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44 the larvae probably fed on liverworts and mosses. The great adaptive radiation of the order probably came at the end of the Cretaceous period and beyond and was correlated with the evolution of the flowering plants.

Though generally aligned with the panorpoids, the Hymenoptera (bees, wasps, ants, sawflies) are quite distinct from all other endopterygotes. Indeed, the recent study by Kukalova-Peck and Lawrence (2004) has the Hymenoptera as the sister group to all other endopterygotes. Fossils are known from the Triassic period, but these were already quite specialized, clearly recognizable as belonging to the extant symphytan family Xyelidae. Members of the suborder Apocrita, which contains the parasitic and stinging forms, are not known as fossils until the Jurassic and Cretaceous periods. The great adaptive radiation of this suborder was, like that of the Lepidoptera, clearly associated with the evolution of the angiosperms (see Section 4.1).

The foregoing discussion of the evolutionary relationships within the Insecta is summarized in Figure 2.6.

3.3. Origin and Functions of the Pupa

As noted in the previous section the Oligoneoptera (endopterygote orders) are characterized by the presence of a pupal stage between the juvenile and adult phases in the life history. The development of this stage, which serves various functions, is a major reason for the success (i.e., diversity) of endopterygotes. Given the importance of the pupal stage, it is not surprising that several theories have been proposed for its origin (Figure 2.8).

FIGURE 2.8. Theories for the origin of the pupal stage. Abbreviations: A, adult; E, egg; L, larva; N, nymph; P, pupa. [Partly after H. E. Hinton, 1963b, The origin and function of the pupal stage, Proc. R. Entomol. Soc. Lond. Ser. A 38:77-85. By permission of the Royal Entomological Society.]

Among the earliest proposals was that of Berlese (1913, cited in Hinton, 1963b) who 45

used the principle of "ontogeny recapitulates phylogeny" to develop his ideas. During its

^ INSECT DIVERSITY

development an insect embryo passes through three distinct stages. In the first (protopod)

stage no appendages are visible; this is followed by the polypod stage (in which appendages are present on most segments); and finally the oligopod stage (when the appendages on the abdomen have been resorbed) (see Chapter 20, Section 7.1). Berlese suggested that the eggs of exopterygotes, by virtue of their greater yolk reserves, hatch in a postoligopod stage of development, whereas the eggs of endopterygotes, which have less yolk, hatch in the polypod or oligopod stages. According to Berlese, the larvae of endopterygotes correspond to free-living embryonic stages, while the pupa represents the compression of the exopterygote nymphal stages into a single instar. The major fault of Berlese's idea is the absence of evidence that the eggs of exopterygotes have a better supply of yolk than those of endopterygotes (Hinton 1963b). Further, the theory implies that abdominal prolegs (see Chapter 3, Section 5.2) are homologous with thoracic legs. Hinton (1963b)

argued that prolegs are secondary larval structures, though this claim is not justified given the multilegged nature of the ancestors of insects (Heslop-Harrison, 1958; Kukalova-Peck,

1991). Truman and Riddiford (1999, 2002) have resurrected interest in Berlese's proposal following their detailed studies of the endocrine control of embryonic development.

Truman and Riddiford compared the subtle shifts in the timing of juvenile hormone activity in embryos of hemimetabolous and holometabolous insects. As well, they examined the effects of treating embryos with extra juvenile hormone or precocene (which destroys the corpora allata) at various stages of development. Truman and Riddiford argued that in ancestral hemimetabolous insects there were three postembryonic stages: pronymph, nymph and adult. These correspond to the larval, pupal, and adult stages, respectively, of holometabous forms. In modern hemimetabolous forms the pronymph has been retained as a short-lived, non-feeding stage, which typically is spent within the egg. By contrast, in holometabolous insects with earlier secretion of juvenile hormone the pronymph took on increasing importance, becoming a long-lived, multi-instar feeding stage while the nymphal instars, as proposed by Berlese, would be reduced to the single (pupal)

stage.

Poyarkoff's (1914) theory (cited in Hinton, 1963b) offers a major advantage over Berlese's, namely, that it provides a causal explanation for the origin and function of the pupal stage. According to this theory, the eggs of both endopterygotes and exopterygotes hatch at a similar stage of development. The adult stage in the exopterygote ancestors of the endopterygotes became divided into two instars, the pupa and the imago. Poyarkoff suggested that the subimago of Ephemeroptera (see Chapter 6, Section 2) and the "pupal" stage of some exopterygotes (see Chapter 8, Sections 4 and 5) are equivalent to the en-dopterygote pupal stage. Further, the pupa (especially that of primitive endopterygotes such as Neuroptera) resembles the adult rather than the larva. In his view the pupal stage evolved in response to the need for a mold in which the adult systems, especially flight musculature, could be constructed. The second (pupal-imaginal) molt was then necessary in order that the new muscles could become attached to the exoskeleton. In Poyarkoff's theory there is no difference between the endopterygote larva and the exoptergote nymph.*

* Because they were considered originally to be quite distinct, the juvenile stages of exopterygotes and endopterygotes were referred to as "nymph" and "larva," respectively. The modern view (see Hinton's theory) is that nymphal and larval stages are homologous, and that pterygote juvenile stages should be called larvae. For clarity of discussion, however, in this chapter only, the traditional distinction has been retained.

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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