What Is Pterthorax

6.14. A beautiful adult male mayfly in mid-Cretaceous amber from New Jersey. AMNH NJ1018; body length 6 mm.
6.15. A mayfly, family Baetidae, in Miocene amber from the Dominican Republic. Many mayflies have very small hind wings, and in baetids the hind wings are often extremely reduced. Morone Collection M3351. Photo: R. Larimer.

posterior tracheation of the pterthoracic legs; and fixation of the anterior mandibular articulation (although present in Ephemeroptera, it is loose and can act as a slider). The extinct superorder Palaeodictyopterida is included here because they lost the caudal filament.

The Odonatoptera have at times been united with the Ephemeroptera owing to the aristate antennae (hence the name Subulicornia for this group). However, as noted by Grimaldi (2001), the homology between the "aristate" condition seen in these two lineages is suspect. The overall morphology of the "arista" suggests that they are not homologous (Figure 6.16). In the Ephemeroptera the flagellum consists of a series of short, nearly indistinguishable flagel-lomeres, appearing almost as annulations rather than distinct units. By contrast, the flagellum of Odonata consists of 2-4 distinct flagellomeres, each longer than wide and with well-defined articulations between them. The basalmost fla-gellomere in odonates is noticeably stouter and more elongate. Moreover, representatives of stem-group lineages of both Odonatoptera (e.g., Namurotypus) and Ephemeroptera (e.g., Protereisma) have relatively long flagella, indicating that the reduction to an aristate condition occurred independently in both groups.

6.16. The aristate antenna of Ephemeroptera and Odonata is frequently cited as a trait uniting the two orders. However, detailed structure of the antenna differs in the two groups, and early fossils of each group had long antennae, indicating that these antennae evolved con-vergently. Scanning electron micrographs; not to same scale.

EPHEMEROPTERA: Heptageniidae

EPHEMEROPTERA: Heptageniidae

6.17. Reconstruction of a fairly typical paleodictyopteridan, Stenodictya lobata (order Paleodictyoptera), from the Late Carboniferous of Com-mentry, France. The mouthparts in this superorder formed a sucking beak, which was apparently used for feeding on plants. Paleodictyopteridans were the dominant insects of the Paleozoic.

6.17. Reconstruction of a fairly typical paleodictyopteridan, Stenodictya lobata (order Paleodictyoptera), from the Late Carboniferous of Com-mentry, France. The mouthparts in this superorder formed a sucking beak, which was apparently used for feeding on plants. Paleodictyopteridans were the dominant insects of the Paleozoic.

PALAEODICTYOPTERIDA: EXTINCT BEAKED INSECTS

The palaeodictyopterids were fascinating, sometimes enormous insects that comprised about 50% of the known

Paleozoic insect species. They radiated into a diversity of forms and presumably niches that, after their extinction at the end of the Permian, appeared to have been filled by the new Mesozoic insect fauna. The Palaeodictyopterida apparently comprised mostly herbivorous insects from

6.18. Phylogeny of the extinct superorder Palaeodictyopterida. This is the only major lineage of insects to have become extinct, which apparently was at the end of the Permian.

the mid-Carboniferous to the Late Permian, having vanished probably at the End Permian Event about 250 million years ago.

Many palaeodictyopterids (Palaeodictyoptera and Eublep-toptera) were remarkable for their prothoracic paranotal lobes, complete with venation resembling that of the flight wings, albeit reduced (Figure 6.17). The lobes of some species were quite large, which provided much of the basis for notions of "six-winged" insects, although the lobes were apparently never articulated as functional wings. Another defining feature of the superorder was the development of a hypognathous head with haustellate mouthparts or beak, with five stylets, and an enlarged postclypeus (indicative of a cibarial pump for sucking fluids). The "beak" was long in most lineages but a few had shorter, stouter proboscides, perhaps used for puncturing tougher plant tissues. Their mouthparts were designed for piercing-sucking and resembled those of the living order Hemiptera. Other features of the palaeodictyopterids were the long, filamentous, multiseg-mented cerci typically covered with dense setae. The legs were simple but slightly short for such stout bodies, primitively with five-segmented tarsi but reduced to three segments in Diaphanopterodea. The antennae were usually long and filiform but could also be considerably shortened, as in some Megasecoptera. The ovipositor was short, stout, and equipped with many serrations in most taxa, suggesting that they oviposited into plant tissues.

Nymphs of palaeodictyopterids were all apparently terrestrial (e.g., Carpenter and Richardson, 1968), with wing pads held in an oblique, lateral position instead of close over the thorax and abdomen. Given that terrestrialization is primitive for insects as a whole, this observation furthers the notion that the aquatic naiads of Odonatoptera and Ephemeroptera were independently developed, particu larly if Palaeodictyopterida is basal to an Odonatoptera + Neoptera clade (this would also be further evidence that aristate antennae are convergent in Odonata and Ephemeroptera).

Handlirsch (1908, 1925) believed the palaeodictyopterids to be the stem lineage from which all other winged insects derived. He argued originally that the winged insects, via the Palaeodictyoptera, evolved directly from Trilobita and that all other insects were derived from individual branches within Palaeodictyopterida (e.g., Megasecoptera gave rise to what we now called the Panorpida). This concept has not held up to scrutiny, and as we have seen the palaeodictyopterids are not basal within insect phylogeny but rather were a specialized lineage that was also the sole superordinal complex to have become entirely extinct. All other superorders have at least a few survivors today. They were also the first major lineage of herbivorous insects.

The Palaeodictyopterida is divided into groups (Figure 6.18), whose relationships are not entirely understood, although the order Palaeodictyoptera retains more primitive traits and may be paraphyletic to all other members of the superorder. Similarly, the Eubleptoptera of the Megasecoptera may be paraphyletic to the remaining taxa, Eumegasecoptera, Protohymenoptera, Dicliptera, and Diaphanopterodea. Extensive cladistic work within the Palaeodictyopterida is needed, and we have made no attempt to resolve relationships within these more or less well-defined groups. Most work on the group has consisted of the descriptions of taxa, the documentation of their character combinations, and detailed accounts of their stratigraphic occurrence.

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