Invertebrate Anatomy OnLine
Copyright 2001 by
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.
Arthropoda P, Mandibulata sP, Tracheata, Hexapoda SC, Entognatha C, Ellipura, Collembola O, Entomobryoidea SF, (Fig 16-15, 20-14, 20-15, 21-23)
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 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.
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.
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, 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).
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.
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 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.
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 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
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.
Examine a specimen with a dissecting microscope. The collembolan body, like that of other hexapods, consists of three tagmata; the head, thorax, and abdomen.
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.
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.
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.
Springtails are too small for dissection in introductory laboratories.
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).
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.
Living or preserved springtail
Microdissecting forceps and minuten nadeln
8-cm culture dish