The Water-measurer

WATER-MEASURERS ARE NEEDLE THIN INSECTS. THEY MOVE SLOWLY AND DO NOT APPEAR IN SUCH GREAT NUMBERS AS THE WATER STRIDERS, WHICH ARE MORE WELL-KNOWN FOR THAT MATTER. But they are common and can be seen on almost any pond now and then. 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 constantly on the water surface and grabs the prey with its front legs. The Water-measurer stands high on its legs like a stilt walker, is sluggish as a walking stick, often hides in the bankside vegetation and stalks its prey, which is immediately 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 well translated with the English name Water-measurer, as indeed it seems to walk on the water with measured steps. It's fascinating to watch the Water-measurer while hunting. Extremely slowly the animal walks the water, the piercing snout is turned slowly from its horizontal resting position under the body to a vertical one, and seemingly probing it is thrusted in the water. There the head goes up and suddenly a water-flea sits empaled 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.

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. 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.

---

When the eggs hatch, the tiny, orange colored nymphs appear. As with all true bugs they already resemble the adults. 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 nymph 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.

---