Monday, 23 September 2013

Week 29: Water Boatmen (‘Corixa’ and ‘Notonecta’)

Water boatmen were @SpeciesofUK from 14th to 24th July, 2013.

Water boatmen are aquatic bugs that live in ponds and slow-flowing streams.[1]

Greater Water Boatman
[Source: Flickr Creative Commons © JRxpo]

In the UK, water boatmen are grouped into ‘lesser’ water boatmen (Corixidae and Pleidae species) and ‘greater’ water boatmen (Notonectidae). The simplest difference between the two is that lesser water boatmen swim on their front and greater on their back.[2]

Great water boatmen are also known as ‘backswimmers’ because they swim upside-down.

Greater water boatmen are called backswimmers in the USA. It’s a name sometimes used here in the UK too, to differentiate them from lesser water boatmen.

The most well-known and common species of greater water boatmen is Notonecta glauca, the ‘common backswimmer.’[3]

Notonecta Glauca
[Source: Wikimedia Commons © Factumquintus]

N. glauca is widespread throughout Europe and inhabits ponds, ditches and canals (still or slow-moving water).[4] It has two long legs which paddle like oars, making it look like a rowing boat.[5]

Notonecta Glauca in action
[Source: Flickr Creative Commons © Katy Wrathall]

N. glauca is greenish or brown. Its eyes are reddish and large. The four front legs are short while the rear two are very long.[6]

N. maculata is another greater water boatman. It is distinguished by its brick-coloured forewings.[7]

Notonecta Maculata
[Source: Wikimedia Commons © Didier Descouens]

And here is an example of yet another great water boatman species, N. obliqua:

Notonecta Obliqua
[Source: Flickr Creative Commons © Drinker Moth]

Greater water boatmen carry a bubble of air with them underwater.

All aquatic bugs have to surface from time to time to breath oxygen, limiting their time underwater. Greater water boatmen get around the problem by carrying extra air in the form of a bubble attached to their body.

You can often see the greater water boatman's bubble of air because it gives it a silvery appearance.[8]

Its ventral side is also covered in hairs which prevent water from covering the spiracles that the greater water boatman uses to breathe.[9]

Greater Water Boatman reflection
[Source: Flickr Creative Commons © Sam Dredge]

Greater water boatman are carnivorous and they punch above their weight.

Despite their small size, greater water boatmen prey on tadpoles, small fish and other aquatic insects.[10]

Greater water boatman hunt by resting on the surface of the water sensing vibrations. They immediately dive when they feel a disturbance.[11]

Greater Water Boatman resting on surface
[Source: Flickr Creative Commons © berniedup]

A greater water boatman will kill its prey by jabbing it with its rostrum (feeding tube) and injecting its toxic saliva. They are very effective predators.[12] The bite from a Backswimmer can be even painful to humans as their saliva is toxic.[13]

Greater water boatmen can fly and will move between different ponds, which helps to maintain genetic diversity in the species.

Greater Water Boatman out of water
[Source: Flickr Creative Commons © oktobrain]

Greater water boatmen can fly when they become adults, allowing them to move to another pond.[14]

Greater water boatmen mate between December and May and eggs are laid between February and May of the following year, in aquatic plant stems.[15]

Here is a video of a greater water boatman clambering out of the water:

Lesser water boatmen are not actually closely related to greater water boatmen.

Lesser water boatmen spend most of their time feeding on algae under water.[16] In the UK, they make up the family Corixidae, which includes several hundred species worldwide.[17]

Lesser water boatmen are extremely common. Approximately half of the species of water bug recorded from UK ponds are different kinds of lesser water boatmen.[18]

The most common lesser water boatman in the UK is the widespread species Corixa punctata.

Corixa Punctata
[Source: Flickr Creative Commons © urtica]

C. punctata is found in most weedy ponds, lakes and slow-flowing rivers, but rarely in water deeper than 2m.[19] They thrive in water with neutral to high alkalinity, and are rarely seen in acidic peaty streams and ponds.[20]

Like greater water boatmen, lesser water boatmen carry a bubble of air they use for breathing. They brush it regularly to access the oxygen.

C. punctata spend most of their time at the bottom of the pond, anchored to the bottom or a plant.[21] If it loses grip it floats upwards then suddenly rushes back down. They come to the surface only to renew their air supply, hurrying upwards and collecting a new air bubble in less than a second.[22]

Corixa Punctata at bottom of pond
[Source: Flickr Creative Commons © Wolfram Sondermann]

C. punctata is brown, has long hairy hind legs that it uses like paddles, slightly shorter middle legs and very short front legs.[23]

C. punctata's front legs are used to scoop up food. They are omnivorous but mainly eat algae and vegetable matter.[24] Their triangular heads have a straw-like mouthpart (a proboscis), which injects plants with digestive saliva.[25]

Corixa Punctata close up
[Source: Wikimedia Commons © Viridiflavus]

Another remarkable talent of the lesser water boatman is its ability to fly with a ‘sprint start’ - rushing upwards through the water surface and straight into flight.[26]

Lesser water boatmen produce courtship ‘songs’ by rubbing body parts together.

In late spring courting begins. Male C. punctatae attract females with a ‘courtship song’, rubbing their front legs against a ridge on their head to make a low chirping noise.[27]

The loudest lesser water boatman, however, is M. scholtzi. It has the unusual talent of being able to ‘sing’ at volumes of around eighty decibels by rubbing its penis against its abdomen. In fact M. scholtzi was been recorded at 99.2 decibels, making it the loudest species on earth relative to body size.[28]

Female lesser water boatmen lay single eggs, which they attach to plant stems or floating algae.[29]

Strange but true...

In Mexico water boatman are actually considered a delicacy, and are harvested and eaten in large numbers![30]

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