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insects were already well established by the Lower Devonian, some 80 million years earlier than previously assumed. This conclusion agrees with a molecular clock study indicating that insects arose in the Early Silurian (about 430 million years ago), with neopteran forms present by about 390 million years ago (Gaunt and Miles, 2002).

By the Upper Carboniferous period, when conditions became suitable for fossilization, almost a dozen paleopteran and neopteran orders had evolved. Most authors, especially paleontologists, consider the Paleoptera to be monophyletic and the sister group to the Neoptera, and list a number of apomorphies in support of this view (Kukalova-Peck, 1991, 1998). Further, a recent study of 18S and 28S rDNA sequences from almost 30 species of Odonata, Ephemeroptera, and neopterans has provided strong support for the monophyly of the Paleoptera (Hovmoller et al., 2002). However, there are those, notably Boudreaux (1979), Kristensen (1981,1989,1995) and'Willmann (1998), who, having undertaken cladis-tic analyses of the extant Ephemeroptera (mayflies) and Odonata (damselflies and dragon-flies), believe the Paleoptera to be paraphyletic. In Boudreaux's view the Ephemeroptera + Neoptera form the sister group to the Odonata, while according to Kristensen the best scenario has the Ephemeroptera as the sister group of the Odonata + Neoptera. This view is supported by Wheeler et al. 's (2001) analysis, though these authors examined only three species each of Odonata and Ephemeroptera. According to Kukalova-Peck (1991), within the Paleoptera, two major evolutionary lines appeared, one leading to the paleodicty-opteroids (Paleodictyoptera, Diaphanopterodea, Megasecoptera, and Permothemistida), the other to the odonatoids + Ephemeroptera. All paleodictyopteroids (Upper Carboniferous-Permian) had a hypognathous head with piercing-sucking mouthparts (Figure 2.3). Adults and large juveniles used these to suck the contents of cones while younger instars probably ingested only fluids (Shear and Kukalova-Peck, 1990). Prothoracic extensions were

Libellule Geante

FIGURE 2.3. Paleodictyopteroids. (A) Stenodictya sp. (Paleodictyoptera); and (B) Permothemis sp. (Permothemistida). [A, from J. Kukalova, 1970, Revisional study of the order Paleodictyoptera in the Upper Carboniferous shales of Commentry, France. Part III, Psyche 77:1-44. B, from A. P. Rasnitsyn and D. L. J. Quicke (eds.), 2002, History of Insects. @ Kluwer Academic Publishers, Dordrecht. With kind permission of Kluwer Academic Publishers and the authors.]

Permothemis

FIGURE 2.3. Paleodictyopteroids. (A) Stenodictya sp. (Paleodictyoptera); and (B) Permothemis sp. (Permothemistida). [A, from J. Kukalova, 1970, Revisional study of the order Paleodictyoptera in the Upper Carboniferous shales of Commentry, France. Part III, Psyche 77:1-44. B, from A. P. Rasnitsyn and D. L. J. Quicke (eds.), 2002, History of Insects. @ Kluwer Academic Publishers, Dordrecht. With kind permission of Kluwer Academic Publishers and the authors.]

prominent in some paleodictyopteroids (Figure 2.3A) and Kukalova-Peck (1983, 1985) 35

suggested that these were articulated. There was no metamorphic final instar as in modern

exopterygotes; that is, wing development was gradual and older juveniles could probably fly. Their extinction at the end of the Permian may be correlated with the demise of the Paleozoic flora (see Section 4.2). Paleodictyoptera formed the largest order of paleodictyopteroids and included some very large species with wingspans up to 56 cm. As noted earlier, the Diaphanopterodea, which may be the sister group of Paleodictyoptera, were unique among Paleoptera in that they were able to fold their wings. Though most diaphanopterodeans were plant-juice feeders, Kukalova-Peck and Brauckmann (1990) observed that some Permian species were remarkably mosquitolike and speculated that these may have fed on blood. Megasecoptera had several features in common with Diaphanopterodea, though these were likely the result of convergence. Contrary to earlier opinions, the Megasecoptera were not carnivores but sucked plant material; a few may have caught other insects and sucked their body fluids. The Permothemistida [formerly the Archodonata and included in the Paleodictyoptera by Carpenter (1992)] were a small group, characterized by having greatly reduced or no metathoracic wings, short mouthparts, and unique wing venation (Figure 2.3B).

Early members of the Ephemeroptera + odonatoid group had biting mouthparts and aquatic juveniles with nine pairs of abdominal gill plates and leglets. Adults of early Ephemeroptera (Upper Carboniferous-Recent) (including the Protoephemeroptera, formerly separated because of their two pairs of identical wings) differed from extant forms in having functional mouthparts. Some very large forms evolved, for example, Bojophlebia prokopi with a wingspan of 45 cm. The nature of their mouthparts suggests that nymphs were probably predators, some perhaps feeding on amphibian tadpoles (Kukalova-Peck, 1985) (Figure 2.4A). The early odonatoids differed from Ephemeroptera in features of their venation and in having nymphs that lacked abdominal gill plates, using instead the rectal branchial chamber for gas exchange (Chapter 15, Section 4.1). The group includes two orders Protodonata (Meganisoptera) (Upper Carboniferous-Triassic) and Odonata (Triassic-Recent) that are evidently closely related, some authorities even including the former in the latter order. However, Kukalova-Peck (1991) presented five wing features, and features of the genitalia and cerci that justify their separation. The Protodonata were superb aerial predators, catching prey in flight or from its perch using their long, strong legs (Figure 2.4B). In this diverse and abundant group were the largest known insects (Meganeuridae), including

Stenodictya Ephemeroptera Larva

FIGURE 2.4. Early Paleoptera. (A) Juvenile of the Early Permian mayfly, Kukalova americana; (B) Arctotypus sp., a late Permian protodonatan; and (C) Early Jurassic dragonfly nymph, Samamura gigantea. Though the nymph had large anal flaps, reminiscent of the caudal lamellae of damselflies, it used a branchial chamber for gas exchange. [From A. P. Rasnitsyn and D. L. J. Quicke (eds.), 2002, History of Insects. @ Kluwer Academic Publishers, Dordrecht.With kind permission of Kluwer Academic Publishers and the authors.]

FIGURE 2.4. Early Paleoptera. (A) Juvenile of the Early Permian mayfly, Kukalova americana; (B) Arctotypus sp., a late Permian protodonatan; and (C) Early Jurassic dragonfly nymph, Samamura gigantea. Though the nymph had large anal flaps, reminiscent of the caudal lamellae of damselflies, it used a branchial chamber for gas exchange. [From A. P. Rasnitsyn and D. L. J. Quicke (eds.), 2002, History of Insects. @ Kluwer Academic Publishers, Dordrecht.With kind permission of Kluwer Academic Publishers and the authors.]

Meganeuropsis permiana with a 71-cm wingspan. Only recently have protodonate juveniles been discovered (Kukalova-Peck, 1991); these had a mask similar to that of odonate larvae (see Figures 2.4C and 6.8). Some also had prominent wings, leading to the possibility that they could fly. A number of Permian fossils originally described as Odonata, specifically in the suborders Archizygoptera and Protanisoptera, have now been reassigned to the Protodonata (Kukalova-Peck, 1991) so that true Odonata are not known before the Triassic. These generally small predators already bore a strong resemblance to the extant Zygoptera and Anisoptera both in form and habits (Figure 2.4C).

In contrast to the Paleoptera, which were inhabitants of open spaces, the Neoptera evolved toward a life among overgrown vegetation where the ability to fold the wings over the back when not in use would be greatly advantageous. The early fossil record for Neoptera is poor, but from the great diversity of fossil forms discovered in Permian strata it appears that the major evolutionary lines had become established by the Upper Carboniferous period.

Two major schools of thought exist with regard to the origin and relationships of these evolutionary lines. The traditional view, proposed by Martynov (1938), is that, shortly after the separation of ancestral Neoptera from Paleoptera, three lines of Neoptera became distinct from each other (Table 2.1 and Figure 2.5A). Based on his studies of fossil wing venation Martynov arranged the Neoptera in three groups, Polyneoptera (plecopteroid, or-thopteroid, and blattoid orders), Paraneoptera (hemipteroid orders), and Oligoneoptera (en-dopterygotes). In a modification of this view Sharov (1966) proposed that the Neoptera and Paleoptera had a common ancestor (i.e., the former did not arise from the latter) and, more importantly, that the Neoptera may be a polyphyletic group. In his scheme (Figure 2.5B) each of the three groups arose independently, a consequence of which must be the assumption that wing folding arose on three separate occasions.

Ross (1955), from studies of body structure, and Hamilton (1972), who examined the wing venation of a wide range of extant species as well as that of fossil forms, concluded that there are two primary evolutionary lines within the Neoptera, the Pliconeoptera and

FIGURE 2.5. Schemes for theoriginand relationships of themajor groups of Neoptera. (A) Martynov's scheme; (B) Sharov's scheme; (C) Hamilton's scheme; and (D) Kukalova-Peck's scheme.
Kukalova Peck

FIGURE 2.6. A possible phylogeny of the insect orders. Numbers indicate major evolutionary lines: (1) Pale-optera; (2) Neoptera; (3) Plecopteroids; (4) Orthopteroids; (5) Blattoids; (6) Hemipteroids; (7) Endopterygotes; (8) Neuropteroids-Coleoptera; (9) Panorpoids-Hymenoptera; (10) Panorpoids; (11) Antliophora; (12) Amphies-menoptera.

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