Water striders do a balancing act on water!

If you have ever had the opportunity of closely observing the surface of a pond, well, marsh or any calm body of water, you may have spotted insects running, hopping or skimming along the water. What exactly these creatures are and how they manage to walk on water may have baffled you. These large, long and slender-legged aquatic bugs are known as water striders and you may be surprised to learn that they are perfectly adapted to living on the surface of water and have the ability to support and balance themselves nicely without sinking,

The creature is an arthropod (invertebrate with an exoskeleton, a segmented body, and joined appendages; includes insects, arachnids and crustaceans) and is also known as pond-skater, water-skater, water bug, magic bug, skimmer, water scooter, water skeeter, water skimmer, water skipper and water spider.

Water striders belong to a family known as Gerridae in the order Hemiptera. Like all other insects, they have a body divided into three segments (head, thorax and abdomen), six jointed legs and two antennae. Its body is long, dark brown or black, and narrow. Some have wings while others don't. Most water striders are over 0.5 inch (5 mm) long. Scientists have identified over 1,700 species of gerrids, 10 percent of which are marine (living in oceans) and are known as Halobates. Water striders have two antennae with each divided into four segments. The antennae have short, stiff bristles in the third segment. Relative lengths of the antennae segments can help identify species within the family Gerridae, but the first segment is generally longer and stockier than the other three. The four segments combined are usually not longer than the length of the strider's head.

The thorax, which is the section between the head and the abdomen, is generally long and narrow. Some bodies are more cylindrical or rounder than others. The outer layer of the thorax can be either shiny or dull depending on the species, and covered with microhairs to help repel water. The abdomen can have several segments.

Their short front legs are used to catch prey; the long, middle legs act like paddles, moving the bug across the water surface, while the long hind legs steer them and act as brakes. The front legs are attached just behind the eyes while the middle legs are attached closer to the back legs which attach mid-thorax, but extend beyond the terminal end of the body. The specialised legs as well as their bodies are covered by tiny water-repellent hairs called microsetae, with small nanogrooves and it is this leg structure and their lightweight bodies that allow them to stand on water, aided by the water's surface tension.The tiny hairs give the water strider resistance to water, preventing drops of water from weighing down the body.

Gerridae use the surface tension of water to their advantage through their highly adapted long, strong and slender legs and weight distributed over a larger surface area to stay afloat while the tiny hairs on their bodies also provide a hydrophobic and larger surface area to spread their weight over the water.

Striders can be seen making dimples or impressions with the ends of their legs on the water surface. If their hair becomes wet due to oil or detergent on the water surface, the surface tension would be broken and the water strider will sink.Their legs also make them agile, helping them to jump quickly to evade a predator or catch their prey. The microsetae, which also cover the underside of their bodies, are also used to detect vibrations in the water. If an insect accidentally falls in the water, the water strider will feel it through the ripples the insect makes in the water.

The water strider has very good vision and can move at speeds of 1.5 metres er second across the water surface, thereby grabbing its prey quickly. It would push its mouthpart called rostrum into its prey and pierce and suck the insect dry.

Its diet mainly consists of living and dead insects on the surface of the water including aquatic insects such as mosquito larvae and land insects, such as butterflies, dragonflies, beetles and worms and spiders that accidentally land on the water surface or fall into the water.They share the prey with other water striders around them. Halobates feed off floating insects or zooplankton, and occasionally resort to eating their own young. Cannibalism (eating their own kind) often occurs, but helps control population sizes and restrict conflicting territories. Skaters themselves become prey to predators such as birds and fish.

Their inability to detect any motion above or below the water's surface is a disadvantage to them in this area. However, the fact that they can move rather quickly on the water helps them escape predators. They usually appear in large groups and prefer the protection of overhanging trees and shade. Though they are found around water most of the time, they fly far from the water to hibernate during winter, returning to their watery homes with the onset of spring. In the breeding season, water striders communicate by sending ripples to each other on the surface of the water.

Males generally produce ripples in the water under three main frequencies: 25 Hz as a repel signal, 10 Hz as a threat signal, and 3 Hz as a courtship signal. Females lay eggs at the water's edge, on stable surfaces such as plant stems or stones, submerged in water. Halobates often lay their eggs on floating ocean debris which are then spread across the ocean. The amount of eggs laid depends on the amount of food available to the mother during the season. They will reproduce throughout the year in warm tropical regions, but only during warm months in seasonal habitats.

Life cycle

Gerrids go through the egg stage and five instar (a stage of development in arthropods) stages of nymphal (immature insects which resemble the adult in basic appearance) forms before reaching the adult stage. Individuals tend to develop at the same rate through each instar stage.

Each nymphal stage lasts 7-10 days and the water strider moults, shedding its old cuticle. Nympths are similar to adults in behaviour and diet, but are smaller (1mm long) and paler. It takes 60 to 70 days for a water strider to reach adulthood, depending on the water temperature the eggs are in. Water striders can live for many months.

The ability for one brood of striders to have young with wings and the next without, allows them to adapt to changing environments. Long, medium, short and non-existent wing forms are all necessary, depending on the environment and season. Long wings allow them to fly to a neighbouring water body when one gets too crowded.

However, long wings can get wet and weigh a water strider down. Short wings may allow for short travel, but limit how far they can disperse. Non-existent wings prevent them from being weighed down, but prevent dispersal.Sudden increases in salt concentration in the water of gerrid habitats can also trigger migration of water striders.

They will move to areas of lower salt concentration, resulting in the mix of genes between brackish and freshwater species.

They also burrow into the mud when their habitat dries up, remaining dormant until the water level comes back o normal.

As habitat, water stridesr prefer an environment abundant with insects or zooplankton and containing rocks or plants for them to deposit eggs on. It has been found that they prefer water bodies around 25 degrees Celsius with temperatures lower than 22 degrees Celsius being considerd unfavourable.

This may be due to the fact that the development of their young are temperature dependent. Some of the most prominent varieties of Gerridae are found in Wales, the former USSR, Canada, USA. South Africa, South America, Australia, China and Malaysia.

They control insect populations, especially mosquitoes, and are therefore helpful to humans. They don't bite humans and are not harmful to them.

- IT