Insect of the week: 21 August 2023

Insects of the week (38). Anthidiini species (Hymenoptera: Apoidea: Megachilidae) and Samba species, Hymenoptera, Apoidea, Melittidae

This week we look at two very interesting bee species.  Each represents a very different family (Meliittidae and Megachilidae). Both possess a morphological feature not found in other bees, and both thrive in hot, dry habitats (the area around Lake Turkana and lowland eastern Kenya).

Lake Turkana (formerly Lake Rudolph), the world’s largest permanent desert lake is situated at the centre of the Lake Turkana basin.  The lake is located almost entirely in Kenya with a small portion in southwestern Ethiopia. The basin itself extends to the xeric north, south, east and west, hot and dry in all directions. Much to the disappointment of Kenya’s earliest European explorers, the lake is a brackish soda lake. For the living, the habitat is not a forgiving one. It is however a paleontologist’s dream, with its rich fossil sites. And while the desert-like environment of the basin can seem devoid of life, within days of a rainfall event the area becomes a rich, flower-covered plain with the many animals, particularly aestivating bees, that depend on this short-lived floral abundance.

In 2014 a team of scientists visited the area led by Dino Martins, Kenya’s answer to David Attenborough and head of the Lake Turkana Basin Institute. As it happened, he was present shortly after one of the unpredictable rains. Bees were plentiful and one species stood out from the others. This turned out to be a rare member of the family Melittidae, an undescribed species of Samba (Samba), one of the six subgenera of Samba and the most easily identified one due to the presence of a single, out-sized, boomerang-shaped tibial spur on the hind leg of females (see attached image of female of another Samba species). The new species was subsequently described as Samba (Samba) turkana. Martins observed that the tibial spur played an important role in harvesting pollen, helping to scrape it off the anthers. As they stated in a subsequent paper describing the new species, the “ …… enormous and single metatibial spur is only found in Samba s.str. “; (=Samba (Samba)).

We beg to differ. In collections made in eastern Kenya in 2020 and 2021 we found a series of a strange-looking bee species that has (yes, you guessed it) a huge boomerang-shaped spur on the female’s hind tibia, so similar to that of Samba it could have been made in the same workshop (see image of anthidiine). The surprise is that our bee is a Megachilidae of the tribe Anthidiini, only distantly related to the Melittidae and Samba. [Megachilidae and Apidae (the latter of honeybee fame) are long-tongue bees. The Melittidae along with the three other Afrotropical bee families are short-tongue bees].

What to make of this? It is possible that this is an example of convergent evolution, perhaps driven by the need to rapidly collect of pollen in hot, dry areas with dehydration a real danger, and where the possession of the huge, curved tibial spur may provide an advantage for Samba turkana over other bees. It would be useful to discover the the pollen-collecting behaviour of the anthidiine species. Do they exploit similar plants as S. turkana? We now know that the anthidiine species is undescribed and that no other species of Megachilidae have this enlarged spur. To the best of our knowledge this feature is not known from the rest of the world’s bee species. How it appears in different families and nowhere else is a mystery. Perhaps we could look for a gene or genes unique to the two species that have it. 

Credits: Dr Robert Copeland