|MadSci Network: Zoology|
A really interesting question... This kind of behaviour isn't uncommon amongst terrestrial arthropods, but I'm not sure what the explanation in harvestmen ( Opiliones), relatives of spiders and other arachnids, is. The name "daddy longlegs" in England refers to something quite different - a long legged dipteran (true fly), which are also commonly called craneflies, so it's a good job you've included the scientific name, especially on an international medium such as the internet! Any insect and arachnid biology books may help explain the phenomenon.
The kind of factors that could be important are environmental conditions, such as temperature, humidity, wind or light. At least some of these are easily measured (or at least estimated), and if you were interested in studying the phenomenon the first step would be to see if the presence of the congregation correlated with any of these factors. You've made a good start by noting that they appear mostly during the day, but also at night and that the behaviour is seasonally dependent. Also, try and work out what is special about that corner, if the behaviour is an environmental response then it might be unusual, is it particularly wet, dark or cold? Terrestrial animals often seek out such places to avoid the problems of dessication, for example, woodlice (slaters in the US) like live under damp, dark logs or rocks. This is important to harvestmen, too, and the most likely explanation for the location of the mass. Another factor to consider is whether the animals are congregating actively (i.e. seeking out one anothers company) or passively (i.e. all looking for the same environmental conditions, which causes them to end up together). The seemingly distinctive behaviour you describe (the leg stroking and avoiding body contact) may suggest the former, which in turn prob. indicates a more interesting explanation than an environmental one, although this is likely to be important in WHERE they congregate, if not WHY they congregate.
It would also be useful to find out if the individuals which make up the mass changes on a day to day level, you could do this by marking them with a tiny spot of paint or nail varnish (be careful!), and seeing if new, unmarked individuals appear. How does the number and size of individuals in the mass change, if they are very similar in size then that suggests that they are of similar age. You could try trapping harvestmen from around your house or neighbourhood (I don't know what the most effective method would be, but you could try burying a glass jar up to its neck overnight - remember to let them go) to see where the individuals making up the mass go when they are not congregating. It would also be useful to determine if the mass was made up of males and females, or both, but this would be difficult to acheive, since it would require clsoe examination with a handlens or micrscope (depending on the size of the individuals). You should easily be able to generate enough data to test hypotheses such as: 1) The behaviour is a random accumulation of harvestmen attracted to the environmental conditions of the corner... in this case the individuals making up the mass will be a random subset of local harvestmen, the corner should be noticeably darker, colder or damper than other areas nearby and you might be able to find similar phenomena in other similar places or 2) The behaviour is a response of a particular group of harvestment to find shelter (probably from dessication), safety (is the corner protective against predators - probably not), or mating (unlikely).
Harvestmen are fascinating animals, the order Opiliones is a large and diverse group of about 5,000-6,000 species. They are known from nearly all climatic regions of the world, but are most abundant in the tropics. most have small bodies (less than 2cm.) but may have very long legs (up to 10cm.). They prefer damp shaded areas and are common in leaf litter, on trees and logs in dense woodland and in caves. They feed on small invertebrates, and dead animal and plant matter. Unlike other arachnids they can ingest small solid particles as well as liquid food. Also, the males have a penis for direct copulation, whereas in most arachnids sperm is transferred using a modified appendage.
There is a lot of information online about harvestmen in particular and arachnids in generals. Try The Arachnology Home Page, which has enormous amounts of information and links about all arachnids, ranging from technical scientific articles to children's nursery rhymes!.
Theres very few books about harvestmen, and there are certainly no non- specialist books that will deal with their behaviour. Try looking for any books on spider or insect bioloy or behaviour at your local library.
I hope you enjoy finding out more about the harvestmen and their behaviour!!Trevor.
Try the links in the MadSci Library for more information on Zoology.