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