My esteemed editor informed me that the 8th of June is World Ocean Day. Initially I thought I’d ignore that, as so few insects live in the ocean, but a bit of reading has shown that some are really interesting animals.
I hope you all realise by now that insects dominate the terrestrial living world (or biome), including freshwater. But for all their success on land they have been a great failure in the sea, with only a few hundred marine insect species known and only five open ocean species (none of which actually live in the water).
Most insects living in the sea do so at the margins, between the tidal limits. For example, worldwide many beetle species burrow in estuarine mud (and are cylindrical) or live under flakes of slaty rock (and are flattened). These species don’t seem to occur here in the Illawarra, possibly because we have the wrong rock or the wrong mud.
We do have a pale sand-coloured beetle, Sartallus, living along the high tide mark where it feeds on dead things or possibly fly maggots. And a few other strandline insects such as seaweed flies. But these are not really marine.
In our rockpools there are a few insects whose freshwater ancestors have jumped into the marine environment. One of the most extraordinary is a non-biting midge, Pontomyia.
I have to say that I’ve never seen one, so if the following seems like I’m pulling your leg I can only say I trust the scientists who’ve studied it. The Pontomyia adult male is only 1mm long and fast moving. It uses its short wings as oars, not for flight, skimming across a pool surface to find a female and mate.
The female is legless and wingless and mating is back to back (they manage). Both adults live less than three hours – yes, that’s hours – with the poor old female laying eggs immediately after mating and living only about an hour after that.
The entire life cycle is 30 days and linked to the moon. So, if you want to find one, remembering it’s only 1mm long, you are recommended to visit a weedy rockpool on an evening with a full moon, wait until 15 minutes after sunset, then shine a strong torch over the water, looking for fast-moving white streaks (the males). I think it best to avoid rain and storms.
There’s a popular science account of Pontomyia online.
The five truly oceanic (pelagic) insects are species of water striders, in genus Halobates. Halobates is also a small insect, with adult body length of about 3mm, and can be relatively abundant (up to 150,000 per km²). Curiously, they are not aquatic at all. Their whole life history is above the sea surface, with the long-lived eggs being laid on flotsam.
Halobates are covered in dense minute hairs, forming a plastron which traps air as a large bubble. So that if splashed or soaked by rain they remain unwetted and don’t sink into the water. Halobates also have the ability to jump away from predators. There’s some interesting film of Halobates online (Mahadik et al. 2020).
For general insect enquiries, contact the Australian Museum’s Search And Discover team
at sand@austmus.gov.au. Have a question specifically for Chris? Email editor@2515mag.com.au
