In the early summer evenings, if you wander around the streams or ponds, you may encounter small insects with a pair of big orange eyes. They are almost everywhere, not only flying around but also resting on the railings near the ponds or on any walls or structure they can land on. They are a local species of mayflies called 𝘊𝘭𝘰𝘦𝘰𝘯 𝘩𝘢𝘳𝘷𝘦𝘺𝘪 Kimmins, 1947, which belong to the family Baetidae. Mayflies are famously known for their incredibly short lifespan as adults, so the name of the order known as Ephemeroptera (with the same root found in Ephemeral). Their exact lifespan, however, may vary among species and it is common for adult mayflies to live for just a few hours to a few days. Let’s delve deeper into their “brief” but fascinating life cycle.
Actually, the whole life cycle of 𝘊. 𝘩𝘢𝘳𝘷𝘦𝘺𝘪 is not that short. Just like other mayflies, they spend the majority of their lives as aquatic nymphs (the larval stage). They have 7 pairs of gills located on both sides of their abdomen, allowing them to breathe in water and thus inhabit freshwater bodies such as streams, rivers, and lakes. Unlike other baetids, you can find 𝘊. 𝘩𝘢𝘳𝘷𝘦𝘺𝘪 in almost all the water bodies in Hong Kong, including Lantau Island and Sai Kung. This is because their nymphs (larvae) can live in standing freshwater like ponds, as long as the water quality is suitable, which makes this species and other mayflies excellent water quality indicators as they are sensitive to pollution. The nymphs are around 6 mm long, so be sure to look carefully in the water to spot these small organisms. The nymphal stage is crucial for the identification of mayflies in the family Baetidae, as the taxonomy of adults is still under investigation. New adult morphological characters must be sought for confidently identifying species. However, the morphology of nymphs of this family shows great diversity and can be used for identification.
Just like many other mayflies, 𝘊. 𝘩𝘢𝘳𝘷𝘦𝘺𝘪 will go through incomplete metamorphosis, which means they go through a transition from nymph to adult without developing a pupal stage. Instead, they undergo many molts and gradually develop into winged adults, a type of development in insects known as hemimetaboly. In completion of the aquatic stage, mayflies will shed off their exoskeleton and step into a unique stage called subimago. Among all the insects, only mayflies have this stage. During the subimago stage, they have fully functioning wings, but they are not sexually mature yet and thus unable to mate. Other differences between subimago and imago (the adult stage) include the presence of hairs in the body, legs, and wing surfaces in the subimago stage, which help them to not get wet as the hairs are hydrophobic. This stage optimizes their water–air transition and facilitates the development required for flight, mating, and reproduction.
After shedding off its “skin”, the subimago has finally entered its brief adult stage. The mouthparts are completely lost so adults won’t be able to feed! And that’s fine because the adult stage has a single purpose: reproduction. It begins with the swarms formed by a few to hundreds of male mayflies in the early morning. Females will fly near the swarms and will be grabbed by one of the males. They will then drop to the vegetation and mate. After mating, males have fulfilled their reproductive role and will soon die, while the female will lay eggs on the water surface in a floating state. In this condition, the rear part, including the abdomen, is in contact with water while the front part of the body and fore legs are kept above water. And then females die as well.
The adult morphology of 𝘊𝘭𝘰𝘦𝘰𝘯 𝘩𝘢𝘳𝘷𝘦𝘺𝘪, is really intriguing. The compound eyes of male Baetidae mayflies are different to that of other mayflies. They are divided into two distinct parts, the lateral and dorsal parts. The lateral part is black, just like the compound eyes of many insects. However, the dorsal part is very interesting, and is cone-shaped and orange in colour. The dorsal eyes are often called turban eyes. You can imagine they are putting orange turbans or orange binoculars above their eyes. The huge dorsal eyes of the males are helpful to identify the females against the dim sky. Since they need to locate and grab the females within the swarms, the larger dorsal eyes can facilitate them in this behaviour.
Do you want to take a closer look at the binoculars-like eyes of 𝘊𝘭𝘰𝘦𝘰𝘯 𝘩𝘢𝘳𝘷𝘦𝘺𝘪? Come and join our tour guide and admire these fascinating animals up close at the Hong Kong Biodiversity Museum.
Text: Tony Ling
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