The curious Autumn spider mating tactics

Female spiders tend to be larger than males. Large females are thought to be selected for as they can produce more eggs. The relative size differences between males and females depend on the type of competition between males, that is, on the importance of intra-sexual selection. When males do not compete much with one another they are smaller than females, whereas when there are male-male contests for females, then large males are selected for, and size differences with females become smaller. The spider, Metellina segmentata, is an orb spider common during September and October, when most mature individuals are found. Males are more powerfully built and have longer legs than females, suggesting that male-male contests occur in this spider. When young, males build their own webs to hunt insects, but upon reaching their final moult, they mature and start searching for female webs. Once they encounter a receptive female web - likely through detecting a pheromone - they adopt a curious sit and wait strategy. They have to wait, often days, until a large fly falls in the web. Only when the female starts eating the fly does he starts his courtship, taking advantage of the diminished cannibalistic tendencies of sated females. Of course, during this wait, other suitors might arrive to the female's web and when two males encounter each other fights follow. Males are often injured or killed in these fights, where the larger male has an advantage and this is what provides the selective pressure for powerfully built males.
 In addition, males are able to monitor several female webs when they are nearby and this results on males guarding webs becoming larger as the season progresses, and a pool of smaller wandering males.
 The mature female Metellina on the top photo has hung her web on a tomato plant since at least mid september. I haven't noticed any males but I shall keep an eye for these in the next days.

Reference
Prenter, J., Elwood, R., & Montgomery, I. (2003). Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment Animal Behaviour, 66 (6), 1053-1058 DOI: 10.1006/anbe.2003.2266

BugBooks: Chinery's Insects

Given that the oncoming months will, by necessity, subdued at BugBlog, I have decided to start a new section. I will present and review my favourite books on British bugs, books that I routinely use to find information or to identify the bugs that feature in BugBlog. Please feel free to comment with your own favourites. My first choice has to be undoubtedly Michael Chinery's Insects of Britain and Western Europe, a very informative, superbly illustrated field guide. An additional advantage is its size, soft bound and small. Its 320 pages are absolutely packed with information, including distribution pattern in Western Europe and in Britain, time of the year where they are found, behaviour and habitat. The colour illustrations are superb, detailed and accurate. Many species are illustrated with adult and caterpillar or larvae and the illustrations often shows common behaviour. A key at the beginning of the book will help you identify what group an unidentified insect belongs to. At the end of the book, there is a section with a selection of species of non-insect arthropod groups such as Spiders, Woodlice, Millipedes, Centipedes Scorpions and Harvestmen, which comes in handy.
My copy from the reprinted first edition in 1986, is getting dated. Since it was put together, many new species have arrived to the British Isles and have become now common and are not described (e.g., the ubiquitous Harlequin or the Horse Chestnut Miner). In addition, the distribution of many other species has significantly shifted northwards (e.g Speckled Wood, Comma) or they have moved to the U.K. from the continent (Tree bumblebee, Ivy Bee). But those are minor quibbles, I couldn't do without this book. There is a more recent revised edition (2007), which is a bit pricey. A proviso, this book does not include all species of European insects (there are over 100,000 described), but a selection of the most common or noticeable. Once you become familiar with an insect group - be butterflies, hoverflies or dragonflies - you will require a specialist guide.
 Overall, a must for the bug lover.

Two white plate orb spiders

The garden is scattered with orb spiders, including the ubiquitous garden spider Araneus diadematus (above) sat in the middle of their webs during the day, and females, now laden with eggs, are very obvious. But there is a smaller species of orb spider can easily pass unnoticed despite being common in gardens, its the beautifully named, more delicate Metellina segmentata, also known as autumn spider (below). It has longer front legs, and there is a round empty space in the middle of her web. But days are shorter and the sun not as strong, and I miss natural light when taking photos. The orb spiders pose a challenge for my usual white bowl spider portrait technique: they are not walking spiders, and they are at their most comfortable hanging from their webs. So, for these shots of spiders in the garden I replaced my usual white bowl by a large white plate, which I carefully held behind each web while I took their portraits.

Knowledge is power for the sun spot owners

Even as the butterfly season comes to an end, Speckled Wood butterflies, Pararge aegeria, are still going strong in my local overgrown cemetery. The hundred year old trees, shade the ground and sunny clearings are highly prized by Speckled Wood males. Yesterday, I watched them squabbling incessantly in a patch of grassy area amongst the trees. At some point, the resident male chased two other individuals, the three butterflies flying around in tight circles away from the prized spot. Another individual comes by oblivious to the ownership contest and takes possession of a sunny dock leaf in prime position (above). By the time the initial "owner" comes back victorious from the skirmish, the newly arrived one believes this is his spot and the subsequent squabble ensues. It is really never ending in this warm autumnal morning with the sunspot owners.
 What determines how is the fight settled? In the late 1970s Davies work suggested that prior ownership is a very strong predictor of success in a fight: residents always won. Later work by Christer Wiklund and collaborators have expanded and refined Davies' findings, indicating that although a good predictor, ownership is not the only factor involved in territorial fights. Age and size are factors that are known to be predictors of fight outcome, but in the Speckled Wood these have little importance. Martin Bergman, Martin Olofsson and Christer Wiklund investigated the effect of motivation on territorial fight outcome. Residents might be more motivated to fight because they have more information on the value of the resource they are fighting for: the resident male might have prior mating experience in that spot, or knows how prized the spot is for other males. The intruder, most of the time lacks such information. This creates an asymmetry in which the resident is more determined to fight than an intruder.
 To test this hypothesis, the researchers set up experiments in large outdoor cages with a prime large sun-spot, and smaller, suboptimal sunspots. They released two males in each cage and waited until there was a clear dominant male that took possession of the large sunspot. They they removed the dominant male and divided the subordinate males in two groups, half of the males repeatedly encountered a female during a 30 min residency period (female encounter group), while the other half (the control group) did not have any female encounters. Then they tested the effect of this prior experience - motivated vs unmotivated males - on contest outcome when they introduced the original dominant individual back in the cage. The results were very clear.

Outcome of contests between Speckled Wood males during the second contest period when the original winner had been reintroduced, and after the original losers had either interacted with a female during 30 min (female encounter group, n = 30) or been alone for 30 min (control group, n = 30); ‘reversal’ (open bars) denotes that the male that lost the contest during the first contest period reversed the outcome and won the contest against the original winner in the second contest period, and ‘no reversal’ (filled bars) denotes that the same male won in both contest periods (from Bergman et al 2010).

Half of the originally subordinate males won their fight with the original dominant in the "female encounter group". In contrast, most of the dominant males won their fight for the prime sunspot when no females had been present. Also, the fights in the treatment group were more intense and took longer to resolve, while the control group the fights were settled quickly. In conclusion, these experiments showed that male experience in a sun spot increase his persistence in a fight, and therefore increase the chances of winning it, even if he was initially a subordinate male, indicating that motivation is an important factor for contest resolution in the Speckled Wood.

References

Bergman, M., Olofsson, M. & Wiklund, C. (2010). Contest outcome in a territorial butterfly: the role of motivation Proceedings of the Royal Society B: Biological Sciences, 277 (1696), 3027-3033 DOI: 10.1098/rspb.2010.0646
Davies, N. (1978). Territorial defence in the speckled wood butterfly (Pararge aegeria): The resident always wins Animal Behaviour, 26, 138-147 DOI: 10.1016/0003-3472(78)90013-1

On the Western Conifer Seed Bug and Christmas trees

This large, striking bug entered the house at night through an opened window. It is the alien species Leptoglossus occidentalis, the Western Conifer Seed Bug, a native from western North America, where is a minor pest of conifers. This species has long antennae and large rear legs, a pale zig-zag mark on the wings, and distinctly flattened and enlarged tibia, which gives the name to its group, the leaf-footed bugs. During last century, this species spread across North America and during the last decade has also been introduced in Europe, where it has quickly spread from the initial introduction point in Italy. It was accidentally transported in timber, and its spread could also possibly been aided by Christmas tree shipments. Nymphs and adults feed on the sap of cone seeds and buds, and therefore the damage is restricted to seed production. In continental Europe, where there are established populations, it does not appear to cause much damage. Every now and then, there are migratory influxes to the U.K. from the continent, as occurred in 2008. The bug often falls in moth traps, and is a good flyer. It is most obvious in the autumn, when it moves about looking for suitable hibernation sites, often entering buildings and sometimes forming large aggregations, attracted by pheromones produced by males. Nymphs have been found in a few sites in the U.K., so it appears to be establishing.
 If you have seen this bug, you can report it here.

References
Lis, Jerzy A., Barbara Lis & Jerzy Gubernator (2008). Will the invasive western conifer seed bug Leptoglossus occidentalis Heidemann (Hemiptera: Heteroptera: Coreidae) seize all of Europe? Zootaxa, 1740, 68-68
Taylor, S.J., G. Tescari & M. Villa (2001). A nearctic pest of Pinaceae accidentally introduced into Europe: Leptoglossus occidentalis (Heteroptera: Coreidae) in Northern Italy. Entomological News, 112, 101-103.
Blatt, S. & Borden, J. (1996). Evidence for a male-produced aggregation pheromone in the Western Conifer Seed Bug, Leptoglossus occidentalis Heidemann (Hemiptera: Coreidae) The Canadian Entomologist, 128 (4), 777-778 DOI: 10.4039/Ent128777-4