The Evolution of Male Tergal Glands in the Blattaria

Abstract

Male tergal glands in the Blattaria serve to maneuver the female into the proper precopulatory position and arrest her movement (while she “feeds” on the gland secretion, or palpates the male's dorsum) long enough for the male to clasp her genitalia. Although the glands occur in almost all families, they are rare in the Polyphagidae and Blaberidae. In the Blaberoidea, tergal glands show the greatest development and variation in the Blattellidae. When they occur in the Blaberidae (usually in the Epilamprinae) they may represent a relic character, from a blattellidlike ancestor. The tergal gland, when present in the Blattidae (Blattoidea), almost always occurs on the 1st abdominal segment only. In the Blattellidae the number of segments which are specialized may vary from 5 to 1. The greatest number of genera in this family have only 1 modification, and the 2 commonest positions are on segment 7 or 1. The distribution and frequency of occurrence of the tergal glands in the Blattellidae suggest that in the course of evolution there has been a reduction in the number of segments which are specialized and a change in the location of the glands from a posterior to a more anterior position on the abdomen. The “best” position for the tergal glands would be anterior, because when the female places her mouthparts in or palpatesa gland situated anteriorly on the abdomen, the genitalia of both sexes are relatively close together and would allow the male to grasp the female more readily. There is some indication that in apterous and brachypterous males that have multiple specializations, those glands that became exposed as a result of the loss of wings became reduced or were eventually lost. Protection of the remaining glands also resulted from a change in their position on the tergite (evolving from the posterior to the anterior margin) or by a reduction in the length and telescoping of the posterior segments, so that the preceding tergite covered the specialized area.