Plecopterida

Three disparate relatives within the Polyneoptera are the very generalized stoneflies and the highly specialized webspinners and zorapterans. The placement of these three orders in the grander scheme of the insects has been troublesome for many years. The crux of the problem is that each is without clear affinity to any other order and each is a recent remnant of an otherwise ancient lineage. As alluded to before, it is exactly for such groups that paleontological data are hypothesized to be most critical for resolving relationships (Gauthier et al., 1989). The plecopterid orders are united principally by numerous reductions from the polyneopteran groundplan. A prognathous head capsule is a potential synapomorphy for the group, but more likely this feature is a more inclusive trait uniting Plecopterida to Orthopterida (hypognathous in some Orthoptera). Plecopterids also share the reduction of the ovipositor and suppression of male styli. Ovipositor reduction is not homologous to the distinctive reduction noted for Dictyoptera. Furthermore, Early Mesozoic and Paleozoic stem-group Dictyoptera (i.e., blattodean "roachoids") had well-developed ovipositors (Figures 7.65-7.67). Instead, in Plecoptera, Embiodea, and Zoraptera the ovipositor is not merely reduced as it is in Dictyoptera (Figure 7.62) but actually entirely lost. Correspondingly, putative stem-group plecopterids from the Paleozoic appear to have had well-developed ovipositors. Alternatively, the reduction in Plecoptera and Embiodea + Zoraptera may be independently derived (discussed further later in this section), thereby eroding the use of this trait to support the more inclusive clade Plecopterida. Other features are not universally understood across ple-copterid orders and therefore have limited significance so far, the most notable being the median, ventral holes (ostia) in the dorsal blood vessel (presently unknown in Zoraptera).

Zwick (1973, 2000) and Hennig (1981) both dismissed a close relationship of Plecoptera and Embiodea, stressing the many remarkably primitive neopteran traits in stoneflies, and they even suggested that Plecoptera were the living sister group to the remainder of Neoptera. It is ironic that Hennig should have argued on such grounds since plesiomorphic traits are not indicative of relationship. Instead, the available evidence suggests that stoneflies are in the Polyneoptera on the basis of the enlarged anal fan in the hind wing as well as sperm and ovarian ultrastructure (e.g., Buning, 1998; Fausto et al., 2001). Hennig (1981) noted that the argument for Polyneoptera monophyly based on the anal fan was weak because Sharov (1966) derived the Paraneoptera and Holometabola from among the polyneopterous orders (i.e., Polyneoptera was paraphyletic to all other Neoptera in his system). This actually reflects more on a confused phyloge-netic interpretation by Sharov than on the homology of the polyneopteran anal fan.

The controversial placement of stoneflies as the living sister group to Paraneoptera + Holometabola, also known as the Planoneoptera hypothesis (Ross, 1955; Hamilton, 1972a,b), was based upon the structure of the mesotrochantin, which is a slender, straplike structure in the latter groups (although highly modified through much of the Holometabola). Interestingly, this same trait may support the Plecopterida as we view them. In fact, Zoraptera and Embiodea both possess similar mesotrochantins while all three differ from the Holometabolan condition, in which the mesotrochantin is rigidly fused at its base to the episternum and is retained merely as a small projection articulating with the mesocoxa. It may be significant that a slender "eumetabolan-like" mesotrochantin that is separated from the episternum by membrane or a sulcus is present in Plecoptera, Embiodea, and Zoraptera. Dermaptera are the only other polyneopter-ans that have a similar trochantin.

Zoraptera can be excluded from the Paraneoptera based on their primitive retention of cerci, the presence of a first abdominal sternum, and the broad "orthopteroid" lacinia. Zoraptera have also been placed as a living sister group to Paraneoptera or as basal Eumetabola (i.e., Paraneoptera + Holometabola). Characters supporting their Eumetabola relationship include the development of a jugal bar (Hamilton, 1972a), which has been oddly assigned to the zorapterans at times (e.g., Wheeler et al., 2001) when it is actually absent. Alternative hypotheses are entirely unfounded, specifically the theory that tubercles on the furcasternum of Plecoptera approximate or transform into the tricondylic articulation seen in Holometabola (Adams, 1958; Rasnitsyn, 1998; but see Engel and Grimaldi, 2000, for Zoraptera). Evolution is replete with examples where structures have been co-opted for novel functions, such as some vertebrate mandibular bones incorporated into the inner ear. However, in each of these instances, there is significant evidence not only for the basic homology but also for the criterion of continuation and what all other characters indicate.

Assuming plecopterid monophyly for the moment, Plecoptera are basal relative to Embiodea + Zoraptera. Plecoptera retain the polyneopteran anal fan, entirely lost in Embiodea + Zoraptera who both exhibit dehiscent, narrow, paddle-shaped wings without any anal region. Furthermore, the webspinners and zorapterans share asymmetrical male genitalia, apterous morphs, reduced cerci (relative to Plecoptera), tarsomere reduction (likely not homologous to tarsal reduction in stoneflies, as evidenced by stem-group plecopterans), loss of gonostyli, gregarious and cryptic life histories, and large hind femora with overdeveloped tibial depressors (in sharp contrast to the developed tibial levators of other orders) (Engel and Grimaldi, 2000).

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