|Small winter stoneflies|
|Eocapnia nivalis on the snow in Nagano (Japan)|
17-20, see text
The Capniidae, the small winter stoneflies, are a family of insects in the stonefly order (Plecoptera). It constitutes one of the largest stonefly families, containing some 300 species distributed throughout the holarctic. Their closest relatives are the rolled-winged stoneflies (Leuctridae).
Many species are endemic to small ranges, perhaps due to the family's tendency to evolve tolerance for cold (isolating populations in mountain valleys) and winglessness (inhibiting dispersal). Indeed, some wingless Capniidae – e.g. the Lake Tahoe benthic stonefly ("Capnia" lacustra, Capnia is not monophyletic and this species is suspected to belong elsewhere) or Baikaloperla spp. – spend their entire lifecycles under water and do not disperse from their native lakes at all.
Description and ecology
Adult Capniidae, as their common name implies, are typically small Plecoptera; while most are less than 1 cm long with some measuring just 4 mm as adults, a few are as large as 25 mm (1 in) at adulthood. The adults emerge from the water in winter and are often found walking around on the snow. Characteristic are the wings with at most one cubital crossvein, and the paraprocts (anal lobes), the inner lobes of which form a tube closed on the underside by the outer lobes.
Nymphs of small winter stoneflies typically have very elongated and slender bodies, similar to those of Leuctridae. However, the groove along the abdomen, from segment 1 to 9, is generally very pronounced.
The nymphs dwell in the hyporheic zone, the interface between stream water and groundwater. Only immediately before moulting into the adult form will the nymphs move out of the substrate and appear on the stream bed. Thus, although they may be plentiful in clean rivers and streams, they are seldom encountered in standard samples of benthos.
Systematics and taxonomy
The family Capniidae is often considered to be monotypic as to subfamily, with the Capniinae being inserted at that rank. However, this is rather pointless; in any case, the phylogeny, systematics, and taxonomy of the small winter stoneflies are highly confused. There appear to be two very basal genera and presumably two larger clades which conceivably could be considered subfamilies, but the phylogeny of the Capniidae is by no means robustly resolved, with about one-third of the named genera of uncertain position. Hence, any subdivision beyond the generic level is premature.
Furthermore, the type genus Capnia is probably not monophyletic and in serious need of splitting. The type species Capnia nigra and its closest relatives would retain their genus name; they are closely related to Capnura. The North American species, at least for the largest part, are closer to Mesocapnia; other "Capnia" seem closest to Bolshecapnia. Finally, a fourth group is of rather uncertain position in the main assemblage of Capniidae.
A rough layout of the suspected phylogeny can be given as:
Suspected clade/subfamily 1
Suspected clade 2/subfamily Capniinae sensu stricto
Incertae sedis as to clade 1 or 2
- Nelson (1996a,b)
- Nelson (1996b)
- Holst (2000)