Invertebrate Anatomy
OnLine
Collembola ©
Springtails
29jun2006
Copyright 2001 by
Richard Fox
Lander University
Preface
This 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
link 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.
Systematics
Arthropoda P,
Mandibulata sP,
Tracheata, Hexapoda SC,
Entognatha C,
Ellipura, Collembola O,
Entomobryoidea SF,
(Fig 16-15, 20-14, 20-15, 21-23)
Arthropoda P
Arthropoda, 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 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. The body is typically divided into a head and trunk, of
which the trunk is often itself divided into thorax and abdomen.
The 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,
and absorption.
The 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.
The 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 nephrocytes. Respiratory organs also
vary with taxon and include many types of gills, book lungs, and
tracheae.
The 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.
Development 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.
Mandibulata sP
Mandibulata 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 aerial.
Tracheata
Myriapods and hexapods share tracheae and a single pair
of antennae and are sister taxa in Tracheata. Crustaceans, which
have gills and lack tracheae, are excluded and form the sister
group.
Hexapoda SC
The 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 acid. There is relatively little
cephalization of the nervous system. Insects are gonochoric with
copulation and internal fertilization.
Entognatha C
Entognatha, the sister taxon of Insecta, comprises
primitive hexapods with mouthparts partly internalized into a
depression on the head. The absence of wings is a primary
characteristic and is not the result of loss. Their ancestors never
had wings. Development is ametabolous. Malpighian
tubules and compound eyes are reduced. The taxon includes Protura,
Collembola, and Diplura (Japygina, Campodeina).
Collembola O
Collembolans are small hexapods rarely exceeding 5 mm in
length. The 7000 described species are found in a variety of
habitats including leaf litter, soil, shorelines water surfaces,
intertidal zones, snow fields, and caves. They are abundant in the
habitats they frequent where they can be the most abundant
arthropods. A single meter of forest floor may support as many as
62,500 individuals.
As a hexapod, collembolans possess a body with head,
thorax, and abdomen. The thorax has three segments, each with a pair
of legs but no wings. The abdomen is short, with only 6 segments.
Most collembolans have a springing mechanism consisting
of a spring, or furcula, on the forth abdominal segment and a catch
mechanism on the third. The furcula (or furca) folds under tension
below the abdomen and is held in place by the catch when not in
use. When the catch is released the furcula swings ventrally and
posteriorly with speed and force, resulting in the springtail being
propelled up to 20 cm into the air. This is the basis for the name
"springtail" for these animals. The furcula is pair of modified
abdominal appendages. Some taxa, especially those living in soil,
have lost the springing mechanism.
The first abdominal segment bears a ventral collophore
that plays a role in osmoregulation and is apparently involved in
water uptake. The collophore is a modified pair of abdominal
appendages.
Springtails exhibit a variety of feeding modes including
herbivory, carnivory, and detritivory. Fungi are important in the
diets of many. Many suck the juices from plants. Some consume
arthropod feces, pollen, decaying plant matter, pollen, or algae.
The eggs are unusual in being microlecithal. Development
is ametabolous and molting continues after sexual maturity is
achieved.
Collembolan Morphotypes
Three morphological types of springtails differ in general
morphology, external segmentation, jumping mechanisms, eyes,
habitat, and other characteristics. This exercise applies to
entomobryomorph springtails.
Entomobryomorph Collembola
Entomobryomorph springtails have obviously segmented
bodies in which the three segments of the thorax and the six of the
abdomen are clearly visible (Fig 1, 21-18A). The prothorax (first
thoracic segment) is small and lacks a pronotum so that the animal
appears to have an obvious neck. The legs are long and the furcula
is well developed. The body often bears tufts of setae or
microscopic scales. The antennae are long and the eyes are well
developed. Entomobryomorphs are characteristic of forest leaf
litter. This exercise is written specifically for entomobryomorph
collembolans. Tomocerus and Entomobrya are
examples.
Sminthurimorph Collembola
In some taxa (e.g. Sminthuridae) the thoracic and abdominal segments
are fused with adjacent segments to produce a short, globose body
form. External segmentation is not evident. The antennae are well
developed and a narrow “ neck” is present between the head and
thorax. The furcula is well developed and these are active
animals. Globular collembolans are characteristic of grassy
vegetation and open habitats and are tolerant of desiccation. Sminthurus and Dicyrtoma are
examples.
Podomorph Collembola
Podomorph species are grub-like with bodies of uniform
diameter. The prothorax is well developed and a pronotum similar to
the mesonotum and metanotum is present. Consequently there is no
neck and the width of the body does not decrease between the head
and thorax. The antennae are short. The furcula is small or absent
and the legs are short. The collophore is absent or poorly
developed. Eyes may be poorly developed or absent. Podomorph species
are typically found in soil and deep litter. They are intolerant of
desiccation.
Anurida maritima, an inhabitant of the marine rocky
intertidal zone, is a typical podomorph. Onychiurus,
Podura, and Hypogastrula are
additional examples.
Figure 1. An entomobryomorph collembolan from deciduous leaf
litter in Greenwood, South Carolina. The abdominal segments are
numbered. Collemb8L.gif
Laboratory Specimens
Collembolans are easily collected from forest leaf
litter using a Berlese funnel (see Techniques chapter). A few
handfuls of leaf litter are placed in a large funnel, supported with
its large opening uppermost. A vial of alcohol (40% isopropanol or
80% ethanol) is attached to the small, lower end of the funnel. A
75-watt light bulb left burning for several days over the top of the
funnel. The heat from the lamp, and the resulting desiccation,
drives the leaf litter animals downward in the funnel until they
drop into the vial from which they can be recovered and studied.
The Berlese technique will yield a variety of small soil
arthropods, typically including many species of mites and
collembolans as well as pseudoscorpions, spiders, millipedes,
centipedes, ants, and many others. Several species of collembolans
will usually be found in any leaf litter sample. This exercise is
written for a member of Entomobryoidea and is best used with species
exhibiting the entomobryomorph body morphology.
Preserved specimens should be immersed in a small glass
dish of tapwater and studied with the dissecting microscope using
incident illumination.
External Anatomy
Examine a specimen with a dissecting microscope. The
collembolan body, like that of other hexapods, consists of three
tagmata; the head,
thorax, and abdomen.
Head
The head is enclosed in a one-piece cuticular head
capsule. Its segments are not visible internally or
externally. In Collembola, and other members of Entognatha, the
bases of the mouthparts are covered laterally by extensions of the
side of the head capsule, or gena. Only the distal tips of the mouthparts are
visible from the side.
A pair of antennae emerges
from the anterior dorsal angle of the head capsule. Collembolan
antennae have three articles but some of these may be subdivided
into additional articles as seen in Figure 1.
Entomobryomorph collembolans typically have a cluster of
eight or fewer simple
eyes at the base
of each antenna (Fig 1). Each eye is a reduced ommatidium similar to
those of the compound eyes of insects.
Thorax
The
collembolan thorax consists of prothorax, mesothorax, and metathorax
as does that of insects. Each thoracic segment bears a pair of
jointed legs.
In the entomobryomorph collembolans the prothorax (Fig
1) is reduced and is not covered by a sclerotized pronotum as it is
in insects. Its cuticle is entirely flexible and membranous. The
prothorax appears to be simply a connection, or neck, between the
head and thorax, but it is a neck with legs.
The mesothorax and metathorax are
covered by a sclerotized mesonotum and metanotum respectively.
Each thoracic segment bears a pair of legs. These are,
in order anterior to posterior, the forelegs, middle
legs, and hindlegs. The
number of articles in the legs varies with taxon but in general they
are the same articles found in insects, viz,
coxa, trochanter, femur, tibia, and tarsus. There may be fusion,
addition, omission, or other modifications of the basic plan.
Abdomen
The abdomen consists of six segments which, in
entomobryomorphs, are clearly visible. Each is covered dorsally and
laterally by a sclerite.
The first
abdominal segment bears
the collophore on
its ventral midline (Fig 1). This is a short cylindrical process
that functions in water uptake. It is especially well developed in
species inhabiting dry habitats. The collophore is derived by the
fusion of a pair of abdominal appendages. The second
abdominal segment is
unremarkable.
The third
abdominal segment bears
on its ventral surface a small catch mechanism, the retinaculum (Fig
1), whose function is to retain the furcula in tension and ready for
jumping.
The fourth
abdominal segment bears
the furcula (Fig
1), or jumping organ. Like the collophore, the furcula is derived by
the fusion of the appendages of its segment. It consists of a basal
piece from which extend two rami. At rest the furcula is held below
the abdomen by the retinaculum on segment 3.
The fifth
abdominal segment is
smaller than the fourth. The genital aperture is on this
segment. The sixth
abdominal segment bears
the anus distally.
Internal Anatomy
Springtails are too small for dissection in introductory
laboratories.
References
Borrer DJ, Triplehorn CA,
Johnson NF. 1989. An
Introduction to the Study of Insects 6 th ed. Saunders. Philadelphia. 875
pages.
Comstock JH. 1930. An
introduction to entomology. Comstock, ithaca, NY. 1044 pp.
Heyman S, Weaver J. 1999.
A pictographic key to leaf litter arthropods from the Missouri Ozark
forest Ecosystem Project (MOFEP).
http:web.missouri.edu/~bioscish.index.shtml
Hopkin SA .
1997. The biology of the springtails (Insects, Collembola). Oxford
University Press, Oxford. 330
pages.
Janssens F. 2001. Checklist
of the Collembola: Families. www.geocities.com/%7efransjanssens/taxa/collembo.htm
Maddison DR . 1995. Collembola.
Ruppert EE, Fox RS, Barnes
RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Supplies
Dissecting microscope
Living or preserved springtail
Microdissecting forceps and minuten
nadeln
8-cm culture dish