Sunday 10 October 2010

Underwater Arthropods

No matter how much you might like the sea and swimming and such, we humans are fundamentally landlubbers. We just lub the land. Quite right too, what with our spindly limbs and conspicuous absence of gills. It's no wonder then that we usually think of arthropods, insects and arachnids as equally at sea in the sea, what with their spindly limbs and, usually, conspicuous absence of gills. But a number of our exoskeletal friends have taken to water and haven't looked back, relying on various adaptations to get around, feed and breathe. Live, basically. Which is the whole idea. It's often fascinating to see how various skills and lifestyles shared with their terrestrial kin are put to use in this new environment. First though, grasping the nettle, let's quickly step over some arthropods that are so incredibly aquatic they require adaptations to stop being quite so mercilessly aquatic... 

Image: Haplochromis
Crabs, lobsters, shrimp, these are the real aquatic arthropods. From tiny parasites less than a millimetre long, to gigantic crabs over 3 metres across, they walk, clamber and swim all over the water world tearing prey apart with huge claws or filtering out tiny morsels with feathery limbs. They have gills with which they can absorb oxygen from the water around them. Some crabs can scamper around on land, keeping a film of water on their gills to help them take oxygen from the air, other crustacean are adapted to be completely terrestrial.

Source: James Lindsey's Ecology of Commanster Site
Water Bug or Water Boatman
Water bugs, or water boatmen in the US, are a kind of aquatic bug. True bug that is, with piercing, sucking mouth parts and leathery wing cases. Their 4 rear legs are long and powerful, hairy and oar shaped, allowing them to swim around the bottom of their freshwater homes. Their front legs on the other hand, are very short. They use their mouthparts to pierce plants and inject saliva that starts to digest them, before sucking in the delicious gloop. They're small enough to simply absorb oxygen through openings along the abdomen, called spiracles, similar to land insects. Only more wet.

Image: Holger Gröschl
Water Boatman or Backswimmer
Perhaps just to be confusing, water boatman in the UK refers to a specific kind of water boatman in the US, where they are known as backswimmers. They are closely related, the most obvious difference being that they swim on their backs at the water's surface, a characteristic that gives them both their British and American names. Another difference is that they are predators, tackling prey as big as tadpoles. Or indeed humans - they can deliver a painful bite, or rather a bloodletting stab. Careful now, this is one insect that isn't messing around on the river.

Image: © Trevor and Dilys Pendleton
Water Beetle
Water beetles are aquatic beetles (what an utterly useless thing to say!). Like the bugs they have legs, long, strong and hairy for powering them through water and their tough wing cases are smooth and streamlined to make them more aerodynamic. Aquadynamic? They rise to the surface to take in air, some store it under their abdomen to breathe and stop water getting into their spiracles. Others can breathe underwater via adaptations to the exoskeleton that allow them to hold a thin layer of air, oxygen being repleted via diffusion with the water. Many water beetles are ruthless predators, others may eat algae.

Image: © 1998 Heather Proctor
Water Mite
I wonder what a water mite could be? I'm gonna take a gamble and say it's an aquatic mite. Which is what it is. They're tiny enough to simply absorb oxygen through their skin and most of them are parasites, a dispicable lifestyle they share with many of their terrestrial kin. Straight from the egg, young mites go forth in search of their first host, usually the larvae of an insect which will grow up into something with wings. The most successful mites wait around for the adult to emerge before commencing vampirism, not only feasting on blood, but also hitching a lift. This enables them to travel far and wide and be horrible and disgusting in new, undiscovered territories. Clever. Not good, but clever.

Image: Jeffdelonge
Water Scorpion
Water scorpions are... you didn't think it was an aquatic scorpion did you? Cos that would be hilarious! What could possibly make you think such a thing? No, the water scorpion is another water bug, but at one or two inches, it's quite a big one. It isn't a good swimmer, prefering to crawl around near the shore or among plants. The front legs are vicious claws used to ambush, grasp and grip other insects, tadpoles and small fish. As with other bugs, digestive enzymes are injected and the nastiness is drawn back in, particularly appalling when it's happening to a gasping fish. They breathe with the use of a kind of snorkel that projects from the back end, simply breaking the surface of the water while the two half tubes are locked together.

Image source
Diving Bell Spider
The diving bell spider also goes by another name, one so unique and imaginitive yet intuitive and informative that it renews one's faith in Humanity and all his endeavours, and will no doubt play a significant role in uncovering the mysteries of Stonehenge, the Pyramids of Giza and the undeciphered scripts of the Indus Valley. That name, is 'Water Spider'. Revel in its glory.

The water spider is the only spider to spend its entire life in... water and, nature not being one to waste a good thing, their web spinning ability is put to good use. Amongst foliage, they use their threads to create a little capsule that they fill with air that clings to the abdomen when they go to the surface. In this haven they can rest and breathe easy, molt as they grow and the female can protect her young. They rush out to capture any prey that touches the diving bell or nearby threads. Trespassers beware!

Image: UC Museum of Paleontology
Naiads are the aquatic young of insects such as dragonflies, mayflies and stoneflies. I've always found the dragonfly ones to be the most impressive and brutal of the lot, which is cool because I've always admired both the power and the stillness of dragonflies. Also those massive eyes and the fact that both the young and the adults eat mosquitos. All good.

Large dragonflies can spend as much as five years of their life as a naiad, in fact most dragonflies spend more time as a youngster in the water than they do as an adult in the air. As a naiad, they capture their prey using extendable jaws, there is no puckering up for a kiss where these beasts are concerned. They are also great swimmers and can give chase even to tadpoles and fish, whereupon they may resort to a secret weapon. Now, I'm a mature, adult man, but the fact that these things breathe via gills in their rectum and can cast themselves forth at remarkable speed by expelling a gust of water from their anus is somewhat on the hilarious side. I mean, they sort of... fart, don't they? They kind of... fart themselves into shocking acceleration.


I guess it wouldn't be at the forefront of their tiny minds, but it must be quite embarrassing for THAT to be what makes you succumb to the charms of the extendable jaws of the naiad of the dragonfly, no?


Des O'Connor and Dusty Springfield. Now this, I like!


Amila Kanchana said...

Very interesting critters! That red prawn is a real cracker!

Joseph JG said...

Glad you enjoyed it!