Invertebrate Anatomy OnLine
Dermacentor variablis ©
Copyright 2001 by
is an exercise from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises are can be accessed through the ling on the left. A glossary and
chapters on supplies and laboratory techniques are also available through this
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Euchelicerata, Arachnida C,
Lipoctena, Apulmonata, Holotrachea, Cryptoperculata, Acarinomorpha, Acari O,
Ixodida, Ixodidae F (Fig
16-15, 18-44, 18-47)
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa such as
segmented body primitively bears a pair of jointed appendages on each segment. The
epidermis secretes a complex cuticular exoskeleton which must be molted to
permit increase in size. Extant
arthropods exhibit regional specialization in the structure and function of
segments and appendages but the ancestor probably had similar appendages on all
segments. The body is typically divided into a head and trunk, of which the
trunk is often further divided into thorax and abdomen.
gut consists of foregut, midgut, and hindgut and extends the length of the body
from anterior mouth to posterior anus. Foregut
and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all
epidermal surfaces of arthropods. The
midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
coelom is reduced to small spaces associated with the gonads and kidney. The
functional body cavity is a spacious hemocoel divided by a horizontal diaphragm
into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes
there is a small ventral perineural sinus surrounding the ventral nerve cord.
hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps
blood to the hemocoel. Excretory
organs vary with taxon and include Malpighian tubules, saccate nephridia, and
organs also vary with taxon and include many types of gills, book lungs, and
nervous system consists of a dorsal, anterior brain of two or three pairs of
ganglia, circumenteric connectives, and a paired ventral nerve cord with
segmental ganglia and segmental peripheral nerves. Various
degrees of condensation and cephalization are found in different taxa.
is derived with centrolecithal eggs and superficial cleavage. There
is frequently a larva although development is direct in many. Juveniles pass
through a series of instars separated by molts until reaching the adult size and
reproductive condition. At
this time molting and growth may cease or continue, depending on taxon.
is a large taxon that includes spiders, scorpions, pseudoscorpions, ticks,
mites, horseshoe crabs, sea spiders, and many others. The
group originated in marine habitats but almost all modern chelicerates are
body is divided into an anterior cephalothorax with six pairs of appendages and
a posterior abdomen which, in most groups, does not bear appendages or has
highly modified appendages. The
first appendages of the cephalothorax are the chelicerae. Antennae
are not present and the brain has two regions rather than the three found in
are primitively biramous but are uniramous in almost all Recent taxa.
segments of the cephalothorax are fused and covered by a dorsal shield, or
carapace. Two median eyes are present.
are the terrestrial descendents of the early aquatic chelicerates. The
taxon includes the mites, scorpions, pseudoscorpions, spiders, harvestmen, and
several other taxa. All
are adapted for a terrestrial existence with internal gas exchange and a
waterproof integument. Some
are capable of silk production and many use toxins to subdue or kill the prey. Most
are carnivores, digestion is usually outside the body, and food is liquefied
is one of the most successful arachnid taxa with 40,000 known Recent species of
ticks and mites. Segmentation is suppressed in acarines and tagmatization
reduced to two unique tagmata with no distinction between the more conventional
cephalothorax and abdomen. By far the larger tagma is the posterior idiosoma
consisting of the old abdomen and most of the cephalothorax. The much smaller
gnathosoma (= capitulum) is derived from the anteriormost region of the old
consists of the chelicerae, pedipalps, preoral cavity, and some of the anterior
exoskeleton. The idiosoma bears the four pairs of legs typical of the
chelicerate cephalothorax. The gonopore is ventral, on the midline between the
posterior pair of legs.
feeding habits are enormously diverse and they may be herbivorous, carnivorous,
or parasitic. Like other arachnids, most liquefy the food externally and ingest
it as a liquid. Ticks parasitize terrestrial amniotic vertebrates and many are
important disease vectors. The hemal system is reduced and usually lack a heart.
Tracheae are present in most. Excretion is accomplished by a pair of saccate
nephridia and 1-2 pairs of Malpighian tubules. The central nervous system is
highly cephalized and all former segmental ganglia are coalesced into the
subesophageal ganglion. Acarines are gonochoric and fertilization is internal,
although sperm transmission is usually indirect with spermatophores. The life
cycle of most acarines, but not hard ticks, includes a larval and three nymphal
instars between egg and adult. Larvae
have three pairs of legs, nymphs and adults have four.
(= Anactinotrichida) is one of three higher acarine taxa. It
includes about 75 families of ticks (Ixodida) and related mites (Mesostigmata,
setae lack actinochitin in contrast with those of mites. The anus is covered by
sclerites and trichobothria are absent. The gnathosoma is encircled by a
are larger than mites (2-30 mm), have a leathery integument, a relatively larger
gnathosoma, and toothed chelicerae. The first tarsus has a sensory pit known as
Haller’s organ. Ticks
are obligatory blood-sucking parasites that spend only most of their life off
the host. The tick leaves the host following the meal and most of its life is
spent off the host. Hard ticks require one blood meal during each instar in
order to molt to the next instar. Each
instar usually feeds on a different host species. Adult
females must have a blood meal to mate and produce eggs and may consume 50 times
their weight in blood. Adult males of some species do not feed. Soft ticks are
similar but may have more than one blood meal per instar. Ixodida is divided
into two noteworthy taxa (and a third with only one species).
the soft ticks, have no scutum and a soft, leathery cuticle. Both sexes become
distended when engorged with blood. The gnathosoma is ventral and is not visible
in dorsal view. Soft ticks exhibit no sexual dimorphism. About 140 species are
known from reptile, bird, and mammal hosts but most are found on birds. Argas and Ornithodoros are
the only large taxa.
the hard ticks, have a stiff dorsal integument, the scutum. Only
females become distended when engorged. The gnathosoma is clearly visible in
dorsal view. Hard ticks are sexually dimorphic, the sexes differing most notably
in the size of the scutum. The basic acarine life cycle is modified in hard
ticks so it consists of egg, larva, nymph, adult, with one, rather than three,
nymphal instars. Hard ticks are chiefly parasites of mammals. Most ticks, about
650 species, belong to this taxon. Some
more common North American genera are Dermacentor,
ticks of several genera can be collected from a variety of mammalian hosts of
which dogs are probably the most convenient. This
exercise is written specifically for the dog tick,Dermacentor variabilis but
can be used with other ixodid (hard) ticks. Dermacentor is
ornate (with silver or white dorsal markings) and has 11 festoons and a pair of
eyes. NoteworthyDermacentor species
are D. variabilis (American
dog tick) and D. andersoni (Rocky
Mountain wood tick, which is the vector for Rocky Mountain spotted fever). Ixodes,
the largest tick genus, lacks festoons, is inornate, and has no eyes. The anal
groove encircles the anus anteriorly, rather than in its more usual posterior
location. A few of its 40 species are I.
scapularis (deer tick, the
vector of Lyme disease),I. pacificus (on
deer, cattle, and dogs in California), I. ricinus (European
castor bean tick, named for its castor bean shape and markings). Rhipicephalus is
inornate with eyes and festoons. Rhipicephalus
sanguineus, the brown tick, occurs worldwide. Amblyomma includes
about 100 species of ornate, eyed ticks with festoons. Amblyomma
americanais the Lone Star tick found in the southern US and Central America
on a variety of hosts, including humans.
Figure 1. Dorsal view of a male Dermacentor
variabilis from Whittier, North
Carolina. Legs have been omitted. Acarina50L.gif
are easiest to manipulate if preserved in 40% isopropyl alcohol. Preservation
is not necessary, however, and specimens can be observed alive if desired.
Students with access to hosts can provide their own ticks. The
study should be conducted with a dissecting microscope with the specimens
immersed in water or alcohol in a 6-cm culture dish. This
exercise is limited to external anatomy. If possible each student should have
male, unengorged female, and engorged female ticks. At a minimum each student
should have either a male or a female unengorged tick. Sexes
are easily distinguished. Males
have a large inflexible dorsal shield (= scutum) over the anterior end of the
body (Fig 1) whereas the scutum of females is much smaller (Fig 4) leaving the
posterior end of the body flexible and distensible to accommodate a large blood
meal. The author is grateful to Taylor and Jake for cheerfully donating the
specimens on which this account is based.
a mature, unengorged tick with the dissecting microscope. Examine
the dorsal surface. Acarines
have suppressed the usual chelicerate tagmatization (cephalothorax and abdomen)
and instead the body is divided into a very large idiosoma,
which bears four pairs of legs, and a much smaller anterior gnathosoma (=
capitulum), which bears the mouthparts (Fig 1, 18-40A). The
idiosoma is derived from the abdomen and posterior cephalothorax of the arachnid
ancestor and accordingly bears the walking legs and contains the brain. The
gnathosoma is the extreme anterior region of the cephalothorax and bears the
chelicerae and pedipalps. The gnathosoma is not identical to the head, rather is
only the anterior part of the head.
hard tick gnathosoma is
distinct and readily visible both dorsally and ventrally. It is a complex
structure but its chief features are relatively simple, although small and not
always readily visible (Fig 1, 18-40A).
your specimen was torn abruptly from its host there is likely to be host skin
still caught in the gnathosoma and it must be removed. Use
fine forceps to extract it. As
you will soon see, the gnathosoma is equipped with recurved (posteriorly
pointing) teeth designed specifically to prevent what you are attempting to do,
i.e. separate the tick from the host's skin. Remember
that the teeth face posteriorly and move the skin in that direction to unhook it
from the teeth. Then pull the skin anteriorly to remove it. Repeat
the process as often as necessary to free the skin. Now use microneedles to
clean away any remaining skin particles that obscure your view.
relevant features of the gnathosoma are the basis capituli, chelicerae,
pedipalps, and hypostome. The
ringlike basis capituli is
the proximal base of the gnathosoma from which the other parts arise (Fig 2). It
fits into a notch on the anterior margin of the idiosoma. Protruding anteriorly
from the basis, the chelicerae are dorsal, the pedipalps lateral, and the
hypostome ventral. They
enclose the preoral cavity and the mouth opens between the ventral hypostome and
dorsal chelicerae. The
pedipalps are the sides of the preoral cavity. The pedipalps curve over the
chelicerae dorsally and make viewing the chelicerae difficult. They also
partially obscure the hypostome ventrally. Use fine forceps, microneedles and
45X to examine the mouthparts.
at the gnathosoma in dorsal view first and find the dorsal part of the basis
capituli, (= tectum, tegulum) (Fig 2). The pedipalps extend
anteriorly from the sides of the basis and enclose the preoral
cavity, chelicerae, and hypostome. The
pedipalps are composed of four articles, the proximal trochanter, femur,
patella, and small distal tibiotarsus, which can be protracted. On the dorsal
midline the basis capituli extends anteriorly to form a tubular cheliceral
sheath that encloses the
long slender chelicerae. The sheath is visible dorsally but will be mostly
covered by the pedipalps until you move them (the pedipalps) aside with your
fine forceps or microneedles . Once
the pedipalps are out of the way the cheliceral sheath is clearly visible and
you can also see part of the toothed hypostome protruding anteriorly beyond the
chelicerae themselves are not visible, being hidden inside the sheath. They
are triarticulate with two articles forming a distal pair of tiny toothed
bladelike fingers (Fig 18-43B). The
toothed edges of both fingers face laterally and are not chelate as they are in
most arachnids. Instead
they are tiny blades used to cut into the host's integument, this being their
only function. The
chelicerae are inactive after penetration is achieved.
distinct circular porose
areas of the cuticle are
evident on the dorsal basis capituli (Fig 2). The porose areas are, like the
fovea, perforated cuticular plates through which dermal glands release their
Figure 2. Dorsal view of the gnathosoma of a female Dermacentor
variabilis with the pedipalps
moved laterally to reveal the cheliceral sheath. Acarina51L.gif
the tick over and look at the ventral gnathosoma. Once again the pedipalps
dominate the view but the hypostome can
be seen as an anterior projection of the basis capituli (Fig 3). Move
the pedipalps laterally to get a better view of the hypostome. The
hypostome has six rows of readily visible recurved teeth on
its ventral surface. With
these teeth the hypostome is an effective holdfast to anchor the tick in the
host's skin. It
is not involved in cutting into the skin.
dorsum of the idiosoma is partially or completely covered by the tough,
inelastic dorsal shield, or scutum.
In females the scutum is relatively small and is restricted to the anterior end
of the idiosoma (Fig 4, 18-40A) whereas in males it covers the entire dorsum of
the idiosoma (Fig 1). The large inflexible scutum of males prevents the idiosoma
from distending when the gut is engorged. Consequently
males consume less than females and do not become distended. Females, with only
a small part of the body covered by the inelastic scutum, can become grossly
distended when engorged (Fig 6). Any swollen individuals among your specimens
are female. The scutum of Dermacentor is
ornate, with silver markings.
posterior margin of the idiosoma of most hard ticks is scalloped with 11
rectangular festoons (Fig
1, 4, 18-40A) (Ixodes has no
festoons). A pair of eyes is
present on the lateral edge of the scutum (Fig 1, 4). Two
inconspicuous, circular, fovea are
found beside the midline in about the middle of the dorsum (Fig 4). Fovea
are porous cuticular plates through which open secretory foveal glands. Fovea
are absent in male Dermacentor
3. Ventral view of the gnathosoma of a female Dermacentor
variabilis with the pedipalps
moved aside. Acarina52L.gif
The venter of
the idiosoma bears the coxae of
the four pairs of legs, the gonopore, anus, and spiracles (Fig 5). In both
sexes, the gonopore (=
genital aperture) is located on the ventral midline between the coxae of the
second pair of legs. It
is surrounded by a lightly sclerotized genital
plate and is at the apex of
a long triangular suture, the genital
groove, whose base is at the posterior margin of the idiosoma. The anus is
also on the ventral midline but farther posterior, although well anterior to the
posterior end. It
too is surrounded by a sclerite, this one the anal
plate. The anal
groove is a semicircular
suture posterior to the anus. (The anal groove is anterior to the anus in Ixodes.) The
two spiracles are
lateral, at about the level of the anus just posterior to the fourth legs (Fig
spiracle consists of a curved slit, the ostium,
surrounded by a large oval spiracular
spiracular plate is perforated by numerous pores. The pores and ostium open into
a subostial chamber and atrium from which tracheae branch to supply the body
with oxygen. Apparently
the ventilating current passes through the pores, rather than the ostium. Water
loss from expired air is less through the pores than through the ostium. Larval
ticks have no spiracles.
Figure 4. Dorsal view of an unfed female Dermacentor
variabilis. The right legs have been omitted. Acarina53L.gif
the four pairs of walking legs in
ventral view. Each leg consists of a linear series of articles which are, in
order from proximal to distal, coxa,
trochanter 1, trochanter 2, femur, patella (=
genu), tibia, tarsus (with
two parts), and the pretarsus (Fig
5). The platelike coxae are embedded in the ventral exoskeleton where they form
two longitudinal lateral rows of dark sclerotized plates surrounded by the pale,
less sclerotized ventral cuticle (Fig 3). The pale median area is wider in
females than in males. The first coxa has two large spurs,
one lateral and one medial. The remaining coxae also have two spurs but they are
much smaller. The
lateral spurs are acute and pointed whereas the medial spurs are short, broad,
and rounded apically.
Figure 5. Ventral view of an unfed female Dermacentor
variabilis. The left legs have been omitted. Acarina54L.gif
tarsus, is divided into two subarticles, or tarsomeres. The pretarsus,
or foot, is pale, delicate and relatively unsclerotized. Its
structure is complex and includes two claws and
an adhesive footpad, or pulvillus (Fig
organ is a sensory pit
located on the dorsum of the first tarsomere of the first leg at the junction
with the second tarsomere (Fig 4). It
is small, but readily discernable with high power of the dissecting microscope.
The first leg has a sensory function and is used to find and recognize an
Figure 6. A distended female Dermacentor
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Preserved ticks; male, unfed female, fed female
40% isopropyl alcohol
6-cm culture dish
Microneedles on applicator sticks