I've got my hooks in you: damselflies and the wheel of life

Instead of one of my usual forays into the morphology and taxonomy of some of our more obscure invertebrate friends, today I've decided to look at the reproduction of a species in one of the more popular and familiar groups, the damselflies (Odonata: Zygoptera), namely the Blue-tailed Damselfly Ischnura elegans, often known as the Common Bluetail outside the UK.

A typical I. elegans showing the black abdomen with blue 'tail-light'

This is one of the commonest damselfly species in Britain and can be found throughout the mainland. It tends to colonise new ponds quickly and is tolerant of pollution and brackish conditions to some extent. Given how some of our freshwater habitats are treated, this may be one reason why it is doing well. However, the identification, distribution and ecology of this species are well documented in the references given below as well as online. Instead, I would like to focus on just one aspect of this species' biology, namely reproduction, and in particular the behaviour and reproductive structures of the adults.

In both males and females, the primary genitalia are located beneath the 8th abdominal segment (near the end of the abdomen). However, males are unique among insect groups in also having secondary genitalia - these are found under the 2nd abdominal segment and consist of a pair of lateral hooks (called 'hamules') plus, usually hidden away in an opening, a flexible penis - more of that later. The presence of these secondary genitalia is the reason why the Odonata are the only insect group to adopt the characteristic 'wheel' position during copulation.

A pair of I. elegans in the wheel position - the male is on top.

Before mating, the male passes a spermatophore (sperm-packet) from his primary to his secondary genitalia. Once this is done, and a suitable female has been located, he grasps her on the pronotum (the top of the thorax just behind the head) using claspers at the end of his abdomen. These claspers have tiny teeth to ensure a tight grip and are shaped to fit onto contours, grooves and notches on the female's head and pronotum. This is not always a gentle process as the hooked appendages have been known to leave females with scarred eyes (Brooks 2004). Once the female has been grasped, the male tries to induce copulation by swinging his abdomen (with the female) forwards to bring her genitalia in contact with his secondary set where the spermatophore is now located. Once this occurs, the wheel position is formed.

This is the starting point for what can be a long copulation - an average of 324 minutes according to a study of I. elegans in southern France (Miller 1987) which is much longer than for most other Odonata. It is unclear exactly why this is the case, although it may be a form of mate-guarding, preventing other males from mating with the same female (Corbet & Brooks 2008).This is important because there is another form of competition between males - sperm competition. This occurs when, during much of the early part of copulation, the penis (which is flexible and tipped with hooked appendages) is used to remove sperm from any male that previously mated with the same female. Only then does the male introduce his own spermatophore. I have seen mating wheels on may occasions in a range of species, however I have not often seen the structures involved in any detail.

The same pair immediately post-copulation - note the male's white secondary genitalia near the base of his abdomen.

The secondary genitalia in close-up. Although rather blurred at this magnification, the hooked appendages are visible.

In the above photo, little detail can be seen, although it is still a rarely captured view. However, for some amazing hi-res detail in a Scanning Electron Microscope image, have a look at the picture in the Science Photo Library here. This is also of I. elegans and clearly shows the hooks used in sperm competition. It also makes me jealous of their microscope.

And so, as far as copulation goes, that's about it... except of course there is a lot more to damselfly reproduction than simply the wheel position. Males have to recognise females even though some have a male (andromorph) colouration which may be a male-mimicing strategy to reduce interference from other males during egg-laying (oviposition), although it risks failure to reproduce as recognition is visual (and not fully understood) and males may not find such females, and may also lead to greater predation risk due to the bright colour. Afterwards there is of course oviposition itself with eggs being laid in a slit cut into aquatic vegetation - unlike most species, this is done by lone females late in the day once male activity has ceased (Brooks 2004). Then, once the eggs hatch, there is the development of the predatory nymphs prior to emergence as a new generation of adults. The flight season ranges from May (sometimes April) to September, peaking between June and August in the UK.

Lastly, these photos were all taken at Highbridge Farm in Hampshire, site of our community farm project, and the location of a new but rapidly developing farm pond - just the sort of habitat favoured by I. elegans!


References

Brooks, S. (2004). Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland. (revised ed.). BWP, Gillingham.
Corbet, P. & Brooks, S. (2008). Dragonflies. Collins (New Naturalist), London.
Dijkstra, K.-D. B. (2006). Field Guide to the Dragonflies of Britain and Europe. BWP, Gillingham.
Miller, P.L. (1987). An examination of the prolonged copulations of Ischnura elegans (Vander Londen) (Zygoptera: Coenagrionidae). Odonatologica 16: 37-56.