Christmas spider

A large volume of search keywords hitting BugBlog are about spiders. "Scary spider" in particular features high on the search keyword ranking. I hope the blog helps dispel the myth of spiders as scary bugs. I thought fitting to finish 2011 with a not very scary spider, one that most of the time is hiding outside on the corner of windows, doors and railings in a little silky retreat, and that weaves a fresh web every morning all through the winter. Zygiella x-notata, the Missing sector spider, or window-frame spider, makes characteristic webs with a missing section, though which its signal threat runs (top left hand corner). This one's web glistened on the sun today, with the backdrop of cotoneaster berries that the blackbirds haven't quite finished yet.

Twelve bugs of Christmas

Today we woke up to a mild, sunny day and I was pleasantly surprised by the diversity of active bugs about. I made a count of species and when I got to twelve I was reminded of the traditional carol and made up a buggy version:

12 ladybirds walking | 11 winter-gnats dancing |10 honeybees buzzing |9 bluebottles basking | 8 leafhoppers leaping | 7 woodlice hiding | 6 hoverflies flying | 5 snails sleeping | 4 spiders weaving | 3 harlequins | 2 drone flies | and a bumblebee on ivy

The honeybees in the local wildlife garden were coming out tripping over each other and ladybirds (7 spots mostly but also harlequins and 22 spots) were awaken by the mild temperature. I counted 5 spider species outside (Tegenaria, Zygiella, Pholcus, Steatoda, Linyphia). The main surprise was the bumblebee worker, extremely fast and active and with pollen baskets full with pollen.

Here is a slideshow of bugs seen and photographed today

Marsupial water slaters

 Water slaters or water-lice, Asellus aquaticus, scurry about the leaf litter of my indoor pond. They are fascinating pond inhabitants, always active, sometimes three individuals walking in a line across the tank walls. Most of the water slaters had broken antennae when I collected them, but they have now grown back, the new segments were still thin and transparent a few weeks ago (above), but have now fully developed. Unlike insects, which have very limited regeneration abilities, crustaceans can regenerate lost or broken appendages - as even when adults they carry on molting, and in each molt, they regenerate a bit more of the missing leg or antenna.

Water slaters tolerate a wide range of ecological conditions and are distributed across much of temperate Europe, Russia and North America. They are often abundant in polluted water and live in lakes, rivers, springs and even caves - where albino subspecies with smaller eyes have evolved. Being scavengers, they just need some detritus and fallen leaves and organic material to subsist.

In the last few days, a female has laid eggs into her brood pouch or marsupium. While she climbed over the leaves beside the tank wall, I took a photo where her yellow eggs are visible through the transparent brood pouch.

The marsupium is made of overlapping flat blades coming out of the female's anterior four pair of legs, and allows the mother to hold the brood under her body. This structure is a common feature of all Isopods - which include also terrestrial woodlice. The female not only carries the eggs but also the developing larvae. The larvae will emerge as miniature versions of the adults (see the figure below).

Fig. I. Marsupial stages of Asellus aquaticus. Top plate: A, stage A showing two membranes (arrows). B, stage B showing one membrane (arrow) and cleft yolk mass. C, stage B shedding membrane to reveal lateral outgrowth (arrow). D. stage C with its comma shape surrounded by a membrane (arrow). Bottom plate: A, stage C shedding membrane (arrow) to reveal stage D with free appendages. B, stage D with lateral outgrowth (arrow) and appendages covered i n a closely-fitting membrane. C, stage E showing setose body and appendages (arrows). It is at this stage that the juvenile leaves the marsupium to become free-living (from Holdich & Tolba 1981)

Occasionally females expel some larvae from the pouch - possibly due to physical limitations of how many she can carry as they grow - and larvae can also develop normally outside the marsupium from developmental stage B. Probably they need the ventilation and extra oxygen produced by the mother in the initial stage. Carrying the eggs and larvae is also bound to offer them some protection from predators, so it probably evolved as a form of maternal care.

Although water slaters normally stop reproducing during the coldest months of the year, my indoor pond is warmer than outside, and this might have triggered the start of reproduction; a situation that also occurs in wild populations downstream thermal power stations where water is warm all year round. 

More information

Vitagliano, G., Fano, E., Marchetti, E., Colangelo, M., & Vitagliano, E. (1991). Importance of longevity, growth, and diapause in the evolution of Asellus aquaticus Bolletino di zoologia, 58 (2), 113-117 DOI: 10.1080/11250009109355740

Holdich, D., & Tolba, M. (1981). The effect of temperature and water quality on the in vitro development and survival of Asellus aquaticus (Crustacea: Isopoda) eggs Hydrobiologia, 78 (3), 227-236 DOI: 10.1007/BF00008519

Sexual roles and gender conflict in pond snails

I caught these pond snails mating in my indoor tank today. The bluish, tongue-like structure between both snails is the penis of the snail on the background, entering the other snail's mantle cavity. So, that snails is acting as a male, (the one with a walrus-like head resting on its partner's shell) and the other as a female. Pond snails are simultaneous hermaphrodites, all individuals have functional ovaries and testis. The snails can self-fertilise, but if sperm from other individual is present, they prefer to use it. Although reciprocal sperm exchange might appear as a logical reproductive strategy, each individual snail might benefit more from acting as a male or as a female in a particular encounter with a potential partner. For example, large snails are able to invest in energetically demanding eggs, and might prefer to act as a female. Individuals acting as females are limited in the frequency at which they produce eggs, so they might prefer not to mate as often. Male reproductive efficiency might depend more on how many sexual encounters he has had, so snails preferring to be males might want to mate more often.

Sexual encounters of pond snails can result in just one of them acting as a male and the other like a female, or in subsequent reciprocation.  But, the snails could disagree, what if both mating partner insist on providing, but not receive sperm? This is called gender conflict, and the resolution depends on one of the individuals compromising and adopting its less preferred role, at least initially.

Petra Hermann and collaborators studied the effect of age on sex role preferences in the great pond snails, Limnaea stagnalis, disentangling it from the effect of size.  They reared three batches of snails that had known ages at the time of the experiments (young, Y, middle age M, and senior, S) and matched them by size.

First, they looked at the effect of age on mating interactions in age-matched couples. The chances of copulation decreased strongly with age, with young snails copulating with much higher frequency (80%) than senior snails (30%). Encounters between young snails normally ended in a reciprocal intromission (the individual acting as a female initially, then acting as a male), middle aged and senior snails, in contrast, had mostly unilateral sperm exchanges.

 Then they set encounters between snails of different ages. When pairing young and middle aged snails, also size-matched, about 25% ended in no copulation, and unilateral encounters were more common than reciprocal. The primary role in the copulation (the snail acting as a male first) was adopted by the young snail in about 80% of the encounters, indicating that the male role is the coveted one for the young snail, and that the fact that reciprocation is rare indicates that the middle ages snail is content acting as a female in unilateral encounters. There is an age related shift in sexual role preferences from male to female.

Fig. 6. Sexual interactions between young and middle-aged Lymnaea.

(A) Most couples of a young and a middle-aged snail (YM) performed a

copulation. The majority of these interactions were unilateral, i.e. the snails

do not reverse roles after finishing the first copulation. Note that in the

cases in which the middle-aged animals acted as primary male all young

partners reciprocated. By contrast, role reversal occurred in only a small

minority (18%) in the couples in which the middle-aged animals acted as

female. (B) Younger snails act significantly more often as primary male than

their middle-aged partner. (modified from Hermann et al 2009)

They concluded:

Animals in the early phase of the species’ life cycle tend to assure that they act as male (either primary or secondary), independent of the age of their partner (Fig. 7A). Senior animals, by contrast, tend to act preferentially as females. The sexual behaviour of middle-aged animals depends on the age of their partner. Combined with similarly aged partners they either act as male or female. Combined with a younger partner they tend to act as female only. Combined with an older partner, they tend to execute both gender roles and will act as males with a similar probability as younger animals

Interestingly, the snails resolved conflict by engaging in reciprocation. When both agreed on a role the encounters tend to be unidirectional. Note that as these snails store sperm and the partners were not virgins, the lack of interest in copulation of the older snails might stem from the fact that they might have accumulated enough sperm to fertilise their eggs. So although the authors removed the confounding effect of age from their experiments, they failed to account for the effect of sperm storage.

 The snails' efforts are resulting on these lovely egg masses appearing attached on the walls of the tank.

More information

Hermann PM, Genereux B, & Wildering WC (2009). Evidence for age-dependent mating strategies in the simultaneous hermaphrodite snail, Lymnaea stagnalis (L.). The Journal of experimental biology, 212 (19), 3164-73 PMID: 19749110

Pholcus phalangioides, the Daddy Long-leg spider

In dark, forgotten corners of houses and outbuildings, a spindly-legged spider hangs upside down - motionless - from a loose, barely visible web made of very fine threads. It is Pholcus phalangioides, the Daddy Long-leg Spider or Cellar Spider. Today, several hang from underneath a wooden shelf in my outside toilet, including the male above. This species is cosmopolitan but has recently expanded its range northwards in the UK, and it is almost always found associated to buildings.

A male showing its palps.

Mating pair. After an initial male approach and web and leg tapping, if the female accepts him, the partners approach their ventral surfaces and the male inseminates the female using his palps.

Pholcus is a very generalist predator and has no trouble subduing large prey. I have seen it with captured Tegenaria (above) even Dysdera (below), the latter a spider with enormous chelicerae. Pholcus is able to do so thanks to its long legs, as it throws silk to its prey and wraps it on silk while keeping a safe distance. It can also invade other spiders' webs and then makes them vibrate simulating the effect of an entangled prey, in order to attract the owners and catch them, a deceptive behaviour known as aggressive mimicry. It will also eat other spider's eggs and trapped prey. If Pholcus is disturbed in its own web it has a defensive behaviour called whirling: it moves its body rapidly in a circle, becoming a blur, while keeping its legs on the web, this might deter other spiders from entering its web but even so, Pholcus can often capture and eat these spiders.

 Pholcus is even able to capture and feed on woodlice, which often walk up the walls in my conservatory.

Female Pholcus are dedicated mothers. They hold their egg clutch of about 20 to 30 eggs by their chelicerae. The eggs in this clutch are close to hatching. The spiderlings' legs are visible through the egg shell as white threads.

 The spiderlings stay close to their mother for some days after hatching. She hasn't fed since she laid the eggs and she will have to wait until the spiderlings disperse.

 Pholcus go through five moults before maturity. The one below is molting.

Apparently, the most effective enemy of these spiders is that noisy generalist predator, the vacuum cleaner.

More information
Check this website for detailed information on Pholcidae.

Maciej Bartos (1998) Quantitative analyses of male courtship behaviour in Pholcus phalangioides
(Fuesslin, 1775) (Araneae, Pholcidae). In: P. A. Selden (ed.). Proceedings of the 17th European Colloquium of Arachnology, Edinburgh 1997. 171-176. here.

Kazuyoshi Miyashita (1988a) Development of Pholcus Phalangioides (Fuesslin) (Araneae, Pholcidae) under Long and Short Photoperiods. Journal of Arachnology, 16 (1), pp. 126-129.

Kazuyoshi Miyashita (1988b) Egg Production in Pholcus Phalangioides (Fuesslin) (Araneae, Pholcidae) under a Constant Temperature and Photoperiod. Journal of Arachnology, 16 (1), 129-131.

Jackson, R., & Brassington, R. (1987). The biology of Pholcus phalangioides (Araneae, Pholcidae): predatory versatility, araneophagy and aggressive mimicry Journal of Zoology, 211 (2), 227-238 DOI: 10.1111/j.1469-7998.1987.tb01531.x

The CSI Blowfly

At this time of the year, when there has been a few frosts and the sun is weak, the most likely insect you are likely to see on the wing are bluebottles. I found this one yesterday enjoying the sun in my conservatory. It is the urban blowfly, Calliphora vicina, a very cold tolerant species which is the most common buebottle in the UK.
  Calliphora vicina is one of the most important species in forensic entomology, especially in investigating human remains. Different fly species arrive at a body at different stages of decomposition and they will lay their eggs on it. Blowflies can smell rotting flesh from large distances and are one of the earliest finding a body. They have a very fast life cycle, and they can produce up to five generations a year, depending on the temperature.

(Figure from Amendt, Krettek & Zehner 2004)

The female lays batches of 150-200 eggs in open wounds, rotten meat, or bodies. These eggs can hatch almost immediately after being laid, but they can take up to 9 days to hatch at 5 oC. The maggost will start feeding immediately. As the duration of each of the three larval stages depend on the temperature and is known in great detail, the age of the oldest maggots together with the average arrival time of the species allows to estimate the post mortem interval and approximate time of death.

Bluebottles will enter houses and lay batches of eggs in exposed meat either cooked or raw, and their attraction to rubbish makes them very abundant in cities. Bluebottles have an important ecological role as carcass decomposers...

... but they are also pollinators of several plants with exposed nectaries, such as ivy, spurges (Euphorbia) and plants from the carrot family. Some companies have even used bluebottles for greenhouse pollination of various crops as they fly at lower temperatures than bees. They also act as dispersers of fungal spores, and some fungi, like Stinkhorns (Phallus impudicus) have specific adaptations to attract blowflies, releasing chemicals that smell like rotting meat. The flies feed on the surface on the fungus and the spores attach to the fly, which can disperse them.
 Next time you are annoyed when a bluebottle enters your house, you might want to give a thought to how useful these flies are.

More information
Amendt, J., Krettek, R., & Zehner, R. (2004). Forensic entomology. Naturwissenschaften, 91 (2), 51-65 DOI: 10.1007/s00114-003-0493-5

Donovan, S., Hall, M., Turner, B., & Moncrieff, C. (2006). Larval growth rates of the blowfly, Calliphora vicina, over a range of temperatures. Medical and Veterinary Entomology, 20 (1), 106-114 DOI: 10.1111/j.1365-2915.2006.00600.x

How do bugs cope with winter?

We've had a couple of frosts in the last few days. Despite the die-hards which allow me to post occasionally these days, most insect species are nowhere to be seen throughout winter. How and where are bugs surviving the cold weather? There are various ways in which invertebrates go through the winter.

Far, far Away
Some - I'd say the lucky ones, avoid winter altogether - they are far, far away during winter, having migrated to warmer areas around the Mediterranean. These include several butterflies, moths, hoverflies and dragonflies (Red Admiral, Painted Lady -above - Silver Y and the Migrant Hawker are examples) which often lack a frost-resistant stage in their life cycle.

Slowing Down
Others stay to brave the elements, a few of them even carrying on more or less as normal, but at a much more sluggish pace. These are the invertebrates that enjoying the occasional sunbathing during rare winter sunny spells: Bluebottles (above), Winter Gnats, the window spider (Zygiella x-notata). Some moth caterpillars, like the Ruby Moth caterpillar, are found active in the middle of winter. A few bumblebee populations are starting to behave this way as the climate warms and they can make use of winter flowering resources.

Dormancy
Many insects enter a dormant state or diapause: this is the scientific term to describe what we often call 'hibernation': organisms enter a physiological state, dormancy or diapause, in which growth and feeding pretty much stop, and metabolism is very reduced. To protect their bodies against the damaging effects of freezing, some chemicals (glycerol and anti-freeze protein are two of them) are produced in their bodies to maintain cell stability and aid survival under low, often freezing, temperatures. Overwintering can occur in any stage of the life cycle depending on the species or taxonomic group  - egg, larva or nymph, pupae or adult (imago).

The beautiful blue silk cocoon of a Enoplognatha spider, ready for winter.

For example grasshoppers, aphids and many spiders overwinter as diapausing eggs. Many dragonflies and damselflies overwinter underwater as nymphs. Many solitary bees overwinter as pre-pupae in their nest cells.

A fully developed Noctua moth caterpillar in early spring

Some butterflies and flies overwinter as pupae, whereas some butterflies (Comma, Small Tortoiseshell, Brimstone), Ladybirds, queen wasps and queen bumblebees hibernate as adults. Many solitary bees are already imagos in their cells, waiting for the right time to emerge.
 I will keep my eyes open for bug life throughout the winter and be on the lookout for any stirrings indicating that springs is coming.

A little red underwater planet

 I have brought a few animals from my half barrel (=mini pond) to see close up to a small tank. Most of them are detritus feeders or vegetarians, and regularly I top up the tank with fallen leaves. Today's water was stained red and the reflection of the light in the water and the leaves made the photos take a surreal tint. A Great Pond Snail (Lymnaea palustris) fed on the algae growing on the tank walls (above).

Another snail, Planorbis planorbis

A group of ostracods, small bivalved crustaceans, and a copepod towards the left of the image.

There are also a few water slaters and planarians, but they didn't show as well today.

UPDATE: Thank you to Richard Comont for the pond snail ID, now corrected.

More information
Online aquatic snail identification guide from the Conchological Society. here.

Leaf sleepers

This is a good time of year to search for bugs in the leaf litter. On the top layer, ladybirds are set for the winter in the cosy, dry folds of the fresh fallen leaves. They usually cuddle up close to other ladybirds of the same species to overwinter, but it is not unusual to see two species together, like the 22 spot and 7 spot above. If you search a bit deeper on the soil, you can find centipedes, earthworms, and woodlice. With the colder temperatures, they are more torpid than usual, and then easier to photograph.

A shiny woodlouse, Oniscus asellus

Centipede, Lithobius forficatus

 A group of 22 spot ladybirds inside a vine leaf

Arion sp. slug

7 spot ladybird and birch shieldbug

Freshwater Flatworms

 If you kneel down on the edge of a clear water pond and look towards the bottom, even at this time of year, you might be able to see some flattened elongated creatures slithering over the sediments, like they are levitating (above). They are planarians or flatworms. On closer inspection they have a quizzical look due to their simple eyespots or ocelli, which appear to be both looking inwards. They are relatively simple organisms, which move over a carpet of cilia and have simple guts, with a single opening, the mouth, which is sometimes placed in the middle of the body. I am trying to get some shots of water slaters and I have set up a little tank at home. Most of the time when I get some decaying leaves from my half barrel (=minipond) a few flatworms are accidentally transferred with them, and then I can take some photos of them while they climb on the walls. There appear to be several species, but yet have to find out some resources to tell them apart.

One of the most striking features of flatworms is their capacity for regeneration. If you cut them in half, each half grows the missing parts. Even a tiny part of a planarian can regenerate a complete individual.

Centipede on a walkabout

It is long, I counted 76 pairs of legs! My young daughter spotted this centipede, Stigmatogaster subterranea, walking on a stone wall out in the open. This is the first time I see this behaviour in this otherwise soil and leaf-litter living centipede. When disturbed, it had no trouble walking backwards, which had me puzzled for a bit, as I confused the tail and the head end!

The centipede lifted its head as if sniffing...

 ...and finally disappeared in the leaf litter.

Foggy morning

The fog painted the spider webs hanging from bushes and fences this morning with pearly dew drops. It made evident that at least three spider species are still active. On the photo an Araneus diadematus web, we also saw Zygiella x notata and Linyphia triangularis webs.

Winter bees on Mahonia

A lovely, still sunny day, I walk to a clump of Mahonia on a corner of an avenue. I have posted about this bug magnet in previous years (here, here and here) At this time of the year, Mahonia (Oregon Grape) is on bloom, its sweet scent and profuse, sunny-bright yellow flower spikes attract all late nectar lovers. Today, the bushes were busy, Bluebottles, Drone flies, honeybees and three (!) bumblebee species were feeding on it. A Bombus pratorum queen about to land, with a dronefly on the foreground on the top photo. This is one of the garden plants that are likely to have contributed to favour winter colonies in bumblebees. Bugs love this bush, and so do I. 

Bombus terrestris queen. I have seen a worker this week too.

Bombus hypnorum

Honeybee

Dronefly and bluebottle