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Water-measurer, Hydrometra stagnorum
enlarge Water-measurer
WATER-MEASURERS ARE NEEDLE THIN INSECTS. THEY MOVE SLUGGISH AND DO NOT APPEAR IN SUCH GREAT NUMBERS AS THE WATER STRIDERS, WHICH ARE MORE WELL-KNOWN FOR THAT MATTER. But Water-measurers are common and can be seen on almost any pond now and then. Just like the Water strider they are surface bugs, yet totally different in appearance and behaviour. The Water strider lies low above the surface as a skelter, has its legs spread out, is fast, hunts almost full-time on the water surface and grabs the prey with its front legs.
Hydrometra larva last instar with Water-flea
enlarge Water-measurer with Water-flea
The Water-measurer stands on his legs like a stilt walker, is sluggish as a walking stick, sits often hidden in the green of the water borders and steals upon its prey, that is directly speared with the long sucking tube (proboscis). The front legs are used for walking, not grabbing. The movement is a slow walking with the six legs, not rowing with two like the Water strider, and Hydrometra never jumps. The name Hydrometra is literally translated to the name Water-measurer, and indeed it seems to walk on the water with measured steps. It's fascinating to watch the Water-measurer while hunting. Utmost sluggish the animal walks the water, the stabbing mouth tube is turned slowly from its horizontal resting position under the body to a vertical one, and seemingly churning it is thrusted in the water. There the head goes up and suddenly a water-flea is pinned on the mouth tube! Water-measurers stick their proboscis also in the midge larva that hang under water surface and suck out insects fallen on the water. The eyes of this bug are best developed on the under side (logically), this is also the case with the Water striders. They also have legs that are sensitive for vibrations of the water surface.

Water-measurer, two couples (Spanje 7-9-2006)
vergroting Water-measurers mating -Spain 2006
In the summer the Water-measurers mate. The females are a bit larger and have a bit a hollow back, like older model surf boards. It seems comfortable for the male that sits so close on the female that only the tangle of legs shows there are two Water-measurers in stead of one.
Hydrometra egg (after Brocher, 1913)
Hydrometra egg (after Brocher, 1913)
They mate in groups one and then. The eggs are totally different from those of the Water striders, in the first place they are large, up to a fifth of the length of the female! They have a beautiful net structure, which is probably important for the air supply. The female first applies a drop of a rubber like glue on a plant stem or leaf. This drop forms a little attachment disc, on this a stalk is made and on that the egg is deposited. I tried hard to find one, up till now unsuccessfully.
young hydrometra larva 17-07-05
vergroting Water-measurer, young larva
When the eggs hatch the tiny, orange colored larva appear. As with all true bugs they already resemble the adults.
Hydrometra larva
enlargement Water-measurer, larva -Spain 2006
From birth on they possess the razor sharp stabbing snout, which they clean thoroughly, see left. The abdomen is relatively short and is held up in a perky little curve. Calmly the tiny animals walk through the jungle of the border. To us: a mirror smooth water surface along the pond margin with only a little branch emerging here and there, but for the larva that same surface is full of lumps and large, slippery slopes of some millimeters high: difficult to conquer... See also the page on the water surface. They undergo a few moultings and the last instars are almost entirely colored black, by that time they walk as a long legged colt over the water. The long antenna are often held downward to the water and then look like a fourth pair of legs.


WEBLINKS:
photos of Water-measurers, also of a WINGED specimen.



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COPYRIGHT:
All pictures on this site were made by Gerard Visser (Almelo, Netherlands), unless stated otherwise. All rights remain with him. These pictures may not be used for purposes any other than private viewing or printing. Do NOT hardlink to these pictures or place them on other websites without the author's approval. Should you need them for purposes which include third parties, you must ask the author permission by e-mail. People, who want to use this pictures for exhibitions or publications or educative material are much encouraged to do so, after approval as mentioned and giving the normal credits.
© G.H. Visser 02-02-2007
rev.16-06-2010


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