The insect order Odonata contains two types of insect, the dragonflies and the damselflies. All living odonates can be placed in one or the other of these groups except for just two species. The two species of the genus Epiophlebia, E. superstes in Japan and E. laidlawi in the Himalayas, are inhabitants of fast-flowing streams in montane rainforests. In overall appearance, they resemble a dragonfly (and are more closely related to dragonflies than damselflies) but they retain a number of primitive features shared with damselflies. Their wings are more like a damselfly's, and like a damselfly they are able to raise their wings directly above their body so that the wings are together at rest. In dragonflies, the wings have a much smaller arc of movement, so when the insect is resting the wings are still held extending outwards. Like all other living odonates, Epiophlebia nymphs are aquatic. Like dragonflies, they have their gills internalised within the rectum* rather than external as in damselflies. However, unlike dragonflies, Epiophlebia nymphs do not jet-propel themselves through the water by firing water out of their rectum, but move about by the slower but more dignified method of crawling. Growth of Epiophlebia nymphs is slow and they can take up to eight years to reach maturity (Tabaru, 1984).
*Yes, that's right, dragonfly nymphs breathe through their arses.
Despite lacking a fossil record of their own (probably due to their montane habitat), Epiophlebia seem to have diverged from the dragonfly lineage very early on, at least as early as the Triassic, as indicated by the presence in that time of Odonata more closely related to dragonflies than Epiophlebia (Grimaldi & Engel, 2005). The Japanese name for Epiophlebia superstes refers to their presumed age: they are mukashitombo, the "dragonfly from long ago".
REFERENCES
Grimldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press.
Tabaru, N. 1984. Larval development of Epiophlebia superstes in Kyushu. Tombo 27: 27-31.
I experience a certain sense of awe at the thought of a lineage, esp. an insect lineage, dating to the Triassic being represented by just two widely separated species. Presumably we're seeing the last stragglers of an ongoing extinction, or a very narrow bottleneck.
ReplyDeleteOf course, the "especially an insect" doesn't really apply here. The development time of seven or eight years means that these animals have a generation time noticeably longer than most vertebrates.
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