High flyers: Silver Y moths

Yesterday I came across some piece of fascinating research on the silver Y (Autographa gamma), a brownish-grey moth so called for its Y shaped mark on each forewing. This moth is relatively large and is often active by day, with a liking for lavender and other flowers, moving its wings non-stop while feeding.

This is another great summer migrant into the UK and other parts of northern europe, where their caterpillars are often an agricultural pest. Jason Chapman and collaborators used an entomological Vertical Looking Radar (VLR) to detect individual flying insects. This radar is very sensitive and able to assess the high at which each insect is flying, its body orientation and flight speed. They analyzed radar data for nights in August 2000 and 2003, peak season for silver Y returning to their Mediterranean wintering grounds. Using trap data they established that the dominant flying moth species during those nights were silver Ys. Their data shows that this moth selects those nights with favorable wind directions for their migration (SSW) and that they adjust the hight they are flying to where the wind is fastest, usually over 100 m high.

Figure 1 from Chapman et at. 2008. Circular Distributions of Directional Data Obtained during Return Migrations of Autographa gamma Mean directions from each event are plotted (small circles at periphery). The bearing of the solid black arrow indicates the mean direction of the dataset, and the length of the arrow is proportional to the clustering of the dataset about the mean. (A)The mean tracks of high-flying migrant A. gamma during the 42 mass-migration events detected by vertical-looking radar (mean direction = 202). (B) The wind direction at 300 m at both radar sites during the migration periods. (C) The mean flight headings of migrant A.gamma during the 37 events with significant common orientation (mean heading = 205).

More astonishingly, their data shows that these moths compensate for their flight direction when the wind doesn't blow exactly in their preferred heading. This last discovery indicates that silver Ys use a compass. Given that the moths fly during nights when the moon is not visible and in overcast conditions, and their visual acuity, they could exclude a moon compass and a star compass. They conclude that the moths must have a magnetic compass, maybe adjusted by the sun at sunset. This fantastic technology illustrates how complex and plastic insect behaviour is, and how relatively slow flyers can get to their winter grounds in a few nights of flight.

More informationCHAPMAN, J., REYNOLDS, D., MOURITSEN, H., HILL, J., RILEY, J., SIVELL, D., SMITH, A., & WOIWOD, I. (2008). Wind Selection and Drift Compensation Optimize Migratory Pathways in a High-Flying Moth Current Biology, 18 (7), 514-518 DOI: 10.1016/j.cub.2008.02.080

Warming up painted lady

Cloudy, with intermittent showers today. A cold Painted Lady sat 'shivering' on a bush in the garden and allowed me a close approach. The shivering appearance was due to the butterfly revving its flight muscles, what I had never seen before. I have to apologize - to the butterfly - that I had to use flash, as it was dark and I wanted to emphasize the effect of its wings movement. A few moments later, the butterfly flew away.

A view of the Painted Lady's wing underside.

On this frontal shot the first modified pair of legs are visible just under the butterfly's eyes. 

Emergence of Winged ants

For a couple of weeks now, sunny spells after showers bring out a nervous anticipation in ant colonies. The workers come out en masse and run frantically around the colony entrance holes, often in cracks on street pavements. I have stopped a couple of times to watch this but so far no sign of winged queens and males. Yesterday though, we got the first glimpse of emergence of winged ants (Black Garden Ant, Lasius niger). Emergence happens during July and August and sometimes is very synchronic around the country. Workers seem to keep them in, while they stand high on their front legs, as if sniffing the air with their antennae, appearing to check that everything will be fine for their reproductive brothers and sisters. They have to sort out predators - sparrows and many other insect eating birds love them - during their mating flight. Queens will mate and then find a suitable hole where to dig their first colony. They might live for over a decade, sheltered in their nest taken care of by their daughters. Males will die shortly after mating.

Home-grown Painted Ladies

All of a sudden, Buddleia blooms across the city today were decorated with fresh-looking bright coloured Painted Ladies (Vanessa cardui). The contrast with the faded, worn first individual I photographed this year (see this previous post and also an extreme example here) was stark. Most of today's butterflies were likely to have been born in the U.K., the offspring of the migrating individuals. It is unclear how many generations of Painted Lady happen every year, as there is a lot of overlapping. According research carried out by the Spanish researcher Constanti Stefanescu, the whole cycle outdoors takes 6 to 8 weeks depending of the temperature, and he estimated 3 to 4 generations for NW Spain. The number of generations must be more limited here, with the cooler climate, and is likely that these fresh-looking butterflies will become the returning generation and they are not interested in breeding anymore, but in fattening up for their lengthly return to North Africa

Feeding against the sky

A fresh Painted Lady sunbathing on a wall

More information
Stefanescu, C. (1997) Migration patterns and feeding resources of the Painted Lady butterfly, Cynthia cardui (L.) (Lepidoptera, Nymphalidae) in the northeast of the Iberian peninsula. Miscel.lania Zoologica, 20: 31-48.

What are slugs up to on your kitchen floor?

Late last night I went to the kitchen and found a couple of large yellow slugs entwined on the kitchen floor. I dashed back to the front of the house to grab the camera but in the few seconds that took me do do it - it is a well rehearsed sprint of mine - they had started going their separate ways. The sudden flash of light when switching on the kitchen light must have shocked them and they went shy. The only indication they had been together was a circle of slime on the tiles. Should I carry my camera with me as I walk around the house? I sometimes think so. I knew nothing about the yellow slug mating habits. The yellow slug Limax flavus is not as good-looking as its relative the Leopard Slug, Limax maximus of fame due to its acrobatic sex featured in David Attenborough's Life in the Undergrowth TV series (see also the fantastic photos in Snail's Tails). I found very little, well, yes, as snails they are hermaphroditic and exhange sperm, although self-fertilisation is not unheard of. I also found a clip of the start of courtship, which happens on the ground. So I am afraid I will have to wait and see if I can catch them again. I cannot believe I am going to look forward to see slugs at home.