Saturday, 22 August 2009

The Hawkers

For a week I have been watching hawker dragonflies hunting over the garden well into the evening. They fly high (3-5 m) and never seem to stop, so I could not identify them. My luck changed today, when early in the morning I came across a male Migrant Hawker (Aeshna mixta) sunbathing on a fence outside. This is the only species of hawker I have identified in my garden and I usually see it in August and September. It is quite approachable and getting close-ups is relatively easy.
The male was a little later joined by another individual (a duller male I think) and both sunbathed quite close to each other high up in a buddleia for a while.
My usual basic photographic equipment was joined by a chair, so that I could take the pictures. At 11:30 both dragonflies started hunting and they didn't stop again.
This species of dragonfly is very sociable and regularly hunts in groups. Is also, as suggested by its name, a regular migrant species. Before the 1950' it was quite a rare species in the south of the UK but steadily increased its range and is now found also in S. Scotland and Ireland.

Friday, 21 August 2009

A terrifying ladybird tale: Dinocampus coccinellae parasitism

It seems like a good year for 7 spot ladybirds. There are many more these days than Harlequins around here. Some ladybirds I see are parasitized. I dug on the web today after taking a few photos of one and noticing the ladybird was actually alive, moving its head and palps, but unable to walk. The indication that it was parasitized was the presence of a silk cocoon underneath it to which it seemed to be tethered. The parasite is a tiny braconid wasp, Dinocampus coccinellae apparently a parthenogenetic species. Shortly after emergence, the wasp lays eggs inside larvae, pupae and adult ladybirds. The larvae hatching from this egg starts by eating the reproductive organs and eggs inside the ladybird, causing little external damage, then it develops a trophic organ that absorbs nutrients from the ladybird. The cycle becomes even more gruesome from now on. When the larvae is ready to pupate it severs the ladybird's leg nerves, by which it becomes paralyzed partially paralises the ladybird through some chemicals. The larvae then emerges from the ladybird through a hole she bores and spins a cocoon between the ladybird's body and the substrate (leaf, twig, etc). The ladybird is alive but paralysed and the parasite pupae enjoys the protection of the ladybird aposematic colors until its emergence a week or so later through the pointy end of the cocoon. Although some ladybirds recover from the parasitism, most presumably die, and it is not known if the survivors will be able to feed and reproduce after this ordeal!
More info and photos here. To be continued here.

Thursday, 20 August 2009

Scary peacock butterflies

Butterflies have high predation rates by birds. I am sure most of you have come across a butterfly with damaged wings suggestive of a beak 'bite' mark. Long-lived butterfly species often rely on camouflage (crypsis) to avoid being attacked in the first place. The Comma, the Small Tortoiseshell and the Peacock are some of these, mimicking shriveled leaves. They remain immobile if resting, with their wings closed. The Peacock (Inachis io) has a second defense mechanism. If discovered, they suddenly flash their wings open, exposing four large eyespots they also may flick their wings repeatedly and at the same time they make a hissing noise and a series of inaudible clicks by rubbing two wing veins together, during this display, they continually adjust the tilt of their bodies to face the potential attacker.
This Peacock was feeding upside down on a Buddleia showing its eyespots
 This intimidating display was described over a century ago and its effect on predators noted, but experimental support for its effects on the survival of the butterflies themselves was lacking until very recently. Adrian Vallin and collaborators tested the effect of eyespots, stridulation and both combined by modifying captive-reared peacock butterflies experimentally. They removed the eyespots by painting over them with a black marker pen and removed the stridulation ability of the butterflies by cutting out the veins responsible for making the hissing noise. They also tested the effect of the combined factors, that is removing both the eyespots and the noise-making wing veins. Unmodified controls, and controls in which a similar area of wing was painted black without touching the eyespots or cut without affecting the ability to make noise were also tested (bottom row below).

The researchers recorded the effect these various treatments had on survival of peacocks upon exposure to a potential predator, blue tits (Cyanistes caeruleus), a small insectivorous bird. The results show conclusively that butterflies without eyespots are more likely to be predated by the blue tits. All except one butterfly with intact eyespots survived by scaring the blue tit away, showing how effective the eyespots are in intimidating the bird. The effects of the sound or the combined effect were not significant. The predated butterflies were readily eaten by the birds, indicating that the Peacock is not distasteful and supporting the view that, in the words of the researchers, 'a harmless prey can increase its fitness by survival through the adoption of intimidation by bluffing'.
Vallin, A., S. Jakobsson, J. Lind & C. Wiklund (2005) Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits. Proc. R. Soc. B. 272:1203-1207.