Gerridae

The Gerridae are a diverse family of insects known commonly as water striders.

They belong to the order Hemiptera and the suborder Heteroptera. These insects are true bugs, meaning they have mouthparts designed for piercing and sucking. A defining characteristic of the Gerridae is their ability to live on the water's surface. This lifestyle makes them pleuston, or surface-living animals. They can be found in many environments, including ponds, rivers, and lakes. While 90% of these insects live in freshwater, about 10% are marine. The genus Halobates is a notable exception because it lives in the ocean.

To stay afloat, water striders utilize the high surface tension of water. They use long, slender legs to distribute their body weight over a large surface area. This prevents them from breaking through the water's surface. Their entire bodies are covered in hydrofuge hairpiles. These are tiny, hydrophobic microhairs with more than one thousand hairs per millimeter. These hairs repel water and prevent droplets from weighing the insect down. If a wave pushes a strider underwater, these hairs trap air. The trapped air creates tiny bubbles that provide buoyancy to bring the insect back up. This position, where the body stays mostly above the water, is called epipleustonic.

Movement for a water strider is a complex mechanical process. They have three pairs of legs: front, middle, and hind. The front legs are the shortest and feature preapical claws. These claws are located halfway down the leg rather than at the very end. These are used to puncture prey. The middle legs are the longest and are used for propulsion and steering. They have well-developed fringe hairs on the tibia and tarsus to increase thrust. During a rowing stroke, the middle tarsi are pressed down and backward. This action creates a semicircular surface wave. The strider uses the crest of this wave as a counteracting force to push forward. This allows them to move at speeds of 1 meter per second or faster.

The Gerridae exhibit a fascinating trait called wing polymorphism. This means a single species can have different types of wings. Some populations are long-winged, or macropterous, while others are short-winged or entirely wingless, or apterous. This is often explained by the oogenesis-flight syndrome. Developing short wings allows an insect to save energy. Instead of building wing muscles, they use that energy for egg production. This increases their reproductive success and fitness. Long wings allow for dispersal to new water bodies if a current habitat becomes crowded. Short wings allow for limited travel. Wingless forms are ideal for stable habitats where the environment does not change much. An environmental switch mechanism, influenced by temperature and photoperiod, helps control these seasonal changes.

Scientists have studied the history and evolution of these insects for a long time. Molecular analysis suggests the family originated about 128 million years ago during the Cretaceous period. They split from a sister group called Veliidae. Both groups share a single origin of rowing as a way to move. Early researchers like Eschscholtz discovered several species, bringing attention to the family. Later, Francis Buchanan White conducted heavy studies on the genus Halobates during the Challenger expedition between 1822 and 1883. These studies helped scientists understand how these insects occupy unique ecological niches.

There is significant variety in the size and shape of Gerridae species. Most species have a body length between 2 and 30 millimeters. The genus Aquarius in the Northern Hemisphere includes some of the largest species. In the streams of northern Vietnam and southern China, the species Gigantometra gigas lives. This species is quite large, with winged females reaching about 30 millimeters. In this specific species, the pattern of size is reversed, as females are usually larger than males. Some males can even reach 30 millimeters in length. Their middle and hind legs can also exceed 30 millimeters in length.

The life cycle of a water strider involves several distinct stages. Females lay eggs on submerged vegetation or rocks using a gelatinous glue. A gravid female may carry between two and twenty eggs. The eggs are initially creamy white or translucent but turn bright orange later. After the egg stage, the insect goes through five instar stages as a nymph. Each nymphal stage typically lasts between 7 and 10 days. During these stages, the insect molts by shedding its old cuticle. Nymphs look similar to adults but are smaller and paler. They eventually reach the final adult stage to complete their development.