Neopteran Blattariageneralized mesothorax lateral

4.6. Major types of wing articulation in Ephemeroptera, Odonata, and Neoptera.

during rest, which is what defines the neopterous condition. The medial plate is perhaps derived from the third axillary sclerite. There are most frequently two medial plates, but sometimes there is only one. From the distal medial plate, veins M and Cu arise. The distal and proximal medial plates are separated by an oblique line over which the fold occurs during the flexion of the wings over the abdomen. The distal plate may be reduced in size or only a slightly more sclero-tized region of membrane (leading to the condition of only one medial plate). The small humeral plate articulates with the base of vein C. The tegula is frequently developed to cover much of the wing base. When it is reduced in size, it lies anterior in the membrane between the humeral plate and the thoracic notum. Although tiny and often overlooked by even the most trained of entomologists, these plates orchestrate the complex suite of wing movements during flight.

In the primitively winged insects (e.g., Ephemeroptera, Odonatoptera) the arrangement is somewhat different. The Ephemeroptera wing base is exceedingly complex and very similar to other pterygote insects, but they have the axillary sclerites less defined and there are no medial plates (Figure 4.6). Odonata, alternatively, have only two larger articular plates, the humeral and axillary plates, the result of the fusion of several smaller sclerites; they are hinged together with the thoracic notum (Figure 4.6).

The cells of the wing are the regions between veins and crossveins. There are numerous naming systems for the cells, none of which are consistent across orders (we indicate these in the systematic sections of the book). One of the more common naming systems for cells uses the name of the vein that marks the anterior margin of the cell, though numerous other names are in use, like marginal, submarginal, discal, and dis-

coidal. Only two terms are universal in entomology when referring to wing cells: Closed cells are those that are bordered on all sides by veins, while open cells are those where part of the cell is not delineated by a vein because it is at the edge of the wing.

To provide weight and strength to the leading edge of the wing, there is frequently a region of either expanded anterior veins (veinal composition differs by group) or highly pigmented and slightly sclerotized cuticle in an anterior cell to form a pterostigma. The pterostigma prevents the wing from fluttering during changes in the stroke of the wing by adding weight to the leading edge.

The most ingenious structures on the insect wing, however, are its folds. Recall that the insect wing does not have muscles within it to control the fine adjustments to its shape that are necessary during flight. The alterations must come about through the structure of the wing itself. The wing is beset with a series of flexion lines that respond to physical stresses and allow, in a very controlled fashion, the wing to partially collapse or fold upon itself, principally to generate vortices when developing lift. Some of these points of weakness are common to most insects. For example, the median flexion line is typically an oblique line of flexion running between Rs and MA and the nodal line is a transverse line of flexion, typically dividing a stiffer proximal region from a more deformable distal wing.

Because insects are dramatically varied, so are their flight mechanics. Hovering insects, for example, have veins near the wing tip coalesced to keep it rigid. As such, different groups require their wings to undergo different changes in shape so as to achieve the type of flight necessary for that particular lineage. Alar fenestra is a generalized term for any of various, shorter lines of flexion in various places on the wing, often seen as areas of weakness running across veins.

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