Welcome to my personal homepage on the Zoological institution at Göteborg University.
My name is Gary Lång and I am a graduate
student in Animal Ecology.
Jag vill se denna sida på
svenska
This webpage presents some general information
about spiders, centered on the conditions in Sweden. It also
contains a short introduction to my thesis work .
I also present some tips on literature and weblinks to other
webpages with educational or interesting content about spiders
and biology.
My address is at the
bottom of the page
There are close to 40.000 known spider species n the world. The actual number is probably more than twice as many. These are currently divided into something like 100 families. Sweden hosts nearly 800 of these species, belonging to about 25 families. In size, they range from dwarf spiders, less than a mm in length, and up to the fishing spider, with a body reaching 25 mm length.
The body of a spider is clearly divided in two main parts, the
front part is called prosoma or cephalothorax and the rear part
is called opisthosoma or abdomen. The cephalothorax, which is the
head and chest region fused together, has the eyes, the mouth
parts and 8 legs attached to it. It has a rigid "skin",
exoskeleton, on which all muscles are attached. The abdomen
contains the reproductive organs, the book lungs and/or tracheal
openings that are used for gas exchange, and the spinnerets. It
is soft and pliable and used as storage organ for ingested food
between moultings. The abdomen also contains spinnerets,
reproductive organs, respirationsystem and excretory organs.
This division of the body makes it possible for the spider to eat
a lot of food relative to it's body weight and store it in the
abdomen, before moulting to a larger cephalothorax size. Moulting
is the process when the spider sheds it's old rigid skin and
producing a new skin that is larger. The new skin is all soft in
the beginning, so the spider can expand the cephalothorax skin by
using the body fluids stored in the abdomen.
When a spider has matured and become an adult with functional
genitalia, there is a change in the behaviour of the male.
Instead of trying to catch prey to eat, they start to search for
females to mate with. In active hunters that do not build webs to
catch prey, e.g. jumping spiders (Salticidae) or wolf spiders
(Lycosidae), there is no large difference. The males just pursue
females instead of stalking prey. In web-building spiders on the
other hand, e.g. sheet-web spiders (Linyphiidae) or orb-web
spiders (Araneidae), adult males leave their webs and starts
searching the surroundings for females.
In the sheet-web spider Linyphia triangularis the adult
males have a short lifespan. The mating period lasts only a few
weeks from the end of July. If they are lucky, they can mate with
several females during this time and get their genes well
represented in the next generation of spiders. Some males may be
seen in the population for a couple of months, but shortly after
the mating period most of the males are dead. They have either
starved to death or been caught by a predator. The females stay
in their webs for as long as it is possible, producing an egg sac
with up to 70 eggs every two or three weeks. Finally they freeze
or starve to death when the temperature drops in winter, leaving
just the egg sacs stuck to the underside of twigs and dead
leaves. The eggs hibernate and hatch again when the temperature
rise around the beginning of April.
Within the order Araneae - spiders - the females are almost always larger than the female. This is particularly clear in web-building spiders, where the female in some species of the genera Nephila may weigh 100 times the weight of the male.
In spiders it has been shown that the female's reproductive effort (total mass of eggs) is directly proportional to her size. It is possible that this is a good example of how strong fecundity selection for large size in females and low competition betweeen males for mating opportunities selects for a female biased Sexual Size Dimorphism (SSD).
An exception to this rule in web-building spiders is the family of sheet-web spiders (Linyphiidae) where males are of about the same size as or even larger than females. Linyphia triangularis (see below) and Pityohyphantes phrygianus is two examples of the latter. P. phrygianus is a biennal sheet-web spider that occupies branches of spruce. It has a skewed sex ratio of one male on about every five females (ranges from 1:3 to 1:10 between years). In spite of this males are larger than females as subadults just before maturity.
How is it possible for the males to out grow females of these
species?
The males mature earlier,thus have a shorter growth period. The
growth of a spider are most probably limited by the access to
food and there is no reason to assume that this would differ
between sexes. If anything, females under fecundity selection
should maximise their energy intake during growth. The studies
done on spiders show that female fecundity are directly
proportional to their size at maturity.
Since linyphiids seem to show the opposite pattern of sexual
size dimorphism relative to other web-building spiders, it has to
exist a strong selection on males to grow large, e.g. through
male-male competition. Besides this there has to exist some
constraint on growth that affect female fitness more than male
fitness. It could be a cost, e.g. higher chance of mortality,
that increases faster with growth rate than the benefits does. If
the benefits of large size increases faster for males than
females, it might pay the males to increase their growth rate.
The problem is that this should incur a cost that would be seen
as higher risk of mortality. Such a cost is found in P.
phrygianus but not in L. triangularis.
The picture shows an adult female of Linyphia triangularis.
It has an annual life cycle in Sweden, overwintering as eggs in a
cocoon. The cocoon, or egg sac, is placed below twigs or under
dead leaves on the ground. The egg hatches around the beginning
of April and the spiders grow during the summer to reach maturity
in the end of July - beginning of August. The males abandon their
webs after maturing and search for unmated females. Males mature
about a week before females so unmated females are usually
equivalent to subadult females. The males guard an unmated female
until she matures and he can mate with her. During this time he
has to defend her from other males that finds her web and fights
betwen males can lea to serious injury or death for the loser.
The female accepts to mate with the male guarding her as soon as
she has matured. After the complicated courtship and copulation
that lasts for several hours, the male deposits a mating plug in
her reproductive opening and leaves searching for other females
to guard. Sheet-web spiders are generally believed to have a
"first-mate priority system", which means that the
first male to deposit sperm in the female's spermatheca also
fertilises most or all of the eggs of that female.
In spite of the 5 - 10 % shorter development time available, the
male of this species grows to a size about 10 - 15 % larger than
the female. This paradoxical relationship is unique among
web-building spiders. My project aim to look at the mechanisms
that make this possible, and to do comparative analyses between L.
triangularis and related species and species from other
families with a "normal" female-biased sexual size
dimorphism.
Arachnology: The only spider link you need! Large and very nice web site that is updated regularly. All the spider links below is found on their link page.
For
the young spiderfriends Australian, but still
interesting.....G'day, mate!
Zoology-index:
A LARGE index of zoological information in cyberspace.
CSICOP: An international
organisation working to counteract pseudoscience and refute
claims of paranormal phenomena.
Vetenskap
& Folkbildning: The swedish equivalent.
GU Safari: Search for
current research at the university.
| Gary Lång |
| Department of Zoology |
| Box 463 |
| SE - 405 30 Göteborg |
| Telefon: +46 - 31 - 773 36 96 |
| E-mail: gary.lang@zool.gu.se |
UPPDATED 2000 - 04 - 11