Functional morphology of the metapleural gland in workers of the ant Crematogaster inflata (Hymenoptera, Formicidae)

Workers of Crematogaster inflata possess the largest metapleural glands (relative to body size) known among ants, with reservoirs extending anteriorly up to the junction between the proand the mesothorax, and with over 1400 secretory cells on both sides together. This large secretory capacity is related to the gland’s defensive function, which, in members of this species, is directed against larger arthropod and vertebrate enemies, and apparently not against microorganisms, in contrast to other ants, where the gland produces antibiotics. The gland is not equipped with any direct musculature. Secretion release is probably caused by contraction of the oblique longitudinal thorax muscles or by passive expulsion caused by external pressure. Additional key words: exocrine glands, social insects, histology Ants live in colonies that represent potentially very valuable resources for predators. The colony as a whole, however, is generally well protected by the combined defensive power of its individual members. The common defense mechanisms of ant workers include both mechanical and chemical elements. They can rely on their mandibles as powerful instruments for biting, spray formic acid if they are formicine ants, or use their sting, which acts as a syringe to inject the contents of the venom gland. Besides these defense strategies that involve the head and the abdomen, the thorax can also play an important role through the secretory products of the metapleural gland. This gland is known for the elaboration of antibiotics that suppress the development of microorganisms (Maschwitz et al. 1970; Schildknecht & Koob 1970, 1971; Beattie et al. 1986; Bot et al. 2001, 2002; Poulsen et al. 2002a,b). In workers of the southeast Asian ant Crematogaster inflata SMITH 1857, the posterior part of the thorax is bright yellow, in contrast to the otherwise black appearance of these ants. The yellow region corresponds with the position of an enlarged metapleural gland, which, in this species, functions in the elaboration of a sticky defensive secretion (Maschwitz 1974; Jones et al. 2005). The obvious black-and-yellow color pattern was also found to be a warning signal for predators, as exemplified by cases of Batesian mimicry by a similarly colored member of the genus Camponotus and of beetles in the genus Drusilla (Maruyama et al. 2003; Ito et al. 2004). We examined the morphology of this gland in workers ofC. inflata, and linked its structural characteristics with the defensive function it performs. In his 1974 paper, Maschwitz also included the congeneric species Crematogaster difformis SMITH 1857, workers of which also have enlarged metapleural glands, but with the production of antibiotics as their main function. Crematogaster difformis is probably a mis-identification of Crematogaster sewardi FOREL 1901 (Hosoishi & Ogata 2009). As we had access to embedded material of C. sewardi, we also examined the metapleural gland of members of this species for comparison.