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Spiders Through the Scanning
Electron Microscope |
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Main | SEM
Gallery | Spiders | Dinoflagellates | Sikh
Conservation |
Spiders are a large (more
than 38,000 described species worldwide), distinct,
and widespread group. The earliest evidence of spiders
comes from a 380 million year old (Devonian) fossil.
Spiders occur in many types of habitats and are often
very abundant. Typical temperate habitats may support
up to 800 individual spiders per square meter. Point
estimates of spider diversity suggest that more than
600 species may be found in a single hectare of tropical
forest.
Unique derived characters
that define spiders include cheliceral
venom glands (rarely lost, e.g., the family Uloboridae),
abdominal spinnerets,
and the modification of the male pedipalps
into sperm transfer organs. Throughout
this section you will see highlighted
words. Hover the mouse cursor over them for more info.
Anatomy of a spider
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Although
spider bites are widely feared, few species are dangerous
to humans. Brown recluse spiders (and their relatives,
genus Loxosceles) and black widow spiders (genus
Latrodectus) are the most medically important
spiders in the United States.
In the United States, brown recluse spiders are found
mostly in the Midwest and Southwest. Brown recluse venom
causes a small, dry, irregular necrotic lesion that
heals very slowly. Contrary to popular belief, brown
recluse bites cannot be conclusively diagnosed from
the wound. Serious medical conditions including Lyme
disease, chemical burn, and Anthrax infection have been
misdiagnosed as brown recluse bites, delaying proper
treatment. It is not uncommon for such misdiagnoses
to occur outside the range of the brown recluse spider.
Widow spiders are widespread in the United States. Black
widow venom is a neurotoxin. Typical symptoms of envenomation
include swelling of the lymphatic nodes, profuse sweating,
rigidity of the abdominal muscles, facial contortions,
and hypertension. Antivenom is available to counteract
the effects of Latrodectus envenomation.
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Chelicera
and fang of Leptorhoptrum robustum (family Linyphiidae)
from Sakhalin Island, Russia. Note the pore at the tip
of the fang from which venom is exuded.
Chelicera
and fang of Uloborus diversus (family Uloboridae)
from Riverside, California. Uloborids lack venom glands.
Note the absence of a pore at the tip of the fang.
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The ability
to produce silk has evolved independently in several
arthropod
lineages. However, spiders are the only group to use
silk throughout their lives. Silk is spun through spinnerets
located on the posterior part of the abdomen. In addition
to its conspicuous use as a snare to trap prey, silk
is used to line burrows, construct retreats and molting
chambers, make sperm webs, protect developing eggs,
and as a dragline.
Individual species are able
to produce up to seven distinct types of silk, each
with a specialized function. Complex snares including
orb webs incorporate several distinct types of silk.
Some spiders periodically eat their web and are capable
of rapidly recycling most of the protein into fresh
silk. Some spiders, especially
small species and immature individuals, use silk in
a form of airborne travel called ballooning. To balloon,
the spider climbs to a high point and releases silk
into the air. When the drag on the silk exceeds the
spider's mass, the spider releases itself into the air.
Silk is a protein fiber produced
in glands that terminate in spigots on the abdominal
spinnerets. In the gland, silk is a water-soluble liquid
protein soup. As the silk is spun, it passes through
an acid bath. The acid hardens the silk by causing the
molecules to reorient. Complimentary regions of the
silk molecule align and bond together in multi-layered
stacks, forming protein crystals. These crystals are
interspersed in a matrix of loosely arranged amino acids.
The protein crystals give the silk its strength while
the loose matrix provides elasticity.
The physical properties of
silk are remarkable. Tensile strength is the greatest
stress a material will tolerate before failure. Silk
is stronger than most natural materials and is about
half as strong as steel. However, silk is extremely
extensible; it can tolerate substantial distortion (i.e.,
strain) before failure. The product of stress and strain
is expressed as toughness and is the total amount of
energy a material will absorb before failure. Silk has
extremely high toughness; steel tolerates very little
distortion in shape, and so is not very tough.
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Spinnerets
of the widow spider Latrodectus hesperus (family
Theridiidae), female specimen from Riverside, California.
Egg
case of the widow spider Latrodectus variolus
(family Theridiidae), from Maryland.
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Basal
araneomorph spiders produce adhesive silk from
a plate-like cribellum just anterior to the spinnerets.
The cribellum is covered in tiny spigots. Cribellate
silk is composed of hundreds of very fine dry silk fibers
around a few thicker core fibers. The physical basis
of stickiness in cribellate silk is not well understood,
but the adhesive force is proportional to the surface
area contact between the silk and the object being held.
Cribellate silk is combed out from the cribellum using
the calamistrum, a group of specialized, curved setae
on the metatarsus of the fourth leg.
Fourth metatarsus of Waitkera
waitakerensis (family Uloboridae) showing calamistrum,
a row of modified setae used to comb silk from the cribellum.
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Cribellum
of Deinopis spinosa (family Deinopidae) from
Gainesville, Florida.
Detail of cribellum of Deinopis
spinosa (family Deinopidae) from Gainesville, Florida.
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Fourth tarsus of Latrodectus hesperus
(family Theridiidae) from Riverside, California. Theridiid
spiders use the tarsal comb to
immobilize
prey with sticky silk before applying a venomous bite.
Thwaitesia bractcata (family
Theridiidae) from Guyana. Modified setae compose the
comb of the fourth tarsus of theridiid spiders.
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Theridiid
spiders usually have a comb of specialized setae
on the ventral part of the fourth tarsus. They use this
comb to through large drops of viscous sticky silk in
the course of subduing prey. This is known as a sticky-silk
wrap attack. Theridiids do not bite their prey until
they are completely immobilized by silk. Tarsal combs
can also be found in other spider families, including
deinopids,
nesticids,
and synotaxids.
The function of the nesticid and synotaxid comb is similar
to that of the theridiid comb; deinopids also wrap prey
in silk. However, they use a different kind of silk
and the role of the deinopid comb in prey-wrapping behavior
is unknown.
Deinopis spinosa
(family Deinopidae) from Gainesville, Florida. A tarsal
comb is present in several spider families including
the Deinopidae. Note differences in the form of the
comb setae in theridiids and deinopids.
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| Adult
male spiders have their pedipalps
modified into sperm transfer organs. The form of the male
palpal organ is extremely variable and is critical in
spider taxonomy. Minimally, the palpal organ consists
of a bulb containing a sperm duct and a terminal embolus.
The bulb is attached to the palpal tarsus, which is called
the cymbium. The cymbium may be ventrally excavated, partially
enclosing the palpal bulb. The testes are located in the
abdomen. There is no connection between the testes and
the pedipalps. Instead, sperm is extruded from a furrow
in the ventral part of the abdomen. A sperm web is typically
spun to receive the sperm. To transfer the sperm, the
spider inserts his embolus into the sperm droplet and
draws the fluid into the palpal organ, probably by capillary
action. During copulation, the male transfers his sperm
to the female by inserting the embolus into the female
genitalia. |
Male palp of the widow spider Latrodectus
hesperus (family Theridiidae) from Riverside, California.
In spiders, the male palp is modified into a sperm transfer
organ.
Epiandrous gland spigots of Deinopis
spinosa (family Deinopidae) from Gainesville, Florida.
These spigots are used to make the sperm web.
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Female genitalia (epigynum) of
Latrodectus hesperus (family Theridiidae) from Riverside,
California. Epigynum was removed from the abdomen and
mounted so that internal structures are visible.
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Adult female
spiders usually have a sclerotized plate called an epigynum
located on the ventral part of the abdomen. The epigynum
typically consists of a pair of copulatory ducts that
conduct the embolus (terminal portion of the male palpal
organ) to the spermathecae, where the sperm are stored.
Separate fertilization ducts conduct the sperm to the
uterus, where the eggs are fertilized. Fertilization is
the prevue of the female, who releases the stored sperm
as the eggs pass from here ovaries to the silken egg case
she has constructed. This system of female genitalia is
known as entelegyne. Some spiders including the most primitive
groups do not have separate copulatory and fertilization
ducts. Instead, the sperm enters and leaves the spermathecae
through the same system of ducts. This system of female
genitalia is known as haplogyne. |
| In
some species of the spider subfamily Erigonine (family
Linyphiidae), males have their head regions modified with
various assortments of lobes, pores, pits, sulci, and
modified setae. In the few species with modified heads
for which mating behavior has been observed, the female
interacts with these modified structures, often inserting
her fangs into the sheath-like sulci of the male. During
copulation, the male may secrete liquid from concentrations
of pores associated with the sulci. It has been suggested
that these secretions have nutritional value and are offered
to the female as an incentive to mate. Note that the mating
biology of the species illustrated here has never been
observed. Similar head modifications can also be found
in some other spiders, most notably members of the theridiid
subfamily Argyrodinae. |
Head region of male Erigone vicana
(family Linyphiidae) from Cuzco, Peru. The head is modified
with sulci behind the eyes.
Detail of sulcus, Erigone vicana
(family Linyphiidae) from Cuzco, Peru. Female erigonines
may insert their fangs into the male sulcus during mating.
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Credits
All images were taken by Jeremy Miller using
the Amray 1810 at the Smithsonian's National Museum of Natural History
Scanning Electron Microscope Facility with assistance from Scott Whittaker.
Most images were taken as part of: "Assembling the Tree of Life:
Phylogeny of Spiders," a National Science Foundation grant (DEB
0228699) to W. Wheeler, L. Prendini, J. Coddington, G. Hormiga and
P. Sierwald. Additional images were taken as part of Miller's dissertation
research, supported by an NSF-PEET grant to G. Hormiga and J. Coddington
(DEB 9712353). Some information on spider biology was modified from
Miller, J. A. & D. Ubick. 2004. Spiders. Pages 105-128 in: Borror
and DeLong's Introduction to the Study of Insects. Seventh Edition.
C.A. Triplehorn & N.F. Johnson.
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