Pond Skaters, Water Striders, Water Measurers, Water Crickets and their kin are all relatively commonly seen insects, they live on the surface film of water, buoyed up by a combination of their water repellent hairs and the surface tension of the water. At this time of year it seems as if every piece of still water has a few Water Skaters rowing around on it. Often you can see couples of the large Aquarius najas riding around in tandem, the male guarding the female for up to several weeks and even sharing her food. While lurking around the edges of the pond are Water Crickets (Velia caprai) and the more sedate Water Measurer (Hydrometra gracilenta). All these animals are True Bugs, which means they are members of the order Hemiptera and the Infra order Gerromorpha, and therefore have piercing and sucking mouth parts. There are about 1 500 species world wide and though most of them live on fresh water about 10% are marine living on the coast or even the open sea and some are even terrestrial.
Pond Skaters and their allies have been around for at least 55 million years because we have found fossil Gerridae and Hydrometridae dating back to the Upper Paleocene and fossil Veliidae and Mesoveliidae from the lower cretaceous. Scientists believe they probably evolved in the early Mesozoic. Pond Skaters are widespread and you can see them anywhere there is clean water except Antarctica, however their greatest diversity is in the tropics in central Africa and the Indo-Australian region.
They are all predators or scavengers feeding on small aquatic arthropods by stabbing them with their proboscis through the surface of the water. Or rapidly devouring terrestrial insects trapped on the surface film of the water. In tropical climes some of them are important biological control agents in rice paddies taking many fallen nymphs of pests such as Plant Hoppers. They will also feed on emerging adults of insects with aquatic nymphs, smaller species feeding on emerging Midges (Chironomidae) while larger species will take emerging Mayflies. Other surface film dwellers are also at risk, Hydrometra australis preys heavily on Collembolans, while Gerrids will feed on larger surface dwellers even cannibalising younger members of their species. In some cases several members of a species may work together to subdue and then devour a larger prey item.
Females are normally larger than males, and though eggs are laid at different times of year depending on species, egg production rate is limited by the food resources available to the female, i.e. if the female gets more to eat she lays more eggs. There are normally 4 but sometimes 5 larval instars, developmental time varies between species and with temperature, taking on average between 40 to 65 days. In temperate species growth ceases once the temperature drops below 5 - 10 degrees C. Many species exhibit a flexible life history strategy. Meaning that from any given ' cohort ' (a year group --- a group of young of one species hatching at approximately the same time) some will reach maturity and reproduce in their first year and others will overwinter before reaching sexual maturity and breeding in their 2nd year. Tropical species tend to be active throughout the year, though they may show a peak in numbers at one particular time, or go into a summer diapause to survive dry periods when the pools they live on dry up.
In temperate climates species that can fly often travel considerable distances to their hibernation sites. Non-flying forms or species hibernate on land near the edges of the pools and streams they live on. In some species the wing length varies between individuals, with some being short-winged and others long-winged. This is called being ' polymorphic ' for wing length, from the Greek ' Poly ' for many and ' Morphos ' for form. Variation in wing length is believed to be regulated by an interaction between genetic, ontogenetic and environmental cues. Wing length is known to be heritable in some species, though the inheritance factors are complex.
Though all water surface bugs are predators themselves they are often considered to be dinner themselves by larger animals. Predators include, Birds, Fish, Dytiscid water beetles, Notonectid backswimmers, Dragonflies and spiders of the genus Dolmedes. They are also attacked by mites such as Limnochares aquatica and several egg parasitoids such as Tiphodytes gerriphagus (Hymenoptera Scelionidae).
Gerrids range in size from 1.6 to 36mm in length (the 36mm Gigantometra spp. Are the largest of the water surface bugs). There are 500 species of Gerridae in the world and they include the truly oceanic Halobates sp.. We have 8 species in the UK, plus one occasional migrant from mainland Europe. Pond Skaters are more highly adapted for life on the surface film than any other bugs. They rest easily on the water, rowing themselves along on with their middle pair of legs, pushing back with both legs simultaneously and steering with their hind ones. Like most surface bugs they are densely clothed in short unwettable hairs which helps keep them on the surface. They also have the claws of their feet set back from the tips of their tarsi a bit, which prevents their claws from breaking the surface tension of the water. Though they mostly row themselves around they can jump both from the water surface and on land if necessary. Winged forms not only fly to winter hibernating areas but also to find new ponds and streams during the summer. Like most surface bugs they are opportunistic predators and scavengers taking advantage any small animal (alive or recently dead) they can find on or in the water surface.
They overwinter as adults and most species lay their eggs by attaching them either to floating vegetation or floating debris. Using floating material to support their eggs prevents them from drying and dying if the water level drops. Some species such as Aquarius najas however go under the water in order to lay their eggs well below the water line. Gerrids and a few Veliids are known to communicate by detecting and generating ripples in the water's surface. Both sexes use the ripples created by insects trapped in the surface tension to locate their prey. Males also use ripple signals to attract females to potential egglaying sites and to stimulate the female when she arrives, as well as to tell other males to stay away from their female. Signals with a frequency of 35-45 hertz tell other males to stay away while those with a frequency of 2-5 hertz stimulate the female to oviposit.
Pond Skaters and Water Striders use four different mating systems: Monogamy i.e. a male mates with a female and then stays with her to make sure no other males can mate with her (this is called postcopulatory guarding); Resource Defence Polygyny i.e. a male defends a potential oviposition site and mates with all the females who come into his territory; Scramble Competition Polygyny where males search out individual females and mate with any they find; and Lek Polygyny where males gather together to display for females who come looking for a male, here it is generally the 'top' male who does most of the mating. Males in some species show a flexible mating regime with three alternatives. Which mating strategy they use depends on the circumstances they find themselves in. The three options are Territorial defence with courtship and guarding, active searching with courtship but little or no guarding, and active searching with no courtship and no guarding. Females accept matings more readily after courtship but become less selective as population density increases. Experiments have shown that females as well as males mate more than once and that the last male to mate before oviposition usually fathers about 80% of the offspring. Males guard females by riding around on top of them, and it has been shown that females with a rider have a higher foraging success because of reduced harassment by mate seeking males but suffer a greater risk of predation.
These are the Water Treaders or Pondweed Bugs, there are about 30 species in the world but we have only 1 in the UK; Mesovelia furcata which is the only green bug associated with the surface film in the UK. Adult females lay 100 or more eggs inside the stems of water plants, as winter progress these plants die and sink to the bottom of the pond, here the eggs remain safe from the bitter cold of winter until next Spring. When the eggs hatch the young nymphs immediately rise to the surface and break through the surface tension to begin their life on the water surface, they grow quickly and there are often two generations in the UK. They live in a variety of habitats including some on the coast and some in moist leaf-litter well away from the water's edge; there are also some species which specialise on living in caves.
Sphagnum Bugs or Velvet Water Bugs are small bugs animals 1.3 to 3.7 mm long, which as their name implies like to live in ponds which contain Sphagnum Moss. There are about 150 species world wide, most of which live in tropical environments particularly SouthEast Asia. We only have two species in the UK Hebrus ruficeps and its more southerly relative Hebrus pusillus. They live in a number of different habitats in the tropics but always tend to be associated with the water's edge, though some live in quiet pools others live in the splash zone of water falls and on rocks in the middle of fast flowing streams and rivers. Little is known about them as they tend to live secretive lives, but we do know that they lay their eggs in a gelatinous mass at the water's edge and that Hebrus ruficeps can survive winter frozen in the ice at the water suface.
Water Measurers or Marsh Treaders are much more common than the two groups I have just mentioned they range from 2.7 to 22 mm long and there are 112 species known in the world, of which two occur in the UK. Many of you will have seen the small thin body of Hydrometra gracilenta when you have gone pond dipping, and a few of you with access to the Norfolk Broads, and the New Forest may have seen our other species Hydrometra stagnorum as well. Our two species live on relatively still water and are often seen close in to the vegetation around the edges of the pond. Several tropical species however have gone one step further and live a semi-terrestrial existence. Like the Gerrids they propel themselves across the water mostly with their middle pair of legs. However they can easily be distinguished from the Gerrids, (Water Striders and Pond Skaters) because when they are moving across the water they move their legs alternately like you do when you are swimming the crawl. Gerrids move their legs synchronously, which is like when you are swimming in breast stroke. Like their cousins the Pond Skaters they are predators feeding on smaller invertebrates on or just below the water surface. They lay their eggs attached to stones or vegetation above the water surface, they have one generation a year.
This family contains the Water Crickets and Riffle Bugs, these are smaller versions of the Gerrids (ranging from 1 to 10 mm in length), there are about 500 species in the world 5 of which occur in the UK. Most species live on still fresh water, though they can be found in an unusual variety of habitats such as the water held in the centre of Bromeliads or the foam masses associated with mountain streams in Venezuela. Most species move their legs alternately but some, like Velia caprai, (the UK Water Cricket) move them synchronously like the Gerrids. They can also move very quietly when they wish to sneak up on something. Instead of rowing themselves with their middle legs, which sets up all sorts of ripples telling everybody else on the pond what you are doing (a bit like walking in squeaky shoes) they can exude a fluid which reduces the surface tension in the water behind them, causing the surface to expand and thus pushing them forward. It looks like they are gliding across the water without any effort at all, however in reality it costs them a lot of effort to make the fluid so they don't do this all the time. The Genus Microvelia contains some very small bugs, only 1.4 to 2.0 mm long. Microvelia reticulata overwinters as an adult so if you look carefully you can find it in the Spring. It lays its eggs in the moss and feeds on even smaller invertebrates such as mosquito eggs. They may have 3 generations in Southern England but only 2 further North. Like all animals that live on the water surface they need clean water to survive so you will not find many of these on streams or ponds that are polluted or oily.
There are three other families in the Gerromorpha all of which are less well known and do not occur in the UK, between them they contain 15 species. The Hermatobatidae range in size from 2.7 to 4.0 mm in length and all ten species are marine. They have oval shaped bodies and tend to be found in the Indian and Pacific oceans. The Macrovellidae range in size from 2.5 to 5.6 mm and live in the leaf litter nearby to streams or in springs with lots of emergent vegetation. There are only 3 species known worldwide and they tend live secretive lives among the vegetation at the water's edge. Finally the Paraphrynoveliidae range in size 1.7 to 2.4 mm, they live in the zone between the water and the land. There are only 2 species known, both of which come from Southern Africa.