Invertebrate Anatomy OnLine
Calpodes ethlius Larva ©
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links to the left. A
glossary and chapters on supplies and laboratory techniques are also available
through this link. 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.
Mandibulata, Tracheata, Hexapoda SC,
Insecta C, Dicondylia,
Pterygota, Metapterygota, Neoptera, Eumetabola, Holometabola, LepidopteraO,
Hesperiidae F, Hesperiinae sF (Fig
16-15, 20-14, 20-15, 21-23)
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 is divided into three tagmata; head, thorax, and abdomen. Appendages
are uniramous and a single pair of antennae is present. Three
pairs of legs and two pairs of wings are found on the thorax of most adults. Hexapod
legs are uniramous although there is increasing evidence that they evolved from
multiramous appendages of their ancestors. Gas
exchange is accomplished by trachea. Excretory
organs are Malpighian tubules and the end product of nitrogen metabolism is uric
is relatively little cephalization of the nervous system. Insects are gonochoric
with copulation and internal fertilization.
hexapods are insects. A
few hexapod taxa (orders) lack wings and have primitive mouthparts recessed into
the head and belong to Entognatha, the sister taxon of Insecta. Insects
have ectognath mouthparts and the adults (imagoes) of most taxa have wings.
winged insects. These insects are derived from a winged common ancestor. Adults
of most taxa have wings although they have been lost in some.
have no ocelli and there are six or fewer Malpighian tubules.
final larval instar pupates and undergoes a radical metamorphosis in which it is
converted to an imago, or adult. The
imago is sexually mature and in most taxa has wings whereas larvae are immature
and wingless. During
metamorphosis many or most larval tissues are dismantled and adult structures
built anew. Wings,
for example, are manufactured from clusters of undifferentiated cells known as
imaginal discs but not from preformed wingpads as in pauro- and hemimetabolous
and moths. Order
consisting of the polyphyletic “moths”, skippers (Hesperoidea), and scudders
and scudders are together known as “ butterflies”. Wings, body, appendages
covered with pigmented, dust-like epidermal scales or hair-like setae. Adults
with highly derived sucking mouthparts and liquid diet. Adult mouthparts consist
of large labial palps and a coiled tubular proboscis derived from the maxillary
herbivorous with typical chewing mouthparts. Larval
labial glands modified as silk glands. Holometabolous complete metamorphosis in
a pupal stage. In many moths
pupation occurs in a silk cocoon but most butterflies have no cocoon and the
pupa is known as a chrysalis, sometimes associated with a few silk fibers. Lepidopteran
pupae are obtect, with the appendages attached to the body over their entire
species of Lepidoptera have larvae large enough to be conveniently dissected as
an introduction to caterpillar anatomy. Chief among them are the Tomato and
Tobacco Hornworms in the genus Manduca (Sphingidae)
but Calpodes ethlius (= Hesperia
ethlius), the Greater Canna Leafroller, should be considered for this
purpose in places and seasons where and when it is available alive. It
has the advantage of having a transparent body wall through which can be
observed most of the internal organs in a living, active animal.
Two species of leaf-rolling
neotropical caterpillars occur on ornamental cannas (Canna) in the
southern United States. One,
the Lesser Canna Leafroller, Geshna
cannalis (Pyralidae), is
currently known only from the southeastern US but is presumed to occur in
tropical America since its host plants (cannas) have a tropical distribution. It
is a bit small for use in the teaching laboratory.
The second, the much larger
larva of which is the subject of this exercise, is the Greater Canna Leafroller, Calpodes
ethlius (Hesperiidae) known from
the Caribbean and much of South America including the Galapagos Islands and also
Bermuda. It has been introduced into the southern United States where it feeds
on ornamental cannas. The
adult is a butterfly known as the Canna Skipper or Brazilian Skipper. It is
largely restricted to members of the plant family Cannaceae but also occurs on
the closely related arrowroot (Maranta arundinacea), which is grown
commercially as a source of culinary arrowroot.
Leafroller larvae roll the
leaf margins of the host plant to form a tube held together by silk and used as
a domicile offering protection from predators and pesticides. Eventually the
domicile and nearby leaf tissue is consumed and another constructed near a fresh
food supply. Unlike the Lesser Canna Leafroller, Calpodes larvae
flick their frass (fecal pellets and debris) away from the domicile and do not
allow it to accumulate, presumably avoiding thereby attracting predators or
parasitoids. The fecal pellets are flipped away by the anal comb in a process
known as “fecal firing”. Adult
Greater Canna Leafrollers are relatively nondescript, fast-flying, brown
butterflies. Females, which are slightly larger than males reach 27 mm in
Greater Canna Leafrollers are
reported to be most common on red-leaved cannas but also occur on those with
green leaves. In
the South Carolina piedmont, I have found the caterpillars on yellow-flowering
cannas and Bengal Tiger Cannas (orange flowers), both of which have green
leaves. Calpodes is
host to several species of parasitoid chalicidid, trichogrammatid, and encyrtid
wasps as well as some tachinid flies (McAuslane & King, 2000).
The family Hesperiidae
consists of about 3500 species of strong fliers known as skippers for their
irregular “skipping” flight pattern. The
larvae usually inhabit domiciles constructed of leaves held together by silk
secreted by the salivary glands. Larvae
have a characteristic appearance with a large head with a narrowed neck and are
easily recognized by this trait.
Like that of most butterflies,
the life cycle consists of egg, five larval instars, pupa, and imago. Butterflies
are holometabolous insects in which the body undergoes a radical metamorphosis
during the pupal instar. During
this metamorphosis wings develop de
novo from imaginal discs
(clusters of undifferentiated embryonic cells) in the larval thorax and muscles
are reorganized for flight. The mouthparts are converted from the primitive
chewing design suited for feeding on plant tissue to highly derived sucking
mouthparts adapted for ingesting liquids. The
gut is reorganized to accommodate differences in larval and adult diets. The
antennae form anew from imaginal discs in the head and the reproductive system
prolegs are completely lost and the adult compound eye develops near the old
larval ommatidia. The nervous system is reorganized, in part through
condensation and cephalization of ganglia.
The fifth instar reaches lengths of 65 mm before pupating,
making it more than large enough for easy dissection. The larvae are
smooth-bodied with little external ornamentation and are tapered at both ends,
being widest in the middle. They
are naked, with scattered, inconspicuous setae. Fifth instar larvae have an
orange head capsule although that of earlier instars is black. The
larva pupates in its leaf nest as a bright green pupa. The
pupa has a black anterior spine, a very large proboscis extending posteriorly
beyond the tip of the abdomen, and is about 35 mm in length. It’s rudimentary
silken cocoon consists a platform of a few threads. Photographs of all life
history stages, as well as a discussion of biology, can be found at McAuslane &
This exercise is written for use with fifth instar living or
freshly sacrificed caterpillars. It
can also be used with other instars and with preserved specimens although color
and texture descriptions are not applicable to preserved material. If
possible the specimens should be freshly collected so the gut is filled with
ingested leaf particles.
Examine a living, active (i.e. unanesthetized) late instar
larva of the Canna Skipper, Calpodes
of the external anatomy is best studied using an active animal and, because of
the transparent body wall, much of the internal anatomy is visible and can be
studied prior to (or without) dissection. The beating heart, pericardial sinus,
perivisceral sinus, tracheae, Malpighian tubules, salivary (silk) glands, and
the enormous gut and its contents can be seen through the body wall of living
a little patience, the external anatomy can be studied without anesthetizing the
specimen and it is desirable to do so to take advantage of the opportunity to
observe motion and behavior. Sometimes
the caterpillar will sit quietly on the tip of your index finger, which can then
be oriented under the lens of a dissecting microscope to provide the desired
view of the specimen.
Figure 1. External
anatomy of a generalized caterpillar. Redrawn
and modified from Snodgrass (1935). Lepid63L.gif
The larva is, naturally, eruciform,
or caterpillar-like having a more or less cylindrical body and well-developed
sclerotized head capsule, thoracic legs and abdominal prolegs (Fig. 1). The body
is smooth and naked,
with inconspicuous, scattered, short setae,
visible only with magnification. The body, like that of other insects, consists
of a small anterior head,
middle thorax, and
large posterior abdomen.
Together the thorax and abdomen are the trunk. The
head capsule is wider than the short neck, or cervix,
connecting it with the thorax, a characteristic of the family Hesperiidae. Paired,
jointed, segmental appendages are
present on the head and thorax but are lacking on the caterpillar abdomen. Paired,
unjointed, segmental, fleshy prolegs,
however, are found on the abdomen. The body wall is transparent and the insect
is the color of the vegetation in the gut.
A healthy specimen in a dish with a piece of fresh canna leaf
will probably construct a domicile, or leaf nest. Watch
as the larva touches its “chin” (with the spinneret) to the leaf surface to
attach a silken thread on one side of the developing nest. It
then swings the head to the other side of the leaf and again touches the
spinneret to the leaf to anchor the other end of the thread. The
silk shrinks and hardens upon exposure to air, drawing the edges of the nest
together to form a tube enclosing the animal.
The head is
enclosed in a rigid sclerotized head
capsule, or epicranium,
which may be black or orange, depending on instar. The
head consists of several fused segments and bears the appendages of those
segments. The component segments of the head are not recognizable and it appears
to be unsegmented.
Dorsally the epicranium is divided into large right and left
halves by a Y-shaped line, the epicranial
forks of the “Y” are made of the two frontal
sutures whereas the stem is
the coronal suture (Fig.
2). The lateral regions of the epicranium are the cheeks, or genae,
whereas the dorsal posterior portion is the vertex.
The triangular frons and
the clypeus lie
between the two frontal sutures. The frons is dorsal and clypeus is situated on
its ventral border. In
fifth instar larvae the epicranium is orange or beige and the frons bears a
black spot. The
bilobed labrum articulates
with the ventral edge of the clypeus. The labrum is the anterior border of the preoral
cavity, which is a space surrounding the mouth and formed by the
mouthparts; labrum anteriorly, mandibles and maxillae laterally, and labium
posteriorly (Fig 21-7*).
Figure 2. Dorsal
view of the head capsule of the Greater Canna Leafroller, Calpodes
Posterior and ventral to the labrum is a pair of large, dark mandibles each
with a smooth, curved, toothless blade resembling a sculptor’s or woodworker’s
gouge. The blades of the two mandibles oppose each other on the midline and are
used to cut discs of tissue from the canna leaf. If your specimen is alive and
active you will probably see the mandibles moving in the transverse plane. The
mandibles form the sides of the preoral cavity and lie beside the mouth.
The maxillae and labium are small, inconspicuous, and best
studied after the specimen is anesthetized. Immediately
posterior to each mandible is a maxilla from
which protrudes a small galea and
a slightly larger maxillary
palp (Fig. 2-4). The
galea is tipped with several setae whereas the palp is biarticulate. During
metamorphosis the galea becomes the large sucking proboscis characteristic of
adult butterflies and moths. The labium lies
on the midline between the two maxillae and bears the median spinneret. This
is a hollow spine at whose tip the salivary (silk) duct from the salivary (silk)
glands opens. A
tiny, vestigial labial palp protrudes
on each side of the spinneret. The
labial palps become greatly enlarged in adults and, along with the maxillary
proboscis, are the only functional adult mouthparts.
A pair of short, inconspicuous, pale, cylindrical antennae can
be seen lateral to the mandibles, between the mandibles and ommatidia (Fig. 2,
antennae and ommatidia should be revisited after you have anesthetized the
specimen and it ceases to move.
Figure 3. Ventral
view of the head capsule of the Greater Canna Leafroller, Calpodes
Posteriorly the head capsule is penetrated by a large
opening, the foramen magnum,
through which the gut, nerve cord, silk ducts, aorta, hemocoel, and tracheae
pass between head and thorax (Fig. 3). The
foramen is filled with soft tissue and is cannot be seen when the head is
attached to the thorax but you can make out its outline by pushing the soft
tissue of the cervix aside.
Figure 4. Lateral view of the right side of the head
capsule of the Greater Canna Leafroller, Calpodes
Antero-laterally each side of the epicranium bears a
pigmented area with the colorless, transparent corneae,
or lenses, of six large, non-contiguous ommatidia,
sometimes erroneously referred to as ocelli but there are no true ocelli in
caterpillars (or adult butterflies) and the ommatidia are derived from a
compound eye. Together
they are known as the ocularium.
The insect thorax consists
of three segments, prothorax, mesothorax,
and metathorax, each
of which bears a pair of sclerotized, jointed thoracic
legs (Fig. 1, 16-2). These
are theforelegs, midlegs,
respectively. Each thoracic leg consists of a series of cuticular rings
decreasing in size distally. The
largest ring is the proximal coxa,
which articulates with the body Fig. 5, 21-1E). It
is followed in turn by the femur,
which is fused with a vestigial trochanter, then the tibia,
and finally a biarticulate tarsus. The
terminal tarsal article is a darkened, sclerotized claw.
The thorax is connected with the head by a short
or neck, which in hesperiids is not as wide as either the head or prothorax. The
thoracic segments increase progressively in width posteriorly with the prothorax
being about the width of the head. The prothorax bears a cuticularized pronotum,
or prothoracic shield, covering its dorsal and lateral surfaces. A
dark brown or black spot marks each lateral edge of the pronotum. A white prothoracic spiracle can
be seen on each side near the edge of the shield (Fig. 1). These
are actually the spiracles of the mesothoracic segment that have migrated
mesothorax and metathorax have neither sclerotized nota nor spiracles.
Figure 5. Forelegs of the Catalpa Caterpillar, Ceratomia
The abdomen (Fig.
1) is a succession of ten segments which increase posteriorly in diameter to
segments five and six, after which they decrease. The
result is a caterpillar tapered at both ends. Most
of the abdomen is loaf-shaped (round above, flat ventrally) in cross section but
posteriorly it is flattened. Abdominal
segments 3-6 and 10 each bear a pair of soft abdominalprolegs (Fig.
1). Those of the 10 th segment
are the anal prolegs.
The prolegs make good landmarks for recognizing the segments by number. Each
proleg terminates in a central sucker surrounded by a circle of sclerotized
hooks, or crochets.
The suckers can be used to grasp smooth surfaces whereas the crochets function
chiefly in attaching to silk or other rough surfaces. Under the influence of
normal blood pressures the crochets engage the substratum and attach. Elevated
blood pressure causes the crochets to disengage. Consequently, at rest the larva
is by default attached to is substratum and unlikely to fall. An active
elevation of hemocoelic pressure is required to disengage.
Abdominal segments 1-8 each bear a pair of oval white spiracles which
open into the common tracheal system (Fig. 1). A typical insect respiratory
system of interconnected tracheae is
clearly visible through the body wall as a series of white tubules in the body
cavity (hemocoel) between the body wall and the gut. Tracheae arise from each
spiracle and branch dendritically (tree-like) to supply nearby tissues and
organs with oxygen (Fig. 6). Most conspicuous are the three sunbursts of
branching tracheae radiating outward from each spiracle and the two unbranched
lateral longitudinal tracheal trunks, which connect successive spiracles on each
side of the body.
The tracheal system is complex and ramifies throughout the
body to provide oxygen and remove carbon dioxide to and from all tissues. The
hemal system plays no role in oxygen transport in insects. Pay attention as you
dissect the caterpillar to the pervasiveness of the tracheal system. You
will find it extending to all tissues.
Figure 6. Cross section of a typical insect abdominal
from Snodgrass (1935). Lepid65L.gif
In Calpodes many
aspects of the tracheal system are visible through the body wall and dissection
is not required to see it. In general, each spiracle opens into a short atrium
that connects with the lateral
longitudinal tracheal trunk (Fig.
6). From this junction arise three major segmental tracheae, each with many
these are the three sunbursts mentioned above. The dorsal
segmental trachea and its
many branches extend to the dorsal body wall and heart musculature. The visceral
segmental trachea sends
branches to the gut, fat body, silk glands, and gonads. The ventral
segmental trachea supplies
the ventral body wall musculature and nerve cord with oxygen. As
you observe an active caterpillar from the side, the dorsal and ventral branches
remain stationary as the gut moves but the visceral branch moves with the gut.
In some segments the right and left dorsal segmental tracheae
join each other across the dorsal midline to form a dorsal transverse tracheal
a dorsal transverse
commissure can be seen
connecting the spiracles of the eighth abdominal segment. Similarly
some ventral segmental tracheae form ventral transverse commissures. There may
be additional longitudinal trunks associated with the gut and dorsal body wall.
These are smaller than the lateral longitudinal tracheal trunks. Find as many of
these branches as possible. You
will have another opportunity to study the tracheal system after you open the
hemocoel but most of its major features can be seen now, prior to dissection.
Note that, because of the numerous longitudinal and
transverse connections, the tracheae form a single interconnected system so that
all parts of the body, whether or not they have their own spiracles, are
supplied with tracheae and oxygen. The
head, two thoracic, and two abdominal segments, for example, lack spiracles but
nevertheless are provided with oxygen by the system.
The gut through
most of its length is only slightly smaller in diameter than the caterpillar
itself and can be clearly seen in constant motion just inside the body wall. If
the caterpillar has fed recently, the gut will be filled with pieces of
vegetation of uniform (bite) size. These
leaf fragments are cut from the host plant by the curved blades of the
mandibles. Posteriorly these are formed into larger, darker, loosely
consolidated fecal pellets consisting
of these same pieces of vegetation compacted into pellets and packages in a
is no triturative proventriculus in Calpodes and
consequently the bites produced by the mandibles retain their original shape for
the entire length of the system, from mouth to anus. The gut visible at present
is mostly the crop, midgut, and hindgut but they are difficult to distinguish
prior to dissection. The crop occupies most of the thorax and the midgut extends
from the posterior thorax through the anterior abdomen to about the level of
abdominal segment six. The hindgut is in abdominal segments 7-10 and is divided
into an intestine in segments 7-8 and a rectum in segments 9-10. The
rectum opens through the anus on segment 10.
The central feature of the insect hemal system is a tubular dorsal
blood vessel that extends
the length of the animal immediately beneath the dorsal midline (Fig 16-7).
Posteriorly the dorsal vessel is contractile and is the heart whereas anteriorly
it is not contractile and is known as the aorta. The heart consists of a series
of sequential segmental expansions, or chambers, each with a pair of ostia. Because
the body wall is transparent, the heart and aorta are
easily visible in living specimens. In
such a specimen you can observe the conspicuous peristaltic waves passing from
posterior to anterior along the heart. The heart and aorta are enclosed in the
The insect body cavity is the hemocoel,
which is filled with circulating blood that bathes the tissues and transports
food, wastes, and hormones (but not oxygen). Most
organ systems, including the gut, nerve cord, and excretory organs, are in the
hemocoel is divided by two horizontal septa (dorsal and ventral diaphragms) into
three sinuses. The
dorsal pericardial sinus encloses the heart, the middle perivisceral sinus is
the largest and contains the gut and other viscera. The
small, ventral perineural sinus surrounds the ventral nerve cord. The lateral
borders of the pericardial
sinus are visible through
the transparent body wall on each side of the heart.
In late instar male larvae a pair of irregularly shaped,
pale, yellowish testes are
easily seen, even without magnification, beside the heart on the anterior border
of the sixth abdominal segment.
The long, looped, slender, yellow or white Malpighian
tubules arise from the
midgut-hindgut junction and are visible laterally in the abdomen (Fig 16-9.
Segments 7 and 8 bear the paired pores of powder
glands whose secretions form
conspicuous, bright white patches of waxy powder on the ventral surfaces of
these segments. This
powder is used to waterproof the pupa and pupal nest and is typical of skipper
Segment 9 lacks either spiracles or prolegs and is short and
inconspicuous (Fig. 1). Segment
10 is dorso-ventrally flattened, bears a pair of anal
prolegs and is covered by a
dorsal sclerotized, but not darkened, suranal
sclerite (suranal plate).
The anus lies
hidden under the suranal sclerite. The anal
comb is a fan-shaped,
toothed sclerite dorsal to the anus, under the suranal sclerite. It
can be seen by lifting the sclerite. It is used to flick fecal pellets away from
If your caterpillar is living, it may present you with one or
more fecal pellets. If
so, dissect one at about 20X to see that it is enclosed in a delicate peritrophic
membrane. Inside the membrane you will find the uniformly sized and
shaped bites of tissue cut from the host plant by the mandibles.
High Magnification Review of
remainder of the exercise should be performed on an anesthetized or recently
sacrificed specimen. It makes little difference which. Anesthetize (or kill) a
caterpillar by immersing it in 7% ethanol (non-denatured) in a small, wax-bottom
dissecting pan. The animal will succumb in 5-10 minutes. Dissecting
pans made from anchovy fillet tins are an ideal size for most insects and fit
conveniently on the stage of a dissecting microscope.
Spend a few minutes observing
features that were difficult to see, such as the head appendages, spiracles,
thoracic legs, prolegs, and crochets earlier while the animal was active. Reexamine
the heart, find the Malpighian tubules, and look once again at the details of
the tracheal system.
If your specimen shows any
signs of life it may periodically extend and retract the crochets of the
a circle of crochets under high magnification of a dissecting microscope during
one of these cycles. At
rest the curved crochets are deployed and would engage silk strands or another
rough substratum. When
withdrawn they release their hold on the silk and permit movement of the proleg.
Examine a spiracle at
about 40X. Each
spiracle consists of a brownish, sclerotized, cuticular
ring surrounding a recessed atrium from
which a short spiracular trachea connects with the longitudinal trunk. The
atrium is covered and protected by a filter
apparatus consisting of a
ring of closely spaced setae. The
atrium is under the filter and cannot be seen from the surface. A slit-like atrial
orifice penetrates the
center of the filter and opens into the atrium below it. Insert
a minuten nadel into
the orifice to demonstrate its presence and continuity with the atrium. Use
the nadel to
demonstrate that the filter is composed of bristles extending into the orifice
from the surrounding ring. In most insects, including Calpodes,
the spiracle is equipped with a valve, under muscular control, that can close
the spiracle to conserve water.
Find the articles (coxa,
femur, tibia, biarticulate tarsus including terminal claw) of a thoracic leg
using 40X. Examine the anal comb with 40X.
begin the dissection, insert one blade of a fine scissors under the posterior
edge of the suranal sclerite and cut anteriorly through the body wall well to
the right of the middorsal line so that the median heart and pericardial sinus
are not damaged. The heart will end up on the left side of the incision. Remember
this so you can find it later after it is pinned to the bottom of the dissecting
pan. Hemocoelic pressure may force a bulge of the gut through the incision but
this problem will diminish as the incision is lengthened. Avoid
cutting deeper than the body wall and be careful that your cut does not
penetrate the gut wall. Extend the incision anteriorly to the posterior border
of the head capsule but do not cut into the capsule. Use
#1 stainless steel insect pins to pin the edges of the body wall against the
floor of the dissecting pan. Begin
at the posterior end and work your way anteriorly, inserting each pin at a 45 ° angle
and stretching the body wall laterally and posteriorly as you pin it. Much of
the space in the hemocoel is filled with the white ribbon-like fat body,
tracheae, and gut. It
may be necessary to detach some of the fat body from the body wall in order to
separate it from the gut and pin it.
The body cavity opened by this
incision is the perivisceral
sinus, a division of the hemocoel. It
is filled with blood (hemolymph)
(Fig 6, 16-7). The
much smaller pericardial sinus, which is also filled with blood, surrounds the
heart and has been pinned aside with the body wall. Organs
and tissues, including the heart, gut, fat bodies, salivary glands, excretory
organs, and nerve cord, are suspended in the hemocoel and surrounded by blood.
The insect hemal system
consists of a tubular dorsal
blood vessel extending the
length of the body immediately under the dorsal midline (Fig. 7, 16-7). Posteriorly,
for most of the length of the abdomen, the vessel is the muscular, contractile,
and ostiate heart. The vessel continues anteriorly through the thorax to the
head as the non-contractile aorta.
You should have observed the
heart earlier before anesthetizing and dissecting the caterpillar. Take
another look at it now. It
should still be present in the pericardial sinus attached to the body wall and
pinned to the wax of the dissecting pan on the left side of your specimen. It
may still be beating weakly. The heart is a transparent, colorless tube on the
dorsal midline. It
would be difficult to see were it not for the parallel rows of opaque, white,
irregularly shaped nephrocytes on
its walls. These special nephrocytes are large cells known as pericardial
cells. The heart can
be recognized as a clear channel between two parallel rows of nephrocytes. A
pair of large, white, ribbon-like fat
bodies lie beside the heart
and may lie over it and obscure your view. That
portion of the dorsal vessel in the thorax is the aorta.
Remove some of the fat body if necessary to improve your view of the heart and
aorta and also to reveal the dorsal body wall muscles. Large,
translucent, strap-like longitudinal
body wall muscles lie on
either side of the heart and are reminiscent of the continuous muscle layers of
an annelid rather than the individual muscles of an arthropod.
In Calpodes the fat
bodies are conspicuous in
the perivisceral sinus as longitudinal, white, convoluted ribbons attached to
the body wall and gut tube. Note
the tracheae extending to the fat bodies.
The fat body is a large,
multipurpose, mesodermal organ consisting of various types of cells involved in
intermediary metabolism, protein synthesis, and storage of lipids,
carbohydrates, proteins, uric acid, and energy. In most insects the chief period
of feeding, growth, and energy storage is as juveniles, for adults feed
relatively little or not at all. The energy accumulated by juveniles is stored
in the fat body and later used to support the activities, chiefly dispersal and
reproduction, of the imago.
Fat bodies are conspicuous in
the perivisceral sinus as seemingly amorphous white or yellow leafy sheets
attached to the body wall and extending throughout the hemocoel. The
system typically consists of a peripheral
fat body forming a layer
attached to the body wall and a perivisceral
fat body associated with the
gut wall as well as smaller concentrations with specific organs. Note the
tracheae extending to the fat bodies. These
thin sheets, each only one or two cells thick, expose a large surface area to
the hemolymph. All cells in contact with the blood for the exchange of
The digestive system consists
of anterior ectodermal foregut, middle endodermal midgut, and posterior
ectodermal hindgut, which may themselves be regionally specialized. Note
the numerous branches of the visceral
tracheal system extending to
the gut. The foregut of Calpodes,
which is lined by cuticle, consists of mouth, pharynx, esophagus, and crop. Its
role is mostly in food storage with some preliminary mechanical and chemical
pharynx and esophagus are of small diameter and are largely enclosed within the
head capsule. Theesophagus can
be seen emerging from the capsule where it widens to become the crop at
the anterior end of prothorax (Fig. 7). The crop is a storage chamber occupying
most of the space in the thoracic perivisceral sinus. Its
walls are transparent and its diameter is almost equal to that of the thorax. It
is probably full of recently ingested food particles cut from the host plant by
The midgut is
about the same diameter as the crop and occupies most of the anterior abdomen
from segment 1 posteriorly to about segment 7. Unlike that of the crop, its wall
is opaque. The
chief functions of the midgut are enzyme secretion, hydrolysis, and absorption.
The midgut narrows abruptly in
segment 7 to form the proctodeal
valve (pyloric valve) which
separates midgut from hindgut. The hindgut,
which is cuticularized like the foregut, consists of a short intestine (anterior
intestine), a short rectum (posterior
intestine), and the anus. The
intestine and rectum are separated from each other by a constriction. The
hindgut functions in water and salts reclamation, a vital role in terrestrial
animals, and feces formation and storage. Later
you will open the gut tube to study its interior.
Figure 7. Internal
anatomy of a generalized caterpillar viewed from the left. Two
of the three left side Malpighian tubules have been truncated for clarity. Redrawn
and modified from Snodgrass (1935). Lepid64L.gif
pair of glands opening on the labium and thus known as the labial glands
function in most insects as salivary glands and secrete saliva with hydrolytic
enzymes adapted for the diet of the species. Such
labial glands are sometimes referred to as salivary glands. In some taxa, most
notably Lepidoptera (butterflies and moths), Trichoptera (caddisflies), and
Hymenoptera (bees, ants, and wasps), the larval labial glands secrete, not
saliva, but a proteinaceous silk used for a variety of purposes and are known as
silk glands. In addition, caterpillars have a pair of mandibular glands, opening
on the mandibular segment, that secrete saliva whose enzymes assist in the
digestive process. The mandibular glands probably will not be seen.
In Calpodes each silk
gland is a long, transparent
or opaque, colorless, white, or yellowish-brown tube lying to the right or left
of the gut in the ventral perivisceral hemocoel (Fig. 7). A
common duct from the spinneret arises within the head capsule, where you cannot
see it, bifurcates to form the right and left silk
gland ducts. These narrow, transparent ducts can be seen emerging
laterally from the foramen magnum to enter the thorax. The ducts are small in
diameter but quickly expand to become the much larger silk
gland extends to the posterior abdomen, then reverses direction and extends back
to the anterior abdomen, only to reverse direction once more and run posteriorly
again, now closely associated with the fat body and smaller in diameter. The
opaque central region of the gland is the largest in diameter and is well
supplied with tracheae. In
the caterpillar of the silk moth, Bombyx
mori, secretory cells in the
epithelium of the posterior region of the gland secrete fibroin, the major silk
protein, composed of about 40% glycine. The central region of the gland secretes
other proteins whose function is to coat the fibroin strands and hold them
tracheal system was studied prior to dissection (Fig 6). You may now review its
major features and look for some of the details you could not find earlier. Study
the internal aspect of a spiracle with 40X and find the major tracheal branches.
fine scissors to remove a piece of a longitudinal trunk and make a wet mount
with it. Examine
the preparation with 100X, then 400X of the compound microscope. Note the
chitinous rings, known as taenidia,
that reinforce the walls of the trachea and hold it open, much like the
cartilaginous rings that hold your own trachea open (Fig 21-10A).
excretory system consists of nephrocytes in the hemocoel and Malpighian tubules,
also in the hemocoel. Lepidopteran
larvae and adults have six Malpighian
tubules, three arising on each side, right and left, of the
midgut-hindgut junction in the vicinity of the proctodeal valve at the
constriction between midgut and intestine (Fig. 7). On each side the cluster of
three arises from a common ampulla evaginated
from the gut. The tubules are supplied with tracheae. It
is difficult to demonstrate the connection of the Malpighian tubules with the
gut in gross dissection.
tubules emerge from the gut at about the level of abdominal segment 6 and extend
anteriorly as long, slender, opaque white or yellow tubules to the anterior
reaching their anteriormost position they reverse direction and loop
posteriorly, sometimes as far as the posterior end of the abdomen. To
further complicate the pattern in Lepidoptera, the distal ends of the six
tubules enter the connective tissue of the rectum wall and end there, out of
sight, and there are thus no free ends of the tubules. The
connections of both ends of the Malpighian tubules with the gut are difficult to
demonstrate in gross dissection.
scattered throughout the hemocoel and are usually not visible in gross
dissection. Nephrocytes are thought to be storage kidneys that absorb and
sequester a variety of particles and dyes but do not phagocytose bacteria. Special
large nephrocytes, known as pericardial
cells, form a row on each side of the heart. You
saw them earlier.
Digestive System Internal
a longitudinal, middorsal incision along the entire length of the exposed gut to
open its lumen,
which is probably filled with small pieces of canna leaf. In
the midgut the food mass of leaf particles is enclosed in a thin, transparent peritrophic
membrane (Fig 21-9). Cut
through the peritrophic membrane and use a Pasteur pipet to remove the leaf
fragments and improve your view of the gut lumen. The
foregut (crop) is separated from the midgut by an inconspicuous stomodeal
valve (cardiac valve), which
in Calpodes, is manifest as
a slight brownish thickening of the gut wall (Fig. 7). In
many insects the valve is more elaborate. The proctodeal
valve, between the midgut and hindgut, is a more substantial thickening
of the gut wall. Find the anus from inside the rectum and note the appearance of
the anal comb from this viewpoint.
The peritrophic membrane is
secreted by cells in the vicinity of the stomodeal valve and thus there is no
peritrophic membrane anterior to the foregut-midgut junction. The peritrophic
membrane is secreted by the epithelium of the anterior midgut epithelium to
enclose the food mass.
The opacity of the midgut wall
is due to its mucosal epithelium. The
epithelium has a brush border of microvilli that extend through the peritrophic
membrane into the gut lumen to absorb food molecules. This
border is visible in fresh specimens as a bluish green iridescence.
its primitive, largely uncephalized condition, the central nervous system
includes the tripartite brain, or supraesophageal ganglion, consisting of
proto-, deuto- and tritocerebrum and the ventral nerve cord consisting of
segmental ganglia joined by paired longitudinal connectives (Fig 16-11). The
brain innervates the eyes (protocerebrum), antennae (deutocerebrum), and labrum
(tritocerebrum). The tritocerebrum is connected with the double longitudinal
ventral nerve cord by a pair of circumesophageal connectives. Immediately
posterior to the connectives is the subesophageal ganglion consisting of the
fused ganglia of the mandibles, maxillae, and labia. The
segmental ganglia of the thorax and abdomen remain independent of each other and
are spaced along the nerve cord. The
CNS of Calpodes exhibits
this primitive condition in most respects and shows little cephalization or
condensation of ganglia.
the tracheae on the right side of the gut, move the gut to the left, and pin it
so the ventral midline is revealed. Remove the large, lateral fat bodies as
necessary to reveal the ventral midline of the thorax and abdomen, being careful
that you do not damage the nerve cord on that midline. Do not attempt to remove
the small ventral fat bodies associated with the nerve cord.
segmental ganglia are bright white and easy to see widely spaced along the
ventral nerve cord. The
cord itself is not as obvious as the ganglia but once you find the ganglia you
can find the nerve cord connecting them. In
the abdomen the parallel connectives are fused and the double nature of the
nerve cord is not apparent although in the thorax the connectives are widely
separated and the cord is quite clearly double.
subesophageal and first thoracic ganglia are far anterior and can best be seen
by lifting the head capsule and looking into the foramen magnum using 20X. The subesophageal
ganglion, which serves the mandibles, maxillae, and labium, is in the
posterior head capsule but it can be seen lying below the gut by looking into
the foramen magnum (Fig. 7, 21-7). The brainmay
also be visible, but dorsal to the gut. The
subesophageal ganglion is connected by a pair of short connectives with the first
thoracic ganglion, which is very close to it. The second
thoracic ganglion is well
separated from the first and connected to it by a pair of long connectives. The third
thoracic ganglion is far
posterior to the second and is connected with it by a pair of long connectives.
abdominal ganglion is very
close to the third thoracic so the connectives between them are very short. Abdominal
ganglia 2-7 are widely
spaced along the length of the abdomen and are connected by coalesced
connectives, which form the double nerve
cord, which appears to be single (Fig. 6). The abdominal part of the
nerve cord is partly hidden by the ventral fat bodies. Abdominal
ganglion eight is very close
to the seventh ganglion and their connectives are very short. Several
nerves radiate posteriorly from the eighth ganglion. The combined connectives,
whether single or double, form the ventral
Examine a ganglion, such as
the first abdominal, with 40X and find the pair of inconspicuous segmental nerves connecting
the ganglion with the periphery. Note that the nerve cord, ganglia, and nerves
are supplied with tracheae.
that ventral longitudinal
body wall muscles, similar to the dorsal muscles seen beside the heart,
lie beside the nerve cord.
reproductive system of caterpillars is rudimentary and will develop in the pupa
during metamorphosis. Small
gonads, however, are present in late instar larvae and spermatogenesis occurs in
these larvae and in the pupa. Eggs develop in the pupa. Development
of both internal and external genitalia occurs during metamorphosis in the pupa.
*Hyphenated figure call-outs,
such as this one, refer to figures in Ruppert, Fox, and Barnes (2004). Those
without hyphenation refer to figures embedded in this exercise.
Borror DJ, Triplehorn
CA, Johnson NF . An
introduction to the study of insects, 6 th ed. Saunders
College Publishing, Philadelphia. 875pp.
Chapman RF. 1998. The
insects, Structure and function, 4 th ed. Cambridge
Univ. Press, Cambridge. 769
Comstock JH. 1930.
An introduction to entomology. Comstock Pub., Ithaca. 1044 pp.
McAuslane HJ, King K. 2000.
Larger Canna Leafroller. Integrated
Pest Management, Featured Creatures. University of Florida, Department of
Entomology and Nematology. Institute of Food and Agricultural Sciences.http://ipm.ifas.ufl.edu/ctgysrch/insects.htm#fc
King HS. 1880.
Internal organization of Hesperia
ethlius Cram. As observed in the
living animal. Psyche 3:322-324.
Ross HH. 1965.
A textbook of entomology, 3 rd ed.
John Wiley & Sons, New York. 539pp.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Scott JA. 1986.
The butterflies of North America. Stanford
University Press, Stanford. 583pp.
Snodgrass RE .
of insect morphology. McGraw-Hill, New York. 667 pp.
1 1iving, freshly collected, late instar Calpodes
1 small (anchovy tin) wax-bottom dissecting pan
1 dissecting microscope
1 compound microscope (can be shared by several
students or the entire class)
1 microscope slide and coverslip
1 microdissecting forceps
1 microdissecting (iridectomy) scissors
nadeln with applicator stick
20 # 1 stainless steel insect pins
7% non-denatured ethyl alcohol
1 plastic Pasteur pipet