The Plecoptera are an ancient an beautiful order of insects with the earliest known fossils dating from the Permian, they are popularly known as Stoneflies (because they are often seen resting on stones), they are soft bodied small to medium sized hemimetabolous insects.
There are about 2 000 named species all with aquatic larva, most of whom live only in cool waters, generally running streams or lakes with a upper temperature limit of 25 degrees C. The adults have little difference in texture between the hind and fore wings, and are rather poor fliers. They also have weak mouth parts, small coxa and generally two long slender cerci. They are seldom noticed by anyone other than entomologists, however they are rather endearing little creatures once you get to know them.
Though the adults are visually similar to some of the Neuroptera can be easily distinguished from them because the hind-wings possessed of a large anal lobe making them far larger than the fore-wings.
The eggs are laid, either loose in the water, in a jelly on the water, or are placed in cracks near the waters edge, ultimately the eggs sink and disperse.
The eggs hatch by splitting down the middle, and the early instars all have generalised mouth parts and feed on organic detritus, as they grow some species become herbivorous and some carnivorous, all go through numerous moults often more than a dozen before they reach adulthood. In some genera the larvae have gills and in others they don't but they all have a high positive requirement for dissolved oxygen in their environment and this explains their restriction to cold waters, and the fact that they are generally abundant as adults early in the year, the larvae hatching in early spring to avoid the warmer water temperatures of summer. Most species are univoltine (having one generation per year) though some are semivoltine (having one generation every two years)
Emergence of the adults usually occurs at the stream or lake edge, often in the space between the ice layer covering the water and the water itself, (this space is caused by the water level dropping after the surface has frozen); in habitats that support large numbers of species their emergence times tend to be staggered/serial and short, reducing the inter-order competion. Though some species that live in special habitats such as springs which have constant conditions throughout the year have emergence times that reflect this and occuppy most of the year. Most species appear to emerge at night and generally the males start to emerge before the females though there is considerable overlap. It is thought that both temperature and photoperiod are important in determining emergence times.
After emerging the adults fly, or in the case of the flightless species crawl up into the trees. Flightlessness occurs in a number of species, sometimes only in the males sometimes in both species, occasionally it is accompanied by winglessness.
In some species the adults feed and in others they don't. The non-feeding species tend to emerge with fully formed eggs, to mate quickly and to lay their eggs and die after only a few days. Males tend to live only a few days in most species anyway, however the females in feeding species may live for 4 to 5 weeks.
Males drum (beat their abdomen on the substrate i.e. what ever they are standing on) to attract females, and females drum in reply, the frequencies are species specific and are transmitted through the substrate not through the air. The normal sequence is 1) male drums, 2) female drums back, 3) male moves a little way towards female and drums again, 4) female drums back again, 5) male moves closer to female and drums somemore, 6) female drums back again, eventually after an indefinate amount of comunication the pair meet and mate. Females only mate once and mated females do not reply to males.
Though two sexes and mating is the normal situation there are some species from lake Biakal for which males have never been found, in this case reproduction is parthenogenic (where ova develop and mature into female nymphs without fertilization).