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