A pleasant surprise – Prosopistoma phoenicium

During my visit to Israel I visited the Golan Heights with colleagues from Israel and Germany. We were looking for mayflies and ground beetles in particular, but I was interested in anything I could find that would be nice to photograph.
We stopped at one of the fast-flowing springs in the Hula Valley and started flipping rocks in search for unique aquatic invertebrates. It wasn’t too long before we found something interesting: small creatures crawling on the surface of submerged rocks. There was no doubt – these were larvae of Prosopistoma phoenicium.

Typical habitat of Prosopistoma phoenicium larvae, fast-flowing streams (or springs, such as this one) with a rocky substrate.

Typical habitat of Prosopistoma phoenicium larvae, fast-flowing streams (or springs, such as this one) with a rocky substrate.

 

It is important to pause for a moment to reflect on the scientific history of this animal. Viewed from above, its appearance bears a striking resemblance to that of tadpole shrimps, branchiopods of the order Notostraca. And indeed, for many years this creature has baffled taxonomists regarding its true identity.
When Prosopistoma was discovered in 1762 by Geoffroy, he initially described it as a species of Binoculus, a crustacean, due to the curved, shield-like mesothorax. This changed in 1833, when Latreille described the genus Prosopistoma and separated it from arguloid crustaceans, but still considered it to be a branchiopod along with the tadpole shrimps. Later in 1868, more than 100 years after the first discovery, Emile Joly realized that Binoculus/Prosopistoma was in fact a mayfly larva. Viewed from below, the animal clearly shows three pairs of legs, in other words – it is an insect, not a crustacean. Finally, Hubbard completed the required transition between the taxonomic groups by providing a revision of the nomenclature in 1979. As of today, the family Prosopistomatidae contains about 20 described species with a distribution primarily in the old world, throughout the Palaearctic, Oriental, Australian and Afrotropical regions, but entirely missing from the New world, the Nearctic and Neotropic regions.

Larva of Prosopistoma phoenicium from the Golan Heights, Israel. Left: dorsal view; right: ventral view.

Larva of Prosopistoma phoenicium from the Golan Heights, Israel. Left: dorsal view; right: ventral view.

 

These insects are rarely seen, but in Israel they seem to be easy to find if one knows where to look. I would like to take this opportunity to thank the Entomology course staff at Tel Aviv University in Israel, who did a splendid job with students in the field (I just realized I took this course as a student more than 10 years ago. Time flies when you’re having fun!). Without them I would not know where to look for and how to recognize this insect, as well as other cryptic species.

The biology of Prosopistoma is poorly known, but it is believed that the larvae scrape and feed on organic matter, such as algae, from the surface of rocks submerged in fast-flowing streams and springs. Adults are almost unheard of from the wild, most of the currently recognized species of Prosopistoma were described from characteristics of the larvae, and the adult mayflies are known from three species only. In the case of our site, the population was very healthy and we could afford to collect quite a few larvae for laboratory rearing at Tel Aviv University. I hope they complete their metamorphosis successfully as I am hoping to see an adult Prosopistoma one day!

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