Morphology of immatures of the thelytokous ant, Monomorium triviale Wheeler (Formicidae: Myrmicinae: Solenopsidini) with descriptions of the extraordinary last-instar queen larvae

The ant genus Monomorium is one of the most species-rich but taxonomically problematic groups in the hyperdiverse subfamily Myrmicinae. An East Asian species, M. triviale Wheeler, produces both reproductive queens and sterile workers via obligate thelytokous parthenogenesis. Here, we describe the immature forms of M. triviale based on light and scanning electron microscopy observations, with a note on the striking caste dimorphism in the last larval instar. The lastinstar queen larvae were easily recognized by their large size, aphaenogastroid body shape, and rows of doorknob-like tubercles on the lateral and dorsal body surface. This type of queen-specific structure has not been found in ants in general, let alone congeneric species found in Japan. In stark contrast to the queen larvae, worker larvae showed a pheidoloid body shape and a body surface similar to other ants. The worker larvae were estimated to have three instars, consistent with previously described congeners. The pupae of both castes had no cocoon, a characteristic commonly described in other Myrmicinae species. In total, the developmental period from egg to adult worker averaged 59 days under 25 degrees C. We discuss possible functions of the tubercles of queen larvae based on previous studies.

Automated summary

This study describes the immature forms of the East Asian ant species Monomorium triviale, with a focus on the remarkable differences between queen and worker larvae. Queen larvae possess unique body features and tubercles not found in other ants, while worker larvae resemble other ant species. Both castes of pupae lack cocoons, a characteristic shared with other Myrmicinae species.