The Indian stingless bee or dammar bee, Tetragonula iridipennis, is a species of bee belonging to the family Apidae, subfamily Apinae. It was first described by Frederick Smith in 1854 who found the species in what is now the island of Sri Lanka. Many older references erroneously placed this species in Melipona, an unrelated genus from the New World, and until recently it was placed in Trigona, therefore still often mistakenly referred to as Trigona iridipennis. For centuries, colonies of T. iridipennis have been kept in objects such as clay pots so that their highly prized medicinal honey can be utilized.
Taxonomy and phylogenetics
Tetragonula iridipennis belongs to the complex genus of stingless bees Tetragonula which contains more than 30 described species from the Indian subcontinent. Until recently, all species in the group were treated as belonging to the single genus "Trigona". Tetragonula iridipennis belongs to the smaller Iridipennis species group which includes four primary species: Tetragonula iridipennis, Tetragonula praeterita, Tetragonula ruficornis, and Tetragonula bengalensis. These four species were proposed in the 19th century and were poorly characterized. They have since not been directly compared via molecular studies; therefore there is uncertainty about whether they are in fact separate species.
Description and identification
Like other bees in its sub-species group, T. iridipennis can be identified by its dark mesoscutum or middle thorax region with four distinct hair bands separated by broad glabrous interspaces. It can be further distinguished from other species in its sub group by its chestnut-brown colored mandible which does not have a black apical area and a few dark brown erect setae or hair-like structures on the margin of its scutellum. Other species in the subgroup have yellow mandibles which have black apical areas and have light colored setae on their scutella. Based on studies of workers, T. indipennis has a total body length ranging from 3.5-4.0 mm and an entire body which is black to blackish brown. Size differences as well as male genitalia are key for identifying the species. They are relatively small sized bees and the male penis valve is very robust and tapers only at the apex. Male genitalia are also key to differentiating between male and female bees because the two sexes are very similar in size and color.
Distribution and habitat
T. iridipennis is a tropical bee species and is found throughout the Indo-Malay region. Its range was once thought to go as far as the Solomon and Caroline islands, but unlike other Tetragonula species, its range is fairly limited. Although it was originally discovered on the island of Sri Lanka, it is predominantly found in India. Specifically, studies have been conducted on T. iridipennis colonies located in Karnataka, Kerala, and Tamil Nadu. T. iridipennis also occurs on the islands of Indonesia where it is a natural pollinator.
Colony cycle
As with other highly eusocial bee species, T. iridipennis colonies are founded by swarming. However, this process is different for stingless bees like T. iridipennis and honey bees. In honeybees, the process is abrupt and a large group of workers leaves the original colony with the old queen, thus few connections are maintained between the old and the new colonies. The process is more gradual for stingless bees such as T. iridepennis. First scout workers from the original nest find a new nest site and gradually transport resources there from the mother colony. A virgin queen from the mother colony then arrives usually accompanied by a mass of workers. It is only after several days to half a year that relations between the mother and daughter colonies are severed.
Behavior
Mating behavior
Like males in other stingless bee species, there is tremendous competition between T. iridipennis males to copulate with virgin queens. Males have been observed to form large aggregations and to engage in mass flights near nests waiting for the emergence of virgin females. Once a virgin queen emerges, it pairs with one of the males and a mating flight ensues, resulting in the fertilization of the queen.
Life history
Males
While no detailed studies have been done on colony composition of T. iridipennis nests, in keeping with other stingless bee species, it is expected that the total number of males produced per colony is lower than that of workers. In some tropical species, male stingless bees stay for an extended time in nests. They then leave the nests and form aggregations.