Pygidial Shield Research Articles

Varied development of trunk segmentation in three related Upper Devonian phacopine trilobites

In many arthropods, the development of the trunk region is a complex part of post-embryonic development. Consideration of fossil ontogeny provides an additional source of data and a broader evolutionary perspective on the evolution of arthropod body patterning. Here, I examine the development of the thoraco-pygidial exoskeleton of three related phacopine trilobites from the Upper Devonian according to the integrated ontogenetic scheme proposed by Hughes et al. Pygidial shields assigned to three ontogenetic series gave us the opportunity to further explore the evolutionary pattern of the trilobite segmentation. The analysis showed a different mode of development in two of three species and thus reveals variability between the related taxa. Comparison of the boundaries of different aspects of ontogenesis ratifies the diversity of the segmentation process among trilobites and even among related phacopine species. Results include (i) in a synarthromeric trunk condition recorded to date, there has consistently been a preceding ‘equilibrium’ phase for the late meraspid pygidium and (ii) two developmental modes, i.e. both hypoprotomeric development and synarthromeric development, occur contemporaneously in closely related taxa. Such developments suggest that aspects of segmental development such as segment accretion and segment articulation were able to vary in a labile manner.

A Technique to Morphologically Differentiate Larvae of Diabrotica virgifera virgifera and D. barberi (Coleoptera: Chrysomelidae)

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte and northern corn rootworm (NCR) Diabrotica barberi Smith are key pests of corn in the north-central United States (Chiang, 1973; Levine and Oloumi-Sadeghi, 1991). Both species are commonly found as larvae in the same cornfields in areas where geographic distributions overlap. Attempts to distinguish larvae of each species using external characters have proven to be difficult. Mendoza and Peters (1964) reported that differences in anal plate (pygidial shield) morphology could be used to separate larvae of each species. The WCR generally has a notched, dark brown anal plate whereas the anal plate of the NCR is oval and pale. However, Piedrahita et al., (1985) found that these characteristics were not consistent in second-third instar larvae reducing the diagnostic utility of the characters (i.e., third instar NCR were misidentified as WCR 52% of the time). Electrophoretic and molecular techniques have proven to be more reliable diagnostic tools. Piedrahita et al., (1985) surveyed 20 enzyme systems and identified differences among NCR and WCR larvae using horizontal starch electrophoresis. Clark et al., (2001) and Roehrdanz (2003) utilized PCR-RFLP and multiplex PCR respectively, to identify mitochondrial differences among each species. This paper reports an alternative and reliable way to differentiate WCR and NCR larvae using morphological characters on the sclerotized head capsule of each species.

Observations on the quill mites (Acari: Syringophilidae) from charadriiform birds

We provide a list of all 19 named species of the family Syringophilidae (Acari: Prostigmata) parasitizing charadriiform birds (Charadriiformes) together with new records and keys to these species. We describe a new species Niglarobia cursoriae sp. nov. from Cursorius temminckii Swainson (Glareolidae) and redescribe Creagonycha totana (Oudemans, 1904) from Tringa totanus L. (Scolopacidae). Niglarobia cursoriae sp. nov. differs from the closely related species N. rhinoptili Fain et al., 2000 in the following characters: in females of N. cursoriae sp. nov. hysterosomal shield is present and fused with pygidial shield; the length ratio of setae g:pg1:pg2 is 1:5:3.8. In females of N. rhinoptili the hysterosomal shield is absent; and the length ratio of setae g:pg1:pg2 is 1:3:2. All known syringophilid records from charadriiform birds are summarized in tabular form.

Diabrotica virgifera

<i>Diabrotica virgifera</i>

Heatherellidae — a new family of Mesostigmata (Acari: Parasitiformes) based on two new species from rainforest litter in Australia

Abstact Forest litter habitats in Australia contain a diversity of early derivative Mesostigmata, including some taxa that cannot be placed in any currently recognised suborder. A new genus, Heatherella, and family, Heatherellidae, are proposed to accommodate two such new species, H. acanthocharis and H. callimaulos, that feed on nematodes and inhabit subtropical rainforest litter in southeast Queensland. Both mites share a number of derived character states otherwise unknown in the Mesostigmata, e.g. the opisthosoma is ringed by nine pairs of spout-like gland openings; the stigmatal opening is contained within a raised, spiky turret; the adults have three setose pygidial shields, and a unique circumventral shield surrounds the intercoxal region. The relationships of the Heatherellidae appear to be closest to the Uropodina; however, the new family lacks major uropodine synapomorphies and shares characters with Epicriina, Microgyniina, and Zerconina (none of which are known to occurin Australia).

The oldest arthropods of the East European Platform

Cassubia Lendzion 1977 and Liwia new name (for Livia Lendzion 1975 preoccupied) found in the subsurface Zawiszany formation of the basal Cambrian in NE Poland are identified as an anomalocarid and a ‘soft-bodied’ trilobite, respectively, each being the oldest representative of its group. The presence of four free thoracic tergites in Liwia, the relatively strong sclerotization, and the apparent segmentation of the pygidial shield (expressed in the metameric distribution of marginal spines) indicate that the ‘soft-bodied’ trilobites achieved their organization by secondary softening of the dorsal exoskeleton and suppression of the development of thoracic tergites in ontogeny.

Phylogeny of the Arthropoda with Especial Reference to the Trilobites

After the above survey, it should appear that the trilo, bites, particularly in respect 'tO their appendages, are more primitive than any other Arthropoda. .The chief modifications in other groups are in the nature of reductions, in the loss of .whole appendages, of branches or of segments.. Extra segments are sometimes added in certain appendages, and new outgrowths, epipodites, are common among the Crustaeea. 'The trilobites have what seems, at first sight, a peculiar and specialized dorsal test, but now that it has been shown that the pleural lob.es may be lost and the pygidium reduced to a single segment, and, chiefly, that the wormlike form is not primi, tire but secondary, they may be viewed in an entirely new light. In the oldest fossiliferous rocks, Lower Cambrian, trilobites are plentiful, brachiopods rare, and no other arthropods present. A greater differentiation is seen in the Middle Cambrian fauna, due, however, entirely to the remarkable assemblage found by Walcott at a single locality in Br...