Despite being a working entomologist, I have to confess that there are some insect groups with which I am not entirely familiar. The stoneflies, Plecoptera, are one of those groups. I work in arid northern Australia, but stoneflies are associated with cool waters. The highest diversity live in temperate regions of the world; those whose ranges extend into lower latitudes are found higher in the mountains, away from the heat.
Stoneflies live in their favoured waterways as nymphs, emerging when they develop to adulthood (at least one species, Capnia lacustra of Lake Tahoe, appears to also be aquatic as an adult). The adults are large, long-bodied insects that are often better runners than they are fliers. Nymphs are primarily detritivores, but many species are carnivorous to a greater or lesser extent. Adults of some species do not feed; others feed on such things as encrusting algae or lichen or rotten wood. Depending on species, adult stoneflies may have full-sized wings, reduced wings or no wings at all; in some species, both flying and flightless morphs may be present. Two European species, Perla bipunctata and Perlodes microcephala, are solely brachypterous in Britain but may be either brachypterous or macropterous elsewhere in their range (Hynes 1976). Winged females of many species lay eggs while in flight, either dropping them into water or gliding to the water surface and letting the eggs be washed off from the end of the abdomen. Other species attach their eggs to stones underwater or insert them into crevices or rotting wood.
Most recent authors have supported a division of the stoneflies between two lineages, the Antarctoperlaria and Arctoperlaria, that are both morphologically and geographically distinct (Zwick 2000). The Antarctoperlaria are found in South America, Australia and New Zealand. The Arctoperlaria, in contrast, are primarily found in the Northern Hemisphere (except for members of two families, the Perlidae and Notonemouridae). Many species of the Arctoperlaria signal to potential mates by drumming the abdomen on a substrate, a behaviour unknown in the Antarctoperlaria.
Stoneflies have often been regarded as one of the most primitive groups of winged insects, and their position remains contentious. The two main theories are that they are the sister group to all other neopteran insects (insects that are capable of folding the wings back flat over the body), or that they belong to the group known as Polyneoptera that also includes grasshoppers and cockroaches. Which of these is correct has been regarded as potentially significant in understanding how flight evolved in insects as a whole. As discussed in an earlier post, it has been suggested that insect wings are homologous with articulated gills in aquatic nymphs. As well as Plecoptera, the two living non-neopteran insect orders Odonata (dragonflies) and Ephemeroptera (mayflies) are aquatic as nymphs, and if Plecoptera are basal to other neopterans then it suggests that this life history may be ancestral for winged insects as a whole. However, differences in nymphal morphology between the three groups may indicate that the aquatic lifestyle has been independently acquired in all three from terrestrial ancestors, which would also be more likely if stoneflies are derived polyneopterans. Molecular studies have supported a polyneopteran relationship for stoneflies, but not with rock-solid support (e.g. Terry & Whiting 2005); morphological studies are equivocal and do not clearly point either way (Zwick 2009). The fossil record is also unclear: while a number of early insect groups have been connected to stoneflies, whether they are true stem-Plecoptera or closer to other polyneopteran lineages is debatable (Béthoux et al. 2011). It is also worth pointing out that while similarities between stonefly and mayfly gills have been cited in relation to their supposed homology with wings, different families of stoneflies have different gill types, and we still do not know whether and what kind of gills were ancestral for Plecoptera. Also, in those stoneflies with plate-like gills, the gills are not articulated like wings and incapable of independent movement (Zwick 2009).
Béthoux, O., Y. Cui, B. Kondratieff, B. Stark & D. Ren. 2011. At last, a Pennsylvanian stem-stonefly (Plecoptera) discovered. BMC Evolutionary Biology 11: 248.
Hynes, H. B. N. 1976. Biology of Plecoptera. Annual Review of Entomology 21: 135-153.
Terry, M.D., & M. F. Whiting. 2005. Mantophasmatodea and phylogeny of the lower neopterous insects. Cladistics 21: 240–257.
Zwick, P. 2000. Phylogenetic system and zoogeography of the Plecoptera. Annual Review of Entomology 45: 709-746.
Zwick, P. 2009. The Plecoptera–who are they? The problematic placement of stoneflies in the phylogenetic system of insects. Aquatic Insects 31 (suppl. 1): 181-194.