Invertebrate Anatomy OnLine
Copyright 2005 by
is an exercise from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links on the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
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.
Arthropoda P, Myriapoda SC,
Progoneata, Dignatha, Diplopoda C,
Chilognatha, Helminthomorpha, Eugnatha, Merochaeta SO,
Xystodesmidae F (Fig
6-15, 20-14, 20-15)
includes Onychophora, Tardigrada, and Arthropoda. These
taxa share segmentation, a hemocoel, saccate nephridia, ecdysis of a secreted
chitinous but non-collagenous exoskeleton, loss of locomotory cilia, a tubular,
dorsal, ostiate heart in a pericardial sinus, a coelom reduced to end sacs and
gonocoel, and paired segmental legs.
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa. The
body is segmented and primitively bears a pair of jointed appendages on each
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. The body is typically divided into a head and trunk, of
which the trunk is often itself 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. 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 and from 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.
includes arthropods in which the third head segment bears a pair of mandibles. As
currently conceived this taxon includes myriapods, hexapods, and crustaceans. Appendages
may be uni- or biramous and habitats include marine, freshwater, terrestrial,
and hexapods share tracheae and a single pair of antennae and are sister taxa in
which have gills and lack tracheae, are excluded and form the sister group.
body consists of a head and trunk with numerous segments but no tagmosis into
thorax and abdomen. Myriapoda
includes the familiar centipedes and millipedes, as well as symphylans and
myriapod taxon comprises millipedes and pauropods, both of which have two pairs
of mouthparts instead of the three of the ancestral tracheate, insects,
centipedes, and symphylans. Millipedes
have a pair of mandibles and a single gnathochilarium, which probably arose
through fusion of the two first maxillae. The
second maxillae are lost in dignathans.
millipedes are elongate, often worm-shaped, mandibulates, although some are
short and resemble woodlice. The
body consists of a head and a trunk of many segments, most of which bear paired
of the trunk segments occur in fused pairs, called diplosegments, each of which
bears two pairs of legs and two pairs of spiracles externally. Internally
each diplosegment has two pairs of ganglia, two pairs of ostia and two pairs of
lateral blood arteries.
head bears a pair of antennae, a pair of mandibles, and a pair of fused first
maxillae. There is no second maxilla (= labium) and its segment is absent. The
exoskeleton is strengthened with calcium salts and is correspondingly harder
than that of most hexapods. The
heart is an elongate ostiate dorsal tube extending most of the length of the
body. Excretion is accomplished via a single pair of Malpighian tubules and a
pair of maxillary glands (= saccate nephridia). Respiration
is via segmentally arranged tracheae with segmental spiracles.
nervous system consists of a three-part mandibulate brain and a double,
ganglionated, ventral longitudinal nerve cord. The
subesophageal ganglion consists solely of the mandibular and maxillary ganglia. Millipedes
are gonochoric with the gonopores located anteriorly, on segment 3. Sperm
transfer is indirect with intromittent organs and spermatophores. Eggs are laid
in the summer and most species overwinter as adults. Juveniles hatch with three
pairs of legs and add body segments and additional legs with successive molts
(Fig 20-12F). Sexual dimorphism is slight.
including polydesmids, are chemically protected by repugnatorial (scent) glands
that produce and release toxic or repellant substances including aldehydes,
phenol, quinones, iodine, hydrogen cyanide, and chlorine.
Figure 1. Dorsal view of the trunk of a female
polydesmid millipede. The
head is hidden by the first trunk segment. Milliped22L.gif
the flat-backed millipedes, are relatively large (up to 13 cm but with some as
small as 3 mm) millipedes with 20 rings ("segments”). Polydesmids range in size
from 0.3 to 13 cm. Pachydesmus contains
the largest North American Polydesmida and some are common and familiar animals
of lawn and garden. The
body proper, although circular in cross section, appears to be, and functionally
is, dorsoventrally flattened due to large lateral plates (paranota) which extend
from the sides of the cylindrical body (Fig 1, 8, 20-8A, 20-10C). The
four exoskeletal sclerites of each segment and diplosegment are fused together
to form a rigid ring. Many
are brightly (aposematically) colored, frequently with a dark, often black,
background with contrasting bright red, orange, yellow, or cream markings.
Polydesmids are blind without ocelli or eyes. This
is the largest millipede order with more than 2700 known species (as of 1992).
in general, are negatively phototactic, nocturnal detritivores that feed on dead
few eat living plants and can be a problem in greenhouses but most consume
decaying leaves and wood. A few are carnivores feeding on other arthropods and
earthworms. Millipedes live in moist habitats including soil, leaf litter, under
stones, decaying logs, under bark, or in caves. Millipedes join earthworms in
assuming the chief responsibility for recycling organic molecules in forest
ecosystems. About 10,000 species have been described but tens of thousands more
are thought to await discovery, especially in the tropics, where millipedes are
most diverse. Five higher taxa (superorders) are recognized.
are dorsoventrally depressed millipedes belonging to an eco-morphological group
known as “flat wedgers”. Wedgers
live in the spaces between flat surfaces such as layers of decaying leaves on
the forest floor. The
flat shape and powerful musculature allows the animal to push its way between
the leaves, wedging them apart.
Figure 2. Ventral view of the head and anterior
trunk. The legs have been omitted from ring 6 to reveal the two spiracular
exercise was written using specimens of Pachydesmus collected
in Greenwood Co. South Carolina but it can be used with any polydesmid
millipede. Living, freshly sacrificed, or preserved specimens can be used. Anesthetization
can be accomplished with chloroform or carbon dioxide. Living specimens will
release an odiferous repugnatorial substance when handled. The study should be
conducted with magnification, preferably that of a dissecting microscope. This
exercise is limited to external anatomy.
millipede body, like that of all arthropods, is covered by a secreted
exoskeleton, or cuticle,
composed of chitin and protein. In some arthropods, notably millipedes and
crustaceans, the cuticle is hardened, not only by sclerotization (= protein
tanning) but also by the incorporation of calcium salts. Consequently
millipede sclerites are often very hard, rocklike, and totally inflexible. Millipedes
play an important role in calcium cycling in many terrestrial ecosystems.
If an extra specimen (dead) is available for the purpose, remove a segment from
its body and place it in a small culture dish of 10% hydrochloric acid. With
fine forceps and needles test the rigidity of the cuticle and note its ability
to resist deformation. Cover the dish and set it aside, looking at it
occasionally during the laboratory period. You
will notice the slow evolution of small bubbles of carbon dioxide resulting from
the reaction between the acid and calcium carbonate in the cuticle. At
the end of the period test the rigidity again, trying to bend the cuticle with
your forceps. Is there
any difference? Try
again after 24 hours or during the next lab period. Now
is there a difference? How
do you account for the flexibility of the cuticle after a day or two in acid? <
Figure 3. Left side view of the head. Milliped25L.gif
elongate body is divided into two distinct tagmata, the short anterior head and
the long, obviously segmented, posterior trunk (Fig
The head consists
of a smooth sclerotized head
capsule (= cranium,
epicranium) to which is fused the labrum and with which the three pairs of head
appendages articulate (Fig 3). The
capsule forms the dorsal and anterior surfaces of the head. The
unusually large mandibles are the sides and the gnathochilarium (= lower lip) is
the ventral surface. Posteriorly the head joins the trunk via a short neck. The
region of the head capsule ventral to the antennae is the epistome (Fig
3, 4). The
small, narrow, toothed labrum (=
upper lip) is immovably fused to the epistome ventrally (Fig 4, 6). Anteriorly
the head capsule is deeply invaginated to form the dorsal wall of the preoral
cavity (more later).
anteriormost head appendages are the two filiform antennae,
each consisting of 7 cylindrical articles (Fig
3, 4, 20-9). The
antennae articulate with the anterior/dorsal surface of the head capsule. The
tip of the distal antennal article has four tiny chemoreceptive cones
characteristic of diplopods, as well as numerous mechanoreceptive setae. A
walking millipede continuously taps the ground with the tips of its antennae.
Figure 4. En
face view of the head.
and pauropods differ from other tracheates (chilopods, symphylans, and insects)
in having only two pairs of mouthparts (mandibles, gnathochilarium) instead of
the usual three (mandibles, maxillae, labium). The
former form a taxa known as Dignatha, in reference to their two pairs of
mouthparts (di = two, gnath = jaw) whereas the latter are trignathan. The
dignathan mandibles, of course, are homologous to those of other tracheates but
the origin of the gnathochilarium is disputed. Some
believe it to be homologous to the fused first maxillae (= maxillae) of
tracheates that have lost the labium (= second maxillae) whereas others think it
arose through the combination of the maxilla and labium of the ancestral
labium-like gnathochilarium (= lower lip) is the third pair of head appendages
and is posterior to the mandibles. It
is best studied before the mandibles, however, because it almost completely
covers and hides the jaws of the mandibles from view. Like the insect labium,
the gnathochilarium is
a single piece formed in development by the median fusion of two lateral
appendages. It is a broad flat plate ventral on the millipede head (Fig 1, 5,
gnathochilarium consists of two large lateral stipes,
a median promentum (=
mentum, prementum), and two medial lingual
plates (= lamellae
linguales) (Fig 5). Each
stipes has two distal sensory
pegs bearing contact
Figure 5. Posterior-ventral
view of the gnathochilarium as
seen in an intact, undissected animal. Milliped29L.gif
mandibles are the second pair of head appendages and first pair of mouthparts.
Millipede mandibles are large, powerful and heavy, making up most of the side of
the head (Fig 3). Each
consists of a heavy, immovable, biarticulate mandibular base and a toothed,
movable, distal gnathal lobe. The mandibular
base is best viewed from the
side (Fig 3) where it forms most of the side of the head. It
consists of a proximal basal article, the mandibular cardo,
and a distal basal article, the mandibular
gnathal lobe is also visible laterally but to see it in more detail use forceps
to pull the free edge of the gnathochilarium posteriorly and look at from the
ventral aspect. The gnathal
lobe has two heavily
sclerotized movable teeth,
the outer tooth and the inner tooth, a rasp-like molar
plate, and a set of pectinate
lamellae (Fig 6). The teeth
are conspicuous in an intact, undissected specimen, the pectinate lamellae are
exposed to view but not conspicuous, and the molar plate is deep in the preoral
cavity and difficult to see without further dissection.
right mandible in ventral view with the gnathochilarium and left mandible
gnathal lobes articulate with the stipes of the base via two condyles (Fig 6)
and, during feeding, are moved transversely to oppose each other. The teeth are
for cutting and the molar is for crushing or grinding. The first three pairs of
legs may function as auxiliary mouthparts to manipulate food items and transfer
them to the jaws (reminiscent of the maxillipeds of crustaceans).
the gnathochilarium pushed aside look into the space thus revealed. It
is the preoral cavity (Fig
7, 6), from which the mouth (which you will not see) opens. The millipede
preoral cavity is bounded anteriorly by the labrum and epipharynx, laterally by
the mandibles, and posteriorly-ventrally by the gnathochilarium. The
epipharynx of the head capsule is the inner surface of the fold of cranial
exoskeleton that forms the anterior part of the head. It
extends from the labrum to the mouth and is mostly a thin unsclerotized membrane
but it is not visible externally. The epistome is the outer surface of this
most millipedes have ocelli (Fig 20-11) laterally on the head, eyes are absent
in polydesmids (Fig 3, 4).
pair of sensory Tömösváry
organs (TIM-ish-vary) is
present on the head capsule just posterior and lateral to the antennae (Fig 4). They
are small flat oval cuticular rings at the bases of the antennae. Tömösváry
organs are thought to be hygroreceptors for assessing humidity, or possibly
Figure 7. Sagittal
section of a millipede head to show the preoral cavity. (Redrawn
from Snodgrass, 1951). Milliped32L.gif
Segments, Diplosegments, and Rings
polydesmids the trunk is divided into 20 trunk rings, of which a few are simple
segments but most are double segments formed during development by fusion of two
simple segments. Trunk rings composed of a single segment are called just that,
segments, those consisting of two segments are known as diplosegments, and the
term ring, or trunk ring, is used here in reference to all trunk annulations,
segments and diplosegments alike. Segments
have one pair of legs or no legs whereas diplosegments, as you would predict,
have two pairs of legs. The presence of diplosegments is, of course, the reason
for the name Diplopoda and it is an apomorphy of the taxon.
exoskeleton of a typical arthropod segment consists of four sclerites (tergite,
two pleurites, and sternite) joined to each other by flexible articular
membranes to form a circle around the outside of the segment (Fig 16-1B). In
millipedes the sclerites of each circle are fused together to form a rigid hoop,
consisting of four regions with no flexible articular membranes or sutures
separating them. The tergal region of the hoop is the largest and forms an arch
covering the dorsum and upper sides of the ring (Fig 8). Each
tergal region bears two lateral flanges, the paranota, that extend to the side
(Fig 1, 8). The
pleural and sternal regions are smaller. The sternal region is midventral and
the coxae of the legs articulate with it. Each of the two pleural regions, which
are intermediate in size, extends from a paranotum to the base of a leg (Fig 8).
Figure 8. Anterior view of a single diplosegment.
Note that the body is circular, like that of a worm millipede, and the
impression of flattening is due to the paranota. Milliped30L.gif
1 differs from the others in
several respects. In the first place, it is a single segment that
has no appendages. Further, its sclerites are not fused to form a rigid circle
and instead are connected by flexible articular
its tergite, tergite 1, is expanded to form a dorsal shield, the collum (=
collar) (Fig 1, 20-9). Sternite
1 is small, yolklike, and
straddles the ventral midline.
first segment is usually considered to have no legs but it may look to you as if
the first pair of legs belong to this first segment and, in fact, there is some
discussion amongst diplopod specialists as to which segment the first pair of
2-4 are also single segments but each has a pair of legs. Ring
2 bears the first pair of
legs arise from sternite 2 (Fig
1) which is not rigidly fused to the other sclerites of the segment.
3-4 likewise are single
segments and each has one pair of legs. The sclerites of these segments are
fused to the other sclerites of their respective segments form rigid circles,
one around each segment. These circles have no independent sclerites and instead
are composed of tergal, pleural, and sternal regions (Fig 8, 20-10C).
4 is another single segment
with one pair of legs, the third. Ring 4 is the anteriormost ring to possess
spiracles and, being a single segment, it has one pair of them. Diplosegments
posterior to ring 4 have two pairs of spiracles (Fig 2, 11, 20-10B).
The spiracles are
ventral and open from the lateral corners of oval spiracular
apodemes on the
anterolateral corners of the sterna supporting each pair
of legs (Fig
11, 2). The
coxae arise from these corners also and may hide the apodemes from your view. Move
the leg aside, if necessary, to reveal the apodeme. The
spiracle is the anterolateral corner of the apodeme and is difficult to discern.
The spiracles of ring four are the largest of all and this is where you should
look to see your first spiracles. Each spiracle opens into an internal atrium. A
tuft of unbranched tracheae arises from each atrium. The
tracheae extend to the tissues to deliver oxygen. Millipede atria are the hollow
interiors of the apodemes. Apodemes in general are invaginations of the surface
cuticle that function as an endoskeleton for muscle attachment (Fig 16-1E). These
special spiracular apodemes have
an additional function in respiration. With
few exceptions millipedes are unable to close their spiracles making them
subject to desiccation in dry environments.
exoskeleton of each diplosegment is conspicuously divided into an anterior prozonite and
posterior metazonite by
a transverse suture (Fig
10, 11, 1). The paranota are on the metazonites and the two pairs of legs
likewise arise from two sterna on each metazonite. Each metazonite bears two
sternites and two pairs of legs. The terga of the metazonites are expanded
laterally by the paranota so that the metazonites are much wider than
prozonites. The prozonite of each ring telescopes into the metazonite of the
preceding ring and is encircled by it. These two regions do not correspond to
the two original segments that fused to form the diplosegment.
Figure 10. Dorsum of ring 10, disconnected from the
adjoining rings to reveal the prozonite. Milliped34L.gif
half the paranota have a pair of small repugnatorial
gland pores through which
the repugnatorial glands (= stink glands) open to the exterior. These
tiny, but clearly visible (with magnification), pores are very near the dorsal
edges of the paranota of rings 5, 7, 9, 10, 12, 13, 15, 16, 17, 18, and 19) (Fig
1, 10, 20-10C). Focus
on the margins of the paranota with high power (45X) of the dissecting
microscope to find one. When
you find a pore confirm that it is an opening by slipping a microneedle into it.
Ring 7 of
males bears a pair of gonopods. These
will be discussed later.
19, the preanal ring, is the last diplosegment. It
has no legs and no spiracles but it does have repugnatorial gland pores.
posteriormost unit of the trunk, which in polydesmids is ring
20 (= anal ring), is often
called the telson (Fig
1, 14). It is not a true segment for it does not arise in the embryo from
teloblast areas as do real segments. It
has no legs but it bears the large anus,
which will be discussed in more detail later, on its ventral surface.
immature individuals of some millipedes there may be a zone of incomplete
legless segments near the posterior end, between the last legged ring and the
telson. This is the growth zone in which the mitotic teloblast areas are
producing new segments.
1-4 are considered by some authors to constitute a separate tagma, the thorax. Justification
for this is based on the single condition of the segments, legs that function as
accessory mouthparts, absence of repugnatorial glands, absence of spiracles in
all but one segment, and the presence of the gonopores. This
view then combines all the diplosegments into another tagma, the abdomen.
Figure 11. Venter of ring 10 disconnected from the
adjoining rings to reveal the prozonite. The legs have been removed for clarity.
present on all trunk rings except 1, 19, and 20. Millipede legs are strongly
homonomous, or similar to each other in size and morphology, with no regional
only exception being the extreme modification of the anterior pair of legs of
the male 7 th ring,
which become the intromittent organs known as gonopods, discussed below. Each
leg (except the gonopod) is a walking leg used in locomotion. The
legs extend laterally then ventrally to support the body as if in a hammock (Fig
8), rather like the legs of reptiles. Polydesmids
have a total of 31 pairs of legs.
leg consists of seven articles. In
the complete absence of consensus regarding the names, and even the number, of
leg articles, the nomenclature of Snodgrass (1951) is followed here. Thus
the articles are, in order from proximal to distal; coxa,
trochanter 1, trochanter 2, femur, tibia, tarsus, and the clawlike pretarsus (Fig
12). Only the tips of the pretarsi contact the substratum when walking.
Figure 12. A typical leg, the anterior left leg of
diplosegment 6. milliped24L.gif
gonopods are usually considered to be modified legs. Each gonopod consists of a
bulbous basal coxa from
which extends a small coxal
endite and a large
13, 20-12B). (A
telopodite is the movable part of an appendage.) In Pachydesmus the
endite is weakly sclerotized and flexible. The
large striking telopodite consists of two slender, gracefully curved, heavily
sclerotized, rigid, brown rami. The
posterior pair of legs on ring 7 are normal unmodified legs in polydesmids.
posteriormost division of the body, following ring 19, is the triangular telson (=
anal ring) (Fig 1). Not having arisen from embryonic teloblasts, is not
considered to be a true segment. The large anus is
on the venter of the telson protected on each side by a pair of clamshell-like
sclerites, the paraprocts (Fig
14, = anal plates). A third sclerite, the hypoproct (=
sub-anal scale) is immediately anterior to the anus on the venter. The
two paraprocts swing open laterally to permit the extrusion of fecal pellets. With
the paraprocts closed the anus is a long, midventral slit flanked by two
cuticular ridges, which are the edges of the paraprocts. With
your forceps open the paraprocts to expose the anus and reveal the spacious rectum,
or hindgut, within. Fecal
pellets are formed by the rectum prior to extrusion.
Figure 14. Ventral view of the posterior end of a
female. Most of the legs of ring 18 have been omitted for clarity.
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in the Kings Mountain region of North Carolina and the Coastal Plain of South
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textbook of arthropod anatomy. Cornell University, Ithaca (Reprinted by Hafner
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Dissecting set with microdissecting tools
Flat millipede, living or preserved
Small dissecting pan (anchovy tin with wax bottom)