The blue lace web spider

I have done a lot of bee watching this week. Several solitary bees species have emerged and males are patrolling the cherry tree, which is now blossoming. A male Red Mason bee landed to bask on the ivy that covers an east facing wall in the garden. Its legs got tangled on the silk threads of a lace web spider, Amaurobius similis. The spider quickly came out of her burrow, deep in the ivy. The bee disentangled itself before the spider got near and flew away, but the spider carried on checking, with short pulls of the web with her forelegs. There is nothing in my view better than sunlight to get a good shot. This is now my favourite Amaurobius portrait. On top of the natural light, the dark ivy background brings out the warm colours of the spider and also its beautiful blue silk.
Amaurobius produce an uncommon type of silk using a silk spinning organ called the cribellum. This special silk emerges from thousands of spigots on one or more plates instead of normal spinnerets. Each fiber is extremely thin, and the spider then cards the fibers with a series of bristles in the shape of a comb on her back legs - the calamistrum - into a woolly silk, which has properties like the woolly side of velcro. Insects provide spines that get easily tangled into this silk with no need of glue. In Amaurobius, the silk is blue when fresh, with the spider working on new threads in the night.

Why are some hoverflies poor mimics?

Merodon equestris, the Narcissus fly, a bumblebee mimic

Eristalis tenax, a dronefly (top left) and honeybee

Volucella zonaria, a large and colourful hoverfly that is an excellent mimic of the european hornet Vespa crabro

Helophilus, a wasp mimic 

 Sericomia silentis, a wasp mimic

The photos above illustrate that hoverflies are amongst the best examples of mimicry. Although harmless, their colours, patterns, level of hairiness, and detailed morphological and behavioural features often matches species of bees, bumblebees and wasps, making them such wonderful impostors that they fool people into thinking they really are stinging bees or wasps. At close range, anybody can learn to tell mimics and models apart (the shape and size of the antenna is a giveaway). But the fantastic mimicry of hoverflies is shown by the repeated identification failures of natural history photographers, and even specialised editors (a book cover on bees featuring a hoverfly). These are evident when you have a look at many natural history photography websites. As an example I googled "wasp flower" and the first hit was a hoverfly.

The selective pressure for mimicry is predation. Many birds are consummate fly predators, but they will avoid bees and wasps. Although birds could benefit nutritionally from preying upon these succulent, soft flies there is a heavy cost to pay if they get it wrong: a painful and possibly debilitating sting. Given this, shouldn’t natural selection perfect mimicry? Why are there some hoverflies that only have a vague resemblance to bees and wasps?
 Here is an example:

Syritta pipiens, an example of a poor mimic

 There had been many hypotheses put forward to explain why there are poor mimics:

1. 'Eye of the beholder': Imperfect mimics are only imperfect to the human eye, they are really much better mimics to bird eyes. The poor resemblance would be a reflection of our own perception bias.
2. 'Multimodel' Maybe mimics could be safer when imitating roughly several models.
3. 'Kin selection' Flies that are abundant might be surrounded by relatives, and kin selection might have selected for these imperfect mimics.
4. 'Trade offs' mimetic perfection cound be trading off with camouflage ability, or thermoregulation.
5. 'Constraints' mimicry could be limited by developmental or phylogenetic constraints.
6. 'Relaxed selection': small hoverflies – which are less profitable to eat – could have lower predation even if they are poor mimics, which will mean that natural selection for improved mimicry will be weaker.

Heather Penney and co-authors tested these hypotheses using a large set of European hoverfly species. They obtained morphological data and human volunteer rankings of mimicry fidelity to bee, bumblebee and wasp models and phylogenetic data to test these hypotheses.

First, they carried out a detailed morphological analysis on a large set of hoverlfy species which shows that human perception of mimicry fidelity indeed corresponds to objective assessments of similarity between mimic and model based on their morphological analysis. This rules out the 'eye of the beholder hypothesis" and also the 'multimodel' as poor mimics did not fall in between different potential models.

They also performed a meta-analysis showing that poor mimics were not more abundant than accurate mimics and therefore rules out the importance of kin selection in the evolution of imperfect mimicry.

The constraints hypothesis, they did not explicitly test even when they had phylogenetic data to evaluate the importance of phylogeny as a constraint.

The core result of the paper was that the degree of fidelity to the model is strongly correlated with hoverfly body size even when correcting for phylogenetic relationships, as shown in their figure:

Relationship between an estimate of body size and human ratings of mimetic fidelity (fHR). Lines 

show the fitted linear regressions. Filled triangles, wasp mimics; open triangles, bee mimics; filled circle, the non-mimetic syrphid Cheilosia vernalis (from Penney et al 2012)

As the benefit of predating a hoverfly (a meal for the bird) is directly related to the size of the fly, while the cost of error should be constant, the higher risk of predation when the hoverfly is large imposes a higher selective pressure to be a good mimic. Therefore, a hoverfly doesn’t need to be a good mimic if it is small, as the benefit of eating a small fly for a bird is too insignificant to risk the potential cost, therefore resulting in a lower predation pressure for small hoverflies. Birds will err on the side of caution and avoid even the poor mimics when they are small. The other way round: Large hoverflies provide a bigger benefit, so the predator might risk being stung and therefore they have been selected for more precise mimicry. 

Although the results are solid and the hypothesis makes intuitive sense, I found two problems with their design: (1) the limited choice of models and (2) the geographically biased choice of some models, both of them resulting in an underestimation of the degree of mimicry of some hoverflies.

First, not all potential models or even model types were available and this will create an alternative explanation for the evolution of "poor" mimicry, if you can't compare the mimic with its model, they it will by necessity result in a poor mimic. There are hundreds of European species of bees, wasps and bumblebees with a broad range of colours and patterns, but they only used ten. Syritta pipiens, the little hoverfly shown above was rated as a poor mimic in their study, but who says it doesn't mimic a small solitary wasp or bee not included as a model?
In addition, of the three bumblebee species they included two were American bumblebees (B. affinis and B. impatiens), which seemed strange given that their hoverfly sample was European. A single European bumblebee, B. lucorum was included. Given that bumblebees differ extensively in coat colour, this might create biases in the evaluation of mimicry fidelity in bumblebee mimics: they cannot be regarded as excellent mimics if their models are not available for comparison. This is likely to have caused a bias in the evaluation of, for example, the bumblebee mimic Merodon equestris, the Narcissus fly, also illustrated above, which is highly polymorphic for coat colour pattern and mimicks different bumblebee species. They included just a single colour form of this hoverfly, which imitates the all-brown Carder Bee B. pascuorum, but not a single all-brown bumblebee was used as a model. This predictably resulted in (1) the misclassification of M. equestris as a honeybee mimic and (2) his rating as a poor mimic, when this species is in fact a fantastic bumblebee mimic which has tricked me in more than one occasion.
   I wonder up to what point the narrow selection of hymenoptera could result in the assessment of many hoverflies as poorer mimics than they really are. We might not really know some hoverfly species are really imitating because the models they mimic might not relevant to us such as large bees and wasps are.
 Although I think they are valid concerns, I don't think the conclusions of this paper would be affected, but they might have ever got stronger results if their model choice had been wider.

More information

Penney, H., Hassall, C., Skevington, J., Abbott, K., & Sherratt, T. (2012). A comparative analysis of the evolution of imperfect mimicry Nature, 483 (7390), 461-464 DOI: 10.1038/nature10961

A new plume moth

Today I came across this plume moth, Amblyptilia acanthadactyla. When I picked it up I thought it was Emmelina monodactyla, a plume moth common in my garden. But the macro shot revealed otherwise: the tufts of scales protruding from the back of the wings are characteristic of the genus Amblyptilia. Its larvae feeds on a range of plants including Hedge Woundwort, Stachys sylvatica, which I have extensively naturalised in my garden to atract the bee Anthophora furcata. It is interesting to see how introducing a single native plant in the garden can result in several bug species becoming more abundant: the woundwort shieldbug, Eysarcoris venustissimus, several other tiny bugs I still have to blog about, and bees, of course, including the Wool Carder bee, Anthidium manicatum. But going back to this moth, it appears to have become more abundant from the 1990s especially in gardens. There are two generations a year, one of them produced in September, hibernates as an adult, so this one, which I disturbed while watering some plants in the garden, has decided it is spring already.

More information in UK Moths

Do spiders find ladybirds distasteful?

 I witnessed a curious interaction today. A 7 spot ladybird was walking up a wall, when a sudden movement caught my eye. It was a large spider, a female Amaurobius similis that had dashed out of her burrow in a hole in the wall, probably alerted by the pull of one of the silk threads that radiate from her burrow. She had caught the ladybird by one leg. The ladybird struggled to free itself, but she need not fight much. After a few moments, the spider released her grip, turned round and retreated into her burrow. The spider obviously had assessed the ladybird and regarded it not suitable for a meal. I doubt it was due to the ladybird size or strength, as Amaurobius are powerful spiders, able to subdue large flying insects such as droneflies and honeybees. The other possibility is that the spider has tasted the alkaloid rich liquid that constitutes ladybirds chemical defence mechanism and that is released from their leg joints when alarmed (the 'blood reflex'). I have previously seen dead ladybirds wrapped on silk caught on the webs of two spider species (garden spiders Araneus diadematus and the false widow Steatoda bipunctata). Indeed, field surveys and experiments carried out by John Sloggett showed that A. diadematus does trap and consume ladybirds, and is apparently immune to the toxic effects of ladybird's chemical defences. In contrast, other spider species do seem to find ladybirds distasteful, so this is a strong possibility for Amaurobius.

Another view of the interaction, not as sharp, but it shows clearly how the spider's chelicerae are pulling and lifting the ladybird's front right leg.

For other spiders, however, ladybirds are not even considered a food item. Take this little wolf spider, enjoying the company of the 7 spot ladybird a few days ago. Both individuals were sunbathing next to each other most of the morning. The spider popped in and out, paying no attention whatsoever to the ladybird.

More information

Sloggett, J. (2010). Predation of ladybird beetles by the orb-web spider Araneus diadematus BioControl, 55 (5), 631-638 DOI: 10.1007/s10526-010-9291-0

Dandelion bugs

 The verges around the streets have started to be brightened by the sunny faces of dandelions. Much hated by gardeners as the epitome of "weeds", I very rarely pull them in the garden, I just love their intense yellow and the fascination they exert on so many different bugs. As dandelions also start to bloom relatively early, they are an important source of nectar and pollen for many insects. And if you wait, by May you'll have lovely clocks for kids to blow. Leave them be and enjoy them!
 And now a gallery of dandelion bugs if you needed any more convincing. First bees, and not only honeybees...

... but many species of bumblebees and solitary bees...

Emerging Queen bumblebees often feed on them: a the red tailed bumblebee Bombus lapidarius

and this Buff tailed bumblebee queen Bombus terrestris

A poor photo showing two bee species, the cleptoparasite Melecta albifrons and an early bumblebee queen Bombus pratorum.

Other early spring solitary bees can often be seen having a dandelion pollen bath

An unidentified solitary bee

A couple of Red Mason Bees.

And butterflies too...

A long distance shot of a Brimstone,

a Green-Veined White,

And Small Tortoiseshells too.

And last, but not least, many hoverflies use dandelions too

The narcissus fly, Merodon equestris, mating

 Episyrphus balteatus often meet on dandelion flower heads

 Two hoverfly species, a Helophilus pendulus and a Sphaerophoria sp.

I am sure I have missed many more. But not only insects, but also goldfinches enjoy dandelions. Here one feeding on white seeds on my doorstep.

Solitary bees are back!

In the last week I have been spotting more and more species of solitary bees around. First on scene, as usual, at the beginning of March, Anthophora plumipes, the Hairy Footed Flower bee. Then, a couple of days ago I saw the first Andrena fulva (Tawny mining bee) and yesterday I saw a couple of unidentified Andrena sp. (below). Today the garden was buzzing with at least 4 males A. plumipes, and a female feeding on Erysimum, and the cast was complete when in the afternoon a male Osmia rufa (Red Mining Bee) turned up and stopped to bask on the wooden frame of the conservatory (above). All of them have broken records as to how early they have appeared in the year compared to my previous records, which is not surprising given how mild this winter has been. From a few days to a few weeks before previous years, just as many flowers are also blooming early. The Red Mason Bees breed in our bee hotel, but we have been making another one from reclaimed floorboards and canes, I will push to finish it this weekend as females won't take long to appear.
 Are your bees early too?

Andrena sp.

Another Andrena enjoying a Dandelion

The new bee hotel in construction

Two large slugs

The current dry weather means I am not coming across as many terrestrial molluscs as usual. While moving some pots I came across these two beauties, the Yellow slug (Limacus flavus, top) and the Irish Yellow Slug (Limax maculatus). After a while they relaxed on a wet white bowl and let me take their portraits before being returned to the plastic pot pile. Until recently, they were considered the same species, but these two look very different, not only in coloration, but in body shape, despite being a similar length. Both are very common out and inside my house, and are active at night.
I found this useful slug key in the field studies website.

Pine ladybirds waking up

 We spotted lots of Pine Ladybirds, Exochomus quadripustulatus, in our street today. They were crawling on tree trunks on the sunny side of the street, a couple of pairs mating, some still in a little group overwintering together and others just stretching their legs. They used to live mostly near pine trees, willows and sallows, but pine trees are very scarce in our area and I haven't seen any in our street. I assume in our street they feed mainly on scale insects, in particular the Chestnut Scale insect, Pulvinaria regalis, which feeds on trees, including Lime and Sycamore, producing occasionally heavy infestations. Some colour forms of the Harlequin can be similar to the Pine Ladybird, but there are always white marks on the Harlequin pronotum, while the Pine ladybird is all black, with 2 or 4 red spots, the frontal ones with a characteristic comma shape. Pine ladybirds are also smaller, 3-4 mm, and have a rim in their elythra reminiscent of a bowler hat as you can see in the photo below.

Unexpected pollinators

One of my spurge plants is in full bloom now. Spurges have complex flowers with exposed nectaries, which are available to insects lacking a long tongue, although they appear not to be very appealing to bees and butterflies. They attract bluebottles, hoverflies, ladybirds and wasps. Later in the season they will be taken over by ants, which will defend them aggressively from competitors. In the last few days, dozens of 7 spot ladybirds have been enjoying this early nectar, in the company of bluebottles and hoverflies. They pay back the plant by acting as its pollinators. Some of the ladybirds were covered in pollen and the one on top seems to be pushing up the plant pistil, so the anatomy of the plant is right for ladybirds to act as pollinators

This 7 spot ladybird, from 2/04/11 was laden with spurge pollen

Bluebottle enjoying the nectar

More 7 spots on the flowers, and possibly a harlequin

A little hoverfly, Meliscaeva auricollis

Meet the common false-widow spider

The Common False Widow Spider, Steatoda bipunctata, is a regular species in or on buildings and gardens. The female (pictured) is a shiny chestnut brown. It is a small spider, up to 7 mm, with two black spots on top of the abdomen which give it its specific name. It is also known as Rabbit Hutch spider. The male has a much smaller abdomen. It is unrelated to the Widow Spider and not particularly venomous. We found this female in the folds of the cover of a tarpaulin outside, but I have found this species on the dry underside of logs, waterbut covers and window frames. Mature specimens are found all year round, surrounded by a web made up of a tangle of silk threads, some of which are sticky and held under tension so that struggling insects might break them and dangle from them. Males stridulate during courtship making an audible sound using a series of ridges in the abdomen which rub against teeth to the front of the caparace. Although native from Europe, the species was introduced in North America in the early 20th century and outcompetes the closely related species S. borealis.

More information
Spider and Harvestman Recording Scheme Website.

Martin Nyffeler, C. D. Dondale, J. H. Redner (1986) Evidence for displacement of a North American spider, Steatoda borealis (Hentz), by the European species S. bipunctata (Linnaeus) (Araneae: Theridiidae). Canadian Journal of Zoology, 1986, 64(4): 867-874, 10.1139/z86-130.

Anthophora mating!

Since the early morning, a couple of beautifully fresh Anthophora plumipes males had been patrolling the garden, occasionally chasing each other or stopping to sunbathe a bit or feed on Erysimum, tete-a-tete daffodils or Hyacinths. After lunch, I noticed a female feeding on the Erysimums, the first of the year (just 5 days after the 1st male of the year). In a few seconds, a male spotted her and mating ensued immediately. It lasted long enough for me to grab the camera, dash outside and take a couple of shots (above and bottom shots). It is the first time I see a completed mating in this species.

Male grooming itself

Silent aliens

Have a look at these:

Harlequin Ladybird

Lily Beetle

Western Conifer Bug

They are invasive insect species in the UK. They are also so bright, or large, that they are hard to miss. You'll come across them even if you don't look. Many, many other alien invaders are harder to notice, they arrived, and before anybody other than specialist taxonomists noticed, they have spread across sizeable portions of the country. Many such silent invasions involve invertebrates that are small and or hard to identify.
  Clutches of snail eggs in the soil of pot plants, or dormant adults in cracks in stones or other cargo are often sent as inadvertent stowaways across countries thousands of miles away. They arrive in the destination, thrive and begin a quiet invasion. As many snails are tiny, and to the untrained eye they look identical to other snails, they are transported about very often. A sizeable fraction of the snails species of Central Europe is now thought to be of alien origin (about 15%), most of Mediterranean origin and the trend is of a recent increase, aided by increasing temperatures and commerce. In some extreme cases the ability of the snails to disperse is extremely limited, or their requirements for specific substrata so high that they scarcely move after introductions. Such is the case of Papillaria papillaris, a minute snail native of Italy, Sicily and Malta. These snails were likely to have been introduced in the UK in the 18th century with Italian ornaments or stonework, then prized by affluent stately home owners. Just two populations (in Brownsea Island Castle and Cliveden House) have been found so far. Amazingly, they have barely moved a few meters in this time, although they form self-sustaining colonies. Likewise, in Spain, this snail is restricted to walls and ruins from the Roman period, two millennia living practically where they were placed!
 Of course, there are many examples of the other extreme of the spectrum. Species that quickly spread on arrival, or after a lag period. An example is the Girdled Snail, Hygromia cinctella, a snail of Mediterranean origin which now is rapidly expanding in the UK. It was first noticed in the South in 1950, and it has now reached Glasgow, that is fast - for a snail. There have been suggestions that the snail might travel as stowaways in cars! Although this might seem outrageous, snails' habit of climbing up vertical surfaces and attaching itself firmly for aestivation or overwintering might facilitate this and it has been documented that this behaviour increases transport by cars. The girdled snail is 1 cm across and triangular when looked at from the side, with a pronounced keel that sports a pale line. At a distance looks like a juvenile garden snail. My daughter, however, noticed this snail on the pavement in my street a few days ago and it was necessary to rescue it from being crushed by passers by. I only noticed it wasn't a garden snail when I picked it up, after it had retreated into its shell.  I will keep a close eye and see if it has already arrived in my garden.

Side view showing the keel and pale edge.

Underside showing the lack of umbilicus

More information

Burçin Aşkım Gümüş and Henk K. Mienis (2010) Records of Papillifera papillaris affinis in continental Spain and their connection with walls and ruins from the Roman period. The Archaeo+Malacology Group Newsletter, 18: 1-4. here.

Janet Ridout-Sharpe (2010) Papillifera papillaris: a second colony is discovered in England. The Archaeo+Malacology Group Newsletter, 18: 1-6. here.

Alena Peltanova, Adam Petrusek, Petr Kment, Lucie Jurˇicˇkova (2011). A fast snail’s pace: colonization of Central Europe by Mediterranean gastropods Biological Invasions : 10.1007/s10530-011-0121