Invertebrate Anatomy OnLine
Scutigera coleoptrata ©
Copyright 2001 by
This 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 to 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.
Panarthropoda SP, Arthropoda P, Mandibulata, Tracheata, Myriapoda SC, Chilopoda C, Notostigmophora SO, Scutigeromorpha O, Scutigeridae F (Fig 6-15, 20-14A, 20-15)
Panarthropoda 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.
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 consists of a head and trunk with numerous segments but no tagmosis into thorax and abdomen. Trunk segments bear paired segmental appendages, always in excess of three pairs. Myriapoda includes the familiar centipedes and millipedes, as well as symphylans and pauropods. Median ocelli have been lost but lateral eyes are present in some taxa. The nervous system is arthropodan with little tendency to cephalization.
Chilopods are commonly known as centipedes. Each trunk segment has one pair of legs. The first trunk appendages are modified as fanglike forcipules equipped with poison glands. The gonopores are posterior (opisthogoneate), typically on the penultimate trunk segment, unlike those of all other myriapods. Unlike millipedes (and pauropods) centipedes are trignathous and have three pairs of mouthpart appendages; mandibles, first maxillae, and second maxillae. About 2800 extant species are known. Centipedes are nocturnal raptors. Respiration is via tracheae with paired lateral, or unpaired dorsal, spiracles. The excretory system consists of a single pair of Malpighian tubules.
Notostigmorphs have unpaired dorsal spiracles on the posterior margin of the tergites. These serve only the dorsal blood vessel and their tracheae do not ramify throughout the body as do. Scutigeromorpha is the only notostigmorph taxon. All other chilopods belong to Pleurostigmorpha and have paired lateral spiracles serving trachea extending to the tissues, rather than the blood.
Scutigeromorphs have long antennae, with as many as 400 flagellar articles, eight large terga, and fifteen pairs of long legs. The extreme length of the legs is due to proliferation of secondary tarsal articles. Scutigeromorphs are swift cursorial predators that rely on speed and quick reflexes to capture prey. The unpaired spiracles are dorsal and open into short tracheae which supply oxygen to the dorsal blood vessel. This highly unusual tracheal system is probably not homologous to that of other tracheates. Unlike that of all other chilopods, the blood contains hemocyanin. The eyes are pseudofaceted, and although they resemble the compound eyes of insect are probably not homologous to them. Scutigeromorpha contains a single family, Scutigeridae. Scutigera coleoptrata is about 2-3 cm in length but one South American species reaches 5 cm with a legspread of 12 cm.
Scutigera coleoptrata, the house centipede, is a common, harmless inhabitant of houses and their basements and environs. It is often seen scurrying rapidly across the floor or trapped in a smooth-sided bathtub. Scutigera requires a relatively moist habitat to avoid desiccation. It is native to Europe but, like so many other inhabitants of human homes and gardens, it has been introduced to and is now widespread in North America. It feeds on small household arthropods such as flies, spiders, silverfish, bedbugs, and juvenile cockroaches. Although all centipedes have fangs and poison glands, Scutigera is not aggressive and rarely bites humans, its fangs being barely capable of penetrating human skin. The non-lethal bite is comparable to a bee sting. Far from being undesirable, house centipedes are beneficial housemates.
Specimens are not available from the major biological supply companies, either preserved or alive, but occasional individuals for laboratory study can be captured, with skill and luck, and preserved in 40% isopropyl alcohol. They can also be studied alive after anesthetization with carbon dioxide or chloroform. Only the external anatomy is considered in this exercise.
Typical of myriapods, the body is divided into only two tagmata. The anterior head is relatively short but the trunk is long and segmented (Fig 20-1C, B). Because careful examination of the mouthparts destroys the head and is likely to damage the anterior trunk, it is best to study the trunk first and the head last. The following account begins at the posterior end of the animal and works its way anteriorly.
Most of the centipede body is a linear series of homonomous segments extending posteriorly behind the head and known as the trunk. The contractile dorsal blood vessel, or heart, can be seen through the translucent tergites. It lies on the dorsal midline (Fig 1, 20-3).
The true segmentation of the trunk is not immediately apparent from the dorsum because of the disparity in the number of tergites and segments. The trunk consists of 18 segments including a terminal so-called telson (segment 18) but only 11 tergites are present (Fig 2). The first trunk segment bears a pair of poison fangs, or forcipules, and the next 15 (segments 2-16) each have a pair of long legs. The last pair of legs, borne on segment 16, are especially long, antenniform, and have a sensory function, as do the cerci of some insects (Fig 20-1C).
Figure 1. Head and anterior trunk of a female house centipede, Scutigera coleoptrata, from Greenwood, South Carolina. T = tergite. Centipede18L.gif
Dorsally the trunk bears a linear series of 10 tergites but these are not arranged in a one to one correspondence with the segments (Table 1). Most tergites cover more than one segment but segments 1, 2, 16, 17, and 18 cover individual tergites. All other segments share a tergite with one or two additional segments. Trunk segment 1 has a small tergite visible as a narrow collar posterior to the head (Fig 1). The first tergite is followed by a series of eight much larger, quadrate or slightly oval tergites. These tergites cover varying numbers of segments as indicated in Table 1. The 10 th and 11 th tergites are less well developed.
The spiracles are dorsal, median and unpaired (Fig 1, 4). Each opens into a median air-filled atrium from which radiate hundreds of short, tubular, branched tracheae. The tracheae extend to the nearby dorsal blood vessel, or heart. Spiracles are present on tergites 2-8. In non-scutigeromorph centipedes the spiracles are paired and their tracheae deliver blood directly to the tissues rather than to the blood.
Table 1. Tergites and the segments they cover.
Sternites, in contrast with tergites, have a 1:1 relationship with the trunk segments. The sternite of trunk segment 1, the forcipule segment, is fused with the coxae of the forcipules to form the coxosternite (Fig 3, 20-2C). Sternite 2 is on segment 2 and sternite 17 is on segment 17. Segment 18 has no sternite but it does have a sclerotized tergite and is sclerotized laterally.
Figure 2. Diagrammatic side view of the house centipede, Scutigera, showing correspondence of segments, and sclerites, and appendages. Appendages are represented by solid triangles, segments by heavy-walled rectangles, tergites by thin-walled rectangles, and sclerites by solid rectangles. A = antenna, F = forcipule, G = gonopod. Segments, tergites, sternites, and appendages are numbered. Centipede17L.gif
Each trunk segment bears a pair of appendages. Those of segment 1 are the forcipules, those of segments 2-15 are ambulatory legs, segment 16 are anal, or sensory, legs, and those of segment 17 are gonopods.
The posteriormost segment, segment 18, is sometimes referred to as the telson (Fig 4, 5, 6). It bears the anus on its posterior margin but it has no appendages. The anus can be visualized by examining the posterior end of segment 18 with magnification. Slip a minuten nadel into it to demonstrate the opening. Segment 18 has a tergite (tergite 11) but no median sternite. It is sclerotized laterally.
Segment 17 bears one or two pairs of gonopods, depending on the sex of the individual. In females the gonopods are relatively large and biarticulate (Fig 5). The gonopods lie on either side of the female gonopore, or vulva. Most of the gonopod is ventral to the telson and is hidden by it but the terminal article extends posteriorly beyond the telson and thus is visible dorsally. Female gonopods are used to manipulate recently laid eggs.
In males segment 17 has two pairs of small setose stylets, which are the male gonopods (Fig 6). The presence of two pairs of male gonopods suggests that the 17 th segment may in fact represent two fused segments. Male gonopods are ventral to the telson and cannot be seen in dorsal view. The male gonopore opens from the posterior margin of segment 17, between the gonopods.
Figure 3. Ventral view of the head and anterior trunk of Scutigera coleoptrata. S = sternite. The articles of the telopod of maxilla 2 are numbered. Centipede19L.gif
Figure 4. Dorsal view of the posterior trunk of Scutigera. T = tergite. Centipede20L.gif
Figure 5. Ventral view of the posterior trunk of a female Scutigera. S = sternite. Centipede21L.gif
Segments 12-15 each bear a pair of ambulatory legs consisting of seven articles. In order from proximal to distal, the articles are the coxa, trochanter (= trochanter 1), prefemur (= trochanter 2), femur, tibia, tarsus, and tarsal claw (Fig 7, 20-2D). The trochanter is very short and careful examination may be required to find it. The prefemur and femur are large, and the tibia, although slender, is the longest article. The tarsus is divided into many smaller secondary articles of which the most proximal is largest. The distal secondary articles are very small and form a whip or lasso used to capture insect prey. The distalmost article is the tarsal claw.
The legs of segment 16 are the anal legs, which in Scutigera are adapted for sensory reception. Accordingly they are long and antenniform and serve as a pair of posterior “antennae” (Fig 20-1C). The anal legs have the same basic structure, including the same articles, as do the ambulatory legs. They differ in that the prefemur, femur, and tibia are long and slender and the tarsus is extremely long, due to its greatly lengthened antenniform region consisting of numerous small secondary articles.
Figure 6. Ventral view of the posterior trunk of a male Scutigera. S = sternite. Redrawn from Snodgrass (1971). Centipede22L.gif
Figure 7. Ventral view of segments 8, 9, and 10 of a female Scutigera. S = sternite. Centipede23L.gif
Although the trunk is largely homonomous, its first segment, and its appendages, are modified and differ from the remainder of the trunk. The appendages are a pair of highly specialized forcipules. Tergite 1 is much reduced and only lightly sclerotized (Fig 3). Sternite 1 is essentially absent although a vestige may be present fused to the coxa of the forcipule to form a coxosternite.
The appendages of the first trunk segment are the two forcipules (= poison fangs, maxillipeds, prehensors) (Fig 3, 8, 20-2A,C). As modified legs, the forcipules comprise the same articles as the ambulatory legs. Most proximal is the flattened, platelike coxa, which is fused with the sternite of this segment to form a coxosternite. A pair of medial, spinose endites extend anteriorly from the coxosternite.
The telopodite, or movable portion of the forcipule, arises from the lateral aspect of the coxosternite. It consists of a small, proximal trochanter, followed in turn by the prefemur, femur, tibia, and tarsus (Fig 3, 8, 20-2A). Within the telopodite is a poison gland (Fig 20-2C) which connects by a duct to a pore at the tip of the fang-like tarsus.
Figure 8. Ventral view of the right forcipule of a female Scutigera. Centipede24L.gif
The head is covered dorsally, anteriorly, and laterally by the sclerotized cephalic shield and bears a pair of antennae, three pairs of mouthparts, and a pair of lateral compound eyes (Fig 2, 1, 11, 20-2A). Anteriorly the cephalic shield curves abruptly ventrally to form a vertical epistome. The labrum is on the ventral margin of the epistome. It is the anterior wall of the preoral cavity from which the ventral mouth opens. The head is connected with the trunk by a short flexible, unsclerotized neck region.
Large lateral eyes (Fig 1) are borne dorsolaterally near the middle of the head Scutigeromorph eyes are described as “pseudofaceted” to distinguish them from true compound eyes of insects, which they resemble but to which they are probably not homologous (since their ommatidia are structured differently). Scutigeromorph eyes are unusually well developed for centipedes and those of Scutigera are composed of about 200 ommatidia each. Each ommatidium is manifest at the body surface as a cuticular facet, or cornea. Non-scutigeromorph centipedes either lack eyes entirely or have loose lateral clusters of a few non-contiguous ommatidia.
The anteriormost appendages are the two unusually long (for centipedes) filiform antennae, which are attached dorsolaterally to the anterior head capsule (Fig 1, 20-1C). Each antenna consists of a short scape and a long flagellum. The scape consists of the two proximal articles whereas the whiplike flagellum is composed of hundreds of small secondary articles, or annulations.
Centipedes, along with hexapods and symphylans, are trignathic with three pairs of mouthparts; the mandibles, maxilla 1, and maxilla 2. This contrasts with dignathic myriapods, such as millipedes and pauropods, which have with only two pairs; the mandibles and one pair of maxillae. The preoral cavity of centipedes is formed by the anterior labrum, lateral mandibles, and first maxillae. The second maxillae are not associated with the preoral cavity.
The mouthparts should be first studied intact, with magnification, while attached to the head. Subsequently, if desired, each mouthpart may be removed, beginning posteriorly with maxilla 2, placed on a slide with a coverslip, and examined in more detail with the compound microscope.
The second maxillae are leglike, moreso than in any other tracheate, and comprise the same six articles found in the ambulatory legs (Fig 3, 9, 20-2A). Each consists of a proximal platelike coxosternite embedded in the integument of the first trunk segment and a long slender 5-articulate telopod that extends anteriorly and looks like a short antenna. The telopod, or movable portion of the appendage, consists of a tiny trochanter, followed in order by long slender prefemur, femur, tibia, and tarsus. Medial extensions of the coxae touch across the midline.
The two large first maxillae (Fig 3, 10, 20-2C) are contiguous along the ventral midline and together form the posterior wall of the preoral cavity, a function performed by the fused second maxillae (i.e. labium) in insects (Fig 21-1A). In centipedes, however, the second maxilla has not become incorporated in the cluster of appendages enclosing the preoral cavity and is independent of it.
Each first maxilla consists of a large proximal coxosternite from which the short telopod extends. Two small endites project anteriorly from the medial border of each coxosternite. The telopod consists of two articles of which the second is setose and extends anteriorly to the labrum.
Figure 9. Ventral view of the left second maxilla of a female Scutigera. Centipede25L.gif
Figure 10. Ventral view of the first maxillae of a female Scutigera. Centipede26L.gif
The paired mandibles are the anteriormost mouthparts and they are situated lateral to the mouth where they form the sides of the preoral cavity (Fig 11). The first maxillae must be moved posteriorly or removed to reveal the mandibles. The large, soft, white hypopharynx protrudes from the ventral head into the preoral cavity (Fig 11). It lies between the mandibles and anterior to the first maxillae. The hypopharynx is an outfolding of the body wall and is not an appendage.
Figure 11. Ventral view of the head of Scutigera with the maxillae removed to reveal the mandibles and hypopharynx. Centipede27L.gif
The long slender mandible lies along the ventral border of the cephalic shield and articulates with it via a condyle (Fig 12), which is not visible in an undissected specimen. The mandible consists of a distal gnathal lobe projecting anteriorly from the proximal basal lobe. Distally the gnathal lobe bears a row of stout, pectinate setae and three tricuspid teeth (Fig 12).
Figure 12. lateral view of the left mandible of a female Scutigera. The three teeth are not visible in ventral view of an intact animal but the pectinate setae are. Centipede28L.gif
Barnes JK. 2003. House centipede. Arkansas Museum Notes 19. www.uark.edu/depts/entomolo/museum/house_centipede.html
Hoffman RL. 1982. Chilopoda, pp. 681-688 in Parker SP (ed) Synopsis and classification of living organisms, vol. 2. McGraw-Hill, New York. 1232 pp.
Lawrence RF. 1984. The centipedes and millipedes of southern Africa, A guide. AA Balkema, Rotterdam. 148 pp.
Lewis JGE . 1981. The biology of centipedes. Cambridge Univ. Press, Cambridge.
Minelli A. 1993. Chilopoda, pp 57-114 in Harrison FW, Rice ME (eds.) Microscopic anatomy of invertebrates, vol 12, Onychophora, Chilopoda, and lesser Protostomata. Wiley-Liss, new York.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Snodgrass RE. 1971. A textbook or arthropod anatomy. Hafner Pub. Co., New York. 363 pp. (Facsimile of 1952 edition).
Preserved or living Scutigera
Preservative (40% isopropanol