Darting Small skippers

A grassy area in the little wildlife garden near where I live is managed like a meadow, cut once a year. The grass is long and lush now, peppered with a range of wildfowers. One of the insects that benefits from this arrangement is the Small Skipper, Thymelicus sylvestris. The males of this golden brown, little butterfly dart around the meadow, stopping to feed or sunny themselves occassionally. The larval food plant of this species is Yorkshire Fog, a common rough grass. The adults emerge in mid June and fly in a single generation until August. They like to feed on clovers, bird's foot trefoil, restarrows, knapweeds, thistles, brambles and hawkbits. This is another butterfly species thought to have benefited from recent climate warming in the U.K. It now inhabits most of England and Wales, with its range having moved north about 100 km in the last 25 years.

A male, the same individual as above, resting on Bird's Foot trefoil. Male skippers can be distinguished from females by their "sex-brand", a dark like in the middle of their forewings.

Brown-Lipped Snails

The Brown-lipped or Grove snail, Cepaea nemoralis has received a lot of attention by evolutionary biologists for more than a century, due to their strikingly variable shell colour - what is called colour polymorphism. In the decades of the middle of the last century it was a very popular research organism. The shiny shell can be yellow, pink or brown. Over each of these background colours there can be no bands, one band or five bands, and the bands can also be fused and be of variable width. The snail above, which we found yesterday feeding on the fallen leaves on the garden path, is a yellow/one banded one. This polymorphism happens within the same population, but what puzzled biologists was the occurrence of sharp changes in the frequency of colour forms from one population to the next, and these differences seem to persist with time. This phenomenon was called "area effects". Many explanations have been proposed through the years to explain how the polymorphism is maintained and how area effects come to be, from differential predation (especially by song thrushes), adaptation to microhabitat, or other forms of selection to chance effects due to colonization after the glaciations, genetic linkage, dispersal between populations, etc. Many of these factors are not mutually exclusive and seem to have different importance depending on the population.
 We found the shells below in the beach in Spurn Head a few years ago, all in a small area. They are a bit bleached by the sun, but you can see yellow and pink snails and three types of banding patterns.

The Brown Lipped snail can be found from dunes to roadsides, gardens and closed woodland. It can live up to 8 years old. They prefer to feed on dead vegetation than fresh, and on average, only 9% of its diet is fresh vegetation, although this percentage can increase during dry spells.

References
Cain AJ, & Sheppard PM (1954). Natural Selection in Cepaea. Genetics, 39 (1), 89-116 PMID: 17247470
Davison, A., & Clarke, B. (2000). History or current selection? A molecular analysis of 'area effects' in the land snail Cepaea nemoralis Proceedings of the Royal Society B: Biological Sciences, 267 (1451), 1399-1405 DOI: 10.1098/rspb.2000.1156
Paul J. Mensink & Hugh A. L. Henry (2011). Rain event influence short-term feeding preferences in the snail Cepaea nemoralis Journal of Molluscan Studies. DOI: 10.1093/mollus/eyr011

Amber Snail Puzzle

While removing an old pot containing a lot of grass and a dead Agapanthus, next to a rainwater filled pot, I stumbled upon this little snail. I was quite surprised as initially, I thought it was a pond snail, but closer inspection revealed the tell-tale eyes-on-top-of-tentacles characteristic of land snails and slugs, while aquatic snails have their eyes at the base of their tentacles. After sifting through a Molluscs guide I found out it was a Common Amber Snail, Succinea putris. Although not aquatic, it usually lives near water or in waterlogged habitats, and it is often found on the stems of aquatic plants. It cannot retract its body completely inside the shell, and the lower pair of tentacles is vestigial. I have no idea how this snail got into our garden, but snails, despite being slow and strongly dependent on humidity, are known to disperse widely. In the Origin of the Species, Charles Darwin believed birds were the most common long range dispersal agents of snails and other aquatic animals and plants. In 1893, Harry Wallis Kew reviewed the dispersal of land and water molluscs, and discusses the evidence for external transport on the feathers of birds:

Sir C. Lyell, remarking on the wide range of Succinea putris, a land-shell which inhabits moist places on the borders of pools and streams suggested that water-fowl might have distributed its ova entangled among their feathers and it seems quite likely that ova of certain terrestrial kinds may be occasionally thus carried, either in the feathers or on the feet of birds; indeed, we have a near approach to proof of such transportal, the Rev. Canon Tristram, as we have seen, having once found ova, believed to be those of a Succinea, upon one of the feet of a mallard shot by him, on the wing, in the desert of Sahara. It is doubtful, however, whether Succinea, from the nature of the localities they often or usually inhabit, ought not, for the present purpose, to be classed with fresh-water, rather than with land-shells. Mr. Darwin suggested that the just-hatched young, possibly, might sometimes crawl upon the feet of ground-roosting birds, and thus get transported; and it certainly seems in the highest degree probable that such is the case, but, as far as I know, no observations in support of such a supposition have yet been made.

A tantalizing possibility, also first put by Darwin, is that of internal transport of organisms in the digestive tract of birds. Many bird species feed on snails, and given that no gastric juices occur in bird's crop, they can potentially survive for a while and maybe be discharged later elsewhere by the bird, or regurgitated by raptor if the bird falls prey to it. Kew stated:

 Twenty specimens of a Succinea, peculiarly packed together, and four of Pupa viuscornvi were once found by Mr. W. H. Dikes in the crop of a bearded titmouse (Parus biarmicus); all the shells, it is said, were uninjured, but it is not stated that any were observed lo be alive.

In 1968, Biggs reported on the recovery of a living Succinea putris from a pigeon's crop at least 8 h after the bird had died. Indeed, although it seems even more unlikely, some snails can survive passage through the whole digestive tract of birds provided the shell is more or less unbroken. This has recently been shown to happen to a small estuarine snail, Hydrobia ulvae, which can survive passage through the digestive tract of Shelducks, and also in some small Japanese terrestrial snails, Tornatellides boeningi a proportion of which were recovered alive in the feces of two species of terrestrial bird they had been fed to.
I don't think any of these forms of dispersal apply to the particular little snail in the above photo. Maybe it travelled on a pot plant we bought some time back in a garden centre, or, stuck to our clothes or shoes during an outing into some wetlands. Maybe, but just that any of the forms of transport Darwin and Kew discussed actually happen to snails shows you don't need wings to fly high.

References
Biggs, H. E. J. (1968). Succinea putris (L.) in a pigeon's crop Conchologist Newsletter, 24: 36.

Gerhard Cadée (2011). Hydrobia as "Jonah in the whale": shell repair after passing through the digestive tract of shelducks alive. Palaios, 26 (4), 245-249 DOI: 10.2110/palo.2010.p10-095r

Darwin, C.R. (1959) On the Origin of Species. Read the book here.
Kew, H.W. (1893) The dispersal of shells: an inquiry into the means of dispersal possessed by fresh-water and land Mollusca. K. Paul, Trench, Trübner & Co., Ltd. Read the book here.
Shinichiro Wada, Kazuto Kawakami and Satoshi Chiba (2011). Snails can survive passage through a bird's digestive system. Journal of Biogeography : doi:10.1111/j.1365-2699.2011.02559.x

Six-spot burnets: chemical weapons as nuptial gifts

Six-Spot Burnets, Zygaena filipendulae, are spectacular day flying moths. The contrast between their colour and the green meadows where they live makes them very obvious. Their body and forewing background are black, with a metallic green-blue sheen. Their forewings have six crimson-red spots. In addition they are large and heavy, and females like to perch conspicuously atop flower heads. You cannot miss a sitting burnet, but a flying one is even harder to miss: when they fly, a slow, buzzing, heavy flight, their crimson rear wings with a narrow black border become visible. As you could predict, this bright, black-with red spots contrasting colouration is a warning sign. Burnet moths are toxic, when they are injured, they release cyanide, a highly poisonous chemical. Cyanide compounds are found at some level in every life stage from egg to adult. The larvae sequester and store cyanogenic compounds from their food plants - Bird's Foot Trefoil, Lotus corniculatus - and all life stages are able to synthesize these chemicals themselves. However, production of the chemicals is costly, as larvae grow much more slowly when reared on varieties of Bird's Foot Trefoil lacking cyanogenic chemicals, as they have to synthesize them all themselves. There is a sudden loss of cyanogenic compounds from the last larval stage to the adult, and there also appears to be large variation in the amount of toxic chemicals in the adults. This could partly be explained by volatile hydrogen cyanide emissions by larvae - possibly as an antipredator strategy. Also, males convert some of their cyanogenic compounds into a pheromone; upon approaching a female, they release it. Females are able to determine how much pheromone a male is producing, the larger the quantity, the more likely the female is to accept him. But things get even more interesting. Mika Zagrobelny and co-workers, from the University of Copenhagen, collected Six-Spot Burnet larvae, pupa and adult from a local fallow field and made detailed measurements of the levels of cyanogenic compounds in the tissues of the different life stages, and also their toxic emissions. They found that females, as the larvae, emit hydrogen cyanide. Males are attracted to these chemical plumes, which form part of a pheromone cocktail produced by the female.

Average total cyanogenic compounds content in virgin and mated Z. filipendulae adults as well as in discarded males (which females would not mate with). Error bars are standard deviation. (figure modified from Zagrobelny et al. 2007).

The researchers then compared the cyanogenic compounds of virgin males and females, as well as mated males and females they paired up in the laboratory (see figure above). Virgin males and females had roughly similar levels of cyanogenic compounds. In contrast, after mating, females had larger levels, whereas males had lower levels. This indicates that during mating, males transfer some of these chemicals to the female, likely with the sperm. The levels of cyanogenic compounds in rejected males (males in an experimental pair that the females refused to mate with) were lower than average levels in virgin males, which suggests that females will mate preferentially with those males better loaded with chemical weapons. Why would the female benefit from acquiring more cyanide compounds. Possibly because the more she puts into eggs, the better defended they will be from predators, so this nuptial gift might be seen as a form of paternal behaviour. Alternatively, the female might gain through using this nuptial gift to produce more pheromone, attract further males and increase the vigour of her offspring.

A Six-Spot Burnet on Bird's Foot Trefoil

References
Zagrobelny M, Bak S, Olsen CE, & Møller BL (2007). Intimate roles for cyanogenic glucosides in the life cycle of Zygaena filipendulae (Lepidoptera, Zygaenidae). Insect biochemistry and molecular biology, 37 (11), 1189-97 PMID: 17916505

Leaf-cutter bee antics

There has been a lot of leaf-cutter bee activity in the last week in the garden. Females collect nectar and pollen from Campanula, wild rocket (female collecting pollen above), everlasting peas, and birds foot trefoil. They have colourful pollen baskets underneath their abdomen.

 Males patrol the females' favourite nectar and pollen sources. Here a male Megachile willughbiella, easily recognisable by their white-golden mittens and moustache, flies by everlasting pea flowers.

They also keep a close eye on the bee hotel, where, for the first time, I have seen nesting activity, and often bask on top of it or nearby on sunny spells.

Males of Megachile willughbiella also "sleep" in the hotel, or seek refuge during cold or rainy spells. On the photo above, two males settle for the night. The plugged nests around them belong to Osmia rufa, the red mason bee.

When they detect a female feeding, males hover in front of them for a little while before attempting mating. The following description is for Megachile fortis, an American species of leaf-cutter by John Neff and Beryl Simpson, may well apply to the british Megachile species:

 When a female was encountered on a sunflower head, the male hovered in front of the capitulum while the female moved around it, probing the circular array of florets. When the female reached the 1200 position (aligned vertically on the capitulum with her head up) he darted forward and grasped her with his fore and mid legs, rapidly curling his abdomen under the tip of her abdomen, apparently attempting copulation. Both bees then dropped from the flower with the female quickly escaping or being released to fly away and resume foraging. The period from pouncing on the female to release was quite brief, typically less than 3 seconds.

 Today I had the chance to observe a female Megachile willughbiella repelling a male. She was basking on top of the bee hotel and the patrolling male spotted her. He pounced on her but she curled her abdomen up as you can see in the photo above. The male flew away and the female adopted a more relaxed position. Leaf-cutter have very mobile abdomens and tend to tip them upwards. I have no idea why, but I wonder if it has something to do with their pollen collecting. However, males also have this tendency to raise their abdomens when feeding or just when preening themselves.
 I have posted before on these species leaf-cutting behaviour itself, but I had never had the chance to look at a finished nest from close up. Two nests were finished in the last couple of days in the bee hotel. As you can see below, nests come in colours. I wonder if the yellow one is made out of Potentilla petal sections.

Both species of leaf-cutters are nesting in the bee hotel, the smaller I believe is M. centuncularis and the large, M. willughbiella.

Here a female M. centuncularis, recognisable by her all orange pollen brush and marked white striped abdomen, works on the yellow nest.

Reference

John L. Neff and Beryl B. Simpson. 1991. Nest Biology and Mating Behavior of Megachile fortis in Central Texas (Hymenoptera:Megachilidae) Journal of the Kansas Entomological Society, 64: 324-336.