Yesterday I tried unsuccessfully to remove a dead Agave from its pot. The dried, brown rosette of leaves made a great winter refuge for many bugs. I have already talked about the hibernating ladybirds that spent the winter in it but yesterday a clutter of spiders (apparently that is the group name!) and some woodlice started to disperse in all directions once disturbed. I knew there was a sac spider living there, but I hadn't managed to get a shot. It is Clubiona corticalis (above), distinguished by its characteristic abdominal pattern. The second spider, Stemonyphantes lineatus, I had never seen before, and its abdomen is pinkish with fine lines and spots. I wasn't expecting to identify them by just browsing the illustrations at a couple of spider guides. I own The Country Life Guide of Spiders of Britain and Northen Europe by Dick Jones and the Collins Field Guide Spiders by Michael Roberts. I double checked it in one of my favourite spider websites eurospiders, with brilliant macro shots. The third spider is the Amaurobius male living in my mop with a true clutter of Dysdera.
This morning I was getting ready to mop the kitchen when a I noticed a recently dead spider under the mop, a female Dysdera crocata. I thought I had just squished it inadvertently, left it aside for later photos, and filled the bucket. A I lifted the mop again, two spiders fell out of it, an Amaurobius and a female Dysdera. I got a couple of drinking glasses and put them over the spiders and started mopping. Up to then I was only a bit surprised by the attractiveness of this old mop for spiders but little else. I moved to the next room while mopping the floor and... yes, you guessed it! another spider fell out of the mop (number 4!) this time a male D. crocata, looking positively angry, lifting his front legs and fangs open! I grabbed another glass and card to fetch him and proceeded to shake the mop thoroughly. Number 5, a small, this one dead, but in good condition otherwise D. crocata fell out. I couldn't carry on with the mopping. I had three live spiders under glass including two adult male and female Dysderas. I had to stage an encounter between them, a breakfast bowl encounter. It was tricky, as they were warm and easily climbed out of the bowl several times, but I finally managed to put both spiders on the bowl under a glass and they settled. The male was only slightly smaller than the female. I slightly touched the male with the glass and he made contact with the female. As a lighting strike, they lifted their front legs and opened their chelicerae to the maximum and faced each other. I lifted the glass carefully and watched. The spiders stood their ground and wrestled with their fangs opened. After a few moments, the female retreated and the male run away. What were so many spiders doing in the mop? Is this a particularly good hiding place in the outside toilet? is it the mating season? or they are part of the same brood that have become trapped in the toilet. I doubt the later as the spiders are good climbing even glass and the window is always slightly open. The spiders did not seem very keen to mate. I wonder, as there are many woodlice in the outside toilet and it might be that these spiders are not territorial and are just exploiting it as the perfect hunting grounds. This post is a very strange end for a kitchen mopping session!
I mentioned the other day that the Pholcus in my outside toilet was holding a wrapped spider and was looking considerably fatter (above). The following day, I picked from the floor the remains of her prey, which she had dropped. I was shocked to find it was a fearsome Dysdera, the woodlice hunting spider. I tried to unwrap it to get a better shot, but it was difficult. Fortunately, the fangs stuck out. I don't think there is any other British spider with such long fangs and chelicers of red colour. These flimsy spiders make good bets for spider battles!
We've enjoyed a few really warm days this week. Bumblebee queens have come out of their winter refuges and are bumbling about looking for nesting sites and feeding on early flowers. Spring bees have also started to emerge, and one of the most striking, the Tawny Mining Bee, Andrena fulva, has also done so. I came across this fresh female flying about and finally settling on the leaf litter to bask. Females are covered on dense and bright orangey-red hairs, contrasting with their black heads and legs. The one below was sunbathing on the bird table in April last year after collecting some cherry pollen. As other Andrena species, females carry pollen in hair brushes on their back legs.
Males are so very different, slimmer, with a large white moustache and more obvious jaws:
This bee flies in a single generation, from late March to late May. The peak flight season on the Tawny Mining bee, April, coincides with flowering times of fruit trees and they can often be seen feeding on them, and contribute to their pollination. They use a diversity of other flowers, including blackthorn, broom and willow.
The Tawny Mining bee nests in bare patches of ground on grassy areas and therefore is known to nest in gardens and parks. Nesting aggregations, in which many bees nest in the same area, can be quite large and the ground is the peppered with miniature volcanoes of loose soil around the nest entrance which they have dug out while they dig the nest. Despite them nesting near each other, the mining bees are solitary and each female digs her own nest and collects nectar and pollen for her own young. I will keep an eye for their nests, the next few weeks, and see if I can catch them as they come and go from their little volcanoes.
It has been quite warm today, 18.5 oC! A great day to start spring, the garden was buzzing and there was a lot of "firsts" of the year. One of those days it is hard to choose what to blog about. I saw my first butterflies of the year, a Red Admiral Peacock (in flight), a Small Tortoiseshell feeding on willow and a male Brimstone; first queen wasp; first pair of 7 spots mating and my first photo of a ladybird taking off. Also, first Bombus hypnorum, B. hortorum and first female Anthophora plumipes. The following shots are the best of the day.
A fortunate consequence of 7 spot ladybird communal hibernation is that you don't have to look far for a partner when you wake up!
A groggy queen wasp landed on yellow plastic
And a hyperparasitoid flightless wasp, possibly Gelis sp. which looks rather like an ant walks on the blue bin
Philodromus spider sunbathing
The first female Anthophora plumipes leaves a daffodil, which has marked her back with yellow pollen
A male A. plumipes briefly resting on a buddleia trunk. Males look velvety this early in the year. They have been patrolling the garden since the 11th of March
A Pholcus spider on the outside toilet looks much fatter than yesterday, after enjoying her spiderly meal.
A Harlequin and a 7 spot look like they are playing peek-a-boo
Despite being so early in the year, aphids are already covering the shoots of many plants. I saw my first Episyrphus balteatus of the year today. Basking on the warm, springy sun and then taking an interest in a small rose. I watched her while it flew away and back to the rose, chosing a fresh shoot and then adopting an egg-laying posture. I was surprised, but the camera was ready in my hands. The first one of the year and it was already mated! The hoverfly left and I looked at the rose stem.
A bright white egg sat surrounded by fat aphids. What a well chosen place! Hopefully the aphid infestation will soon be under control thanks to the voracious hoverfly larvae.
The breakfast bowl technique is making me much braver handling spiders. The sides are slippery, but spiders can get out if they try, and I have to nudge them in several times with my fingers. Above, a male wolf spider, Pardosa (possibly amentata, from today) and below, a Clubiona sp. I found on the 26th of February in the house.
As soon as there is a sunny, mild day in March, wolf spiders come out of their hiding places and are easy to spot basking on stones or the low vegetation. In my garden, the bottom frame of the conservatory is a favourite spot (above, today). Wolf spiders (Family Lycosidae) are mainly diurnal spiders that do not build a web. It is common to see more than one, sometimes many in the same place and because they tend to move at the same time and in the same direction as they run away from you on low grass or on the ground, they were thought to hunt in "packs" therefore their name. However, they do hunt alone. They are highly visual and have three rows of eyes; the top row is made up of large eyes facing sideways, then two large forward-facing eyes, and the lower one is made of four small eyes in a line. Wolf spiders are robust and quite cryptic, typically brown with lighter and darker markings. They run fast and can even jump, but they are "sit-and-wait" hunters and they do not move much unless disturbed.
Males are in general smaller and darker than females and have dark pedipalps (above). Come May they can be seen courting females by moving in front of them, tiptoeing on their legs and waving his pedipalps to her. Fertilised females spin a pale, lentil-shaped cocoon around their eggs, and carry it around attached to their spinners. They become quite obvious now.
Every now and then, the female repositions the cocoon, checking for humidity inside and replenishing it if necessary. Once the spiderlings are born, the female opens the cocoon and lets them crawl on her abdomen.
The "fuzzy" looking spider carries around the spiderlings until they are able to fend by themselves. Females can spin a second cocoon, and by that time males in the population have already died. There are many British species (37) and they are difficult to determine without capturing them. The ones in my garden belong to the genus Pardosa and they appear most similar to the common "spotty wolf spider", Pardosa amentata.
I was moving some pots outside when I found a female Dysdera, the Woodlice hunting spider: an excellent subject for my breakfast bowl technique. I carefully put the spider in a little plastic bug pot, grab the bowl and camera and go outside. Despite the sun, with no flash the spider colours do not come out very well so I use a flash. I want a shot of the fangs so I gently tickle them with a piece of grass and voila! the spider delivers. I can check later that it has got spines on the upper side of the femur in the back legs (see photo below), which is the character that identifies it as a D. crocata, and separates it from Dysdera erythrina, the other British species. D. crocata is also more often found in gardens, and reaches a more northern distribution than D. erythrina.
I find invasive species fascinating. Either on their own steam or with a little help from us they have expanded their geographical distribution and, with time, they start forming part of the network of ecological links in each environment. Furthermore, - although they might have a negative impact on the economy and/or native species - they provide unique natural experiments that allow us to witness evolution in action and to investigate which evolutionary forces are be involved. One such invasive species is the Harlequin ladybird. This beautiful ladybird has gone from a native Central and Eastern Asian species to be considered a worldwide plague, via its use by us as an agent for the biological control of aphids. Despite the Harlequin having been used as such control in the U.S for decades, its range did not immediately expand and there seemed to be little room for concern. However, in 1988 the Harlequin started to become invasive and went to invade large areas in the U.S, Europe, South America and South Africa. In the U.K. the first sightings occurred in 2004 and it appears that by 2010 the ladybird had expanded into most available habitat.
A surprising fact of biological invasions is that they are successful, despite, stemming from a presumably small number of introduced individuals, what is termed a population bottleneck. Population bottlenecks are known to result in breeding between relatives and lead to strong inbreeding depression, a loss of fitness due to consanguineous matings. Why are then invasions successful? shouldn't they end in extinction? There are two ways out of this paradox. The first one, that there is no paradox at all. Invasive populations might actually not suffer a bottleneck, maybe because many individuals are introduced at once, or because repeated introductions replenish the population genetic diversity before the alien population becomes extinct and restores its fitness and invasibility. A second more interesting possibility stems from the small population surviving long enough to "purge" or remove from the population the alleles responsible for inbreeding depression as inbred individuals carrying two copies from these alleles die out. Purging can take place given certain combilations of population features such as population size, fertility and duration of the bottleneck. This possibility, although backed up by the theory and laboratory experiments with fruit flies, had not been documented to occur in the wild. In a recent paper, Facon and colleagues actually test (1) that bottlenecks actually occurred in the Harlequin ladybird and (2) that purging has happened in the invaded range. Their theoretical model, based on the genetic diversity of native and introduced populations, points to a most likely population bottleneck of 150 individuals lasting 20 generations; features that are compatible with purging to be able to occur, and fitting the large time gap between introduction and invasion. Then, they carried out experiments to compare the fitness (measuring generation time and lifetime number of offspring) of inbred and outbred invididuals from three invasive and three native populations.
The results were astounding (see Figure above): inbred invasive ladybirds have similar fitness than outbred invasive ones and they even match native outbred ladybirds. Native ladybirds, as expected, have strong inbreeding depression. This shows that invasive harlequin populations have got rid of the negative consecuences of mating between relatives, that is, purging has taken place. As in the frontline of an invasion, when population densities are low, and mating is more likely to happen between siblings, the evolution of the ladybirds during the lag period ended giving them their invasive "edge". It would be interesting to see if inbreeding depression accumulates again once harlequin populations become dense and settled in an area.
Facon B, Hufbauer RA, Tayeh A, Loiseau A, Lombaert E, Vitalis R, Guillemaud T, Lundgren JG, & Estoup A (2011). Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis. Current biology : CB, 21 (5), 424-7 PMID: 21333536
Maderspacher, F. (2011). The benefits of bottlenecks Current Biology : CB, 21 (5)
I was out and about this weekend taking advantage of our "mini-spring". When repotting some self-seeded daisies I came across a few millipedes in the soil. I seem to always encounter the same species (maybe the same genus I should say, as there are several similar looking species) of millipede: a long, thin type of various sizes, which, upon encounter rolls itself into a spiral. This is the aptly named millipede Cylindroiulus. The commonest UK species is C. punctatus and it is likely this is my garden's species. Millipedes (Class Diplopoda) like insects and spiders are arthropods, and they are thought been amongst the first animals colonising land, some time in the Silurian Period, over 400 million years ago. There are around 10,000 described species, although there are probably several times that number of species to be described. Just 52 of them are known from the U.K. Millipedes are mainly detritus-feeders, and are often found in the soil, leaf litter, or on dead wood, where they contribute to the recycling of vegetal matter.
Millipedes means "A thousand legs" but millipedes, although having more legs than centipedes, don't have a thousand legs, the record holder is, apparently, 750! Each of the millipedes body section has two pair of legs, except the few rings behind the head. Their antenna are elbowed and clubbed, and they constantly tap the ground while they move. Cilindroiulus has eyes, but many millipedes, especially those permanently living in the soil or in caves, lack eyes. A curious feature of millipedes eyes is that they add ocelli as they moult, so that adults have much large eyes, and likely much better eyesight, than juveniles. The ocelli are added so precisely to each moult than the growth stage can be determined in some species by counting the rows of ocelli.
Millipedes can deter predators, physically, curling up into a spiral which shelters the head and ventral area (above), exposing only the chitinous, hard dorsal rings, in a similar way to an armadillo or pill woodlouse. At the same time they curl up, they display their chemical defence: they secrete cocktails of repellent and/or toxic chemicals through glands present in each body ring when attacked. In large centipedes the smell may be quite obvious, in small ones, you have to sniff them to detect the smell. Some small predators might be killed by these chemicals if confined with the millipede in a small space, or deterred by the smell. Despite this defence, millipedes fall prey to birds who like to explore the leaf litter, such as blackbirds.
During winter, Cylindroiulus moves down into the soil, and sometimes several individuals roll up together during winter or hide under bark. That is what the usually wood dwelling millipede was doing under the pot soil.
Stephen P. Hopkin, Helen J. Read. 1992. The biology of millipedes. Oxford University Press, 1992 233 pp.
Gordon Blower. 1985. Millipedes: keys and notes for the identification of the species. Brill Archive. 242 pages.
You have probably come across ladybirds clustered under leaves or bark during winter. To spend the winter, seven spot ladybirds - otherwise solitary creatures - they seem to actively seek each other. I took the photo above a few minutes ago in my garden. I counted 16 ladybirds - most were 7-spots, with two Harlequins - on the shady side of an agave killed by this winters' harsh frosts. Before I go on to explain this communal overwintering behaviour I have to explain why ladybirds are so colourful. Ladybirds are aposematic, a term describing an antipredator adaptation by which organisms have evolved bright, contrasting colours (think on the yellow and black stripes in cinnabar moth caterpillar or wasps) to warn predators of dangerous behaviour (stinging) or distastefulness. Ladybirds belong to this later group. Their beautiful glossy red and yellow elithra with black spots is the first line in a complex defence system, a warning signal to predators, probably birds, of their foul taste. Their bodies contain a bitter tasting alkaloid. If the predator ignores the warning signal and attacks the ladybird - or if you handle them a bit roughly - they release a yellow liquid from their leg joints, with high concentrations of the alkaloid, coccinelline, in what is called "reflex bleeding".
Given that ladybirds can secrete up to almost a quarter of their body weight during the reflex bleeding, it is an energetically demanding defence mechanism. During the winter, when ladybirds do not feed and need to save precious resources, they do not reflex bleed, although they still taste bitter. It is because of this that they cluster together: as other distasteful prey that lack a mechanism to let know of their foul taste to a potential predator, clustering allows individuals with warning coloration in a group - even if they are unrelated - to benefit from just one of them being injured or killed by a predator, as the predator is unlikely to attack further ones in the cluster.
Reference Holloway, G., Jong, P., Brakefield, P. & Vos, H. (1991). Chemical defence in ladybird beetles (Coccinellidae). I. Distribution of coccinelline and individual variation in defence in 7-spot ladybirds (Coccinella septempunctata) Chemoecology, 2 (1), 7-14 DOI: 10.1007/BF01240660
Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males. Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.
(from Verrell, 1985)
References Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticusJournal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127