety of environmental factors, including day length. Crowding is the major factor that influences

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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