Colony-level stabilization of soldier head width for head-plug defense in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)
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Termite colonies invest in the production and maintenance of soldiers, which specialize in defense and must be fed by workers. In Reticulitermes termites, however, the role of soldiers as colony defenders has been questioned. I found that Reticulitermes speratus Kolbe soldiers use phragmotic defenses (blocking openings with their heads) that prevent enemies from invading an undamaged part of the nest. In experimental nest-like conditions, Reticulitermes soldiers effectively blocked nest access of the termite-hunter ant Brachyponera chinensis Emery, using a combination of head-plug defense and mandibular threats. Phragmotic defense requires that soldiers have heads wide enough to block the small nest openings. This suggests that stabilizing selection should act on soldier head width. Comparison of the coefficient of variation of head width with that of other body parts showed that head width is a stable character. Analysis of the relative growth of head width and head length suggested that stabilizing selection on soldier head width operates at the colony level. In contrast, soldier head width was as variable as other body parts in non-phragmotic Coptotermes formosanus Shiraki and Nasutitermes takasagoensis Shiraki soldiers. Within-colony uniformity of soldier head width is likely an adaptation of Reticulitermes soldiers for phragmotic defense.
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