Invertebrate Anatomy OnLine
Periplaneta americana ©
American Cockroach
30jun2006
Copyright 2004 by
Richard Fox
Lander University
Preface
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 in the column 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.
Systematics
Arthropoda P, Mandibulata sP, Tracheata, Hexapoda SC, Insecta C, Dicondylia, Pterygota, Metapterygota, Neoptera, Blattaria O, Blattoidea SF, Blattidae F, Blattinae sF (Fig 16-15, 20-14, 20-15, 21-23)
Arthropoda
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
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
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.
Insecta
Most 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.
Pterygota
Pterygotes are the winged insects. These insects are derived from a winged common ancestor. Adults of most taxa have wings although they have been lost in some.
Blattaria O
Roaches are fast running insects probably closely related to termites (Isoptera ). Some primitive taxa have endosymbiotic gut zooflagellates similar to those of termites. The tarsi are five-articulate. The oval body is dorsoventrally depressed and most the head is hidden from dorsal view by the large pronotum. The antennae are long and filamentous. Most taxa have wings, in at least one sex, but they are not always functional. Eggs are enclosed in a proteinaceous ootheca which may be gestated internally, brooded externally, or deposited without further maternal attention. Most cockroaches are tropical and several species are pests in or near buildings especially in warm climates. It is often found in homes and in my experience in the southeastern US, it is common in zoology laboratories, especially in those in older buildings.
Biology
Periplaneta americana
Periplaneta americana, the American cockroach, the largest widespread North American cockroach, reaches 44 mm in length. It is dark reddish-brown with a pale peripheral band around the pronotum. Adults have large wings and can fly, although they do not often do so. Nymphs, of course, are wingless. It is easily, albeit unintentionally and regretfully, spread by human commerce and is worldwide in warm climates and, in heated buildings, cosmopolitan. Periplaneta is nocturnal, negatively phototactic, and prefers dark warm, moist habitats. It is acutely sensitive to vibrations and is one of the world’s fastest running insects, as you know if you have tried to stomp one in the kitchen late at night. Periplaneta americana is one of several cockroaches found near (peridomestic) or in (domiciliary) human habitations. Such insects are referred to as synanthropic (= with man). Several of the most common North American synanthropic species are discussed below. Any of these species could be used for this exercise but other species of Periplaneta differ the least from this account and for our purposes are interchangeable with P. americana.
The life cycle consists of egg, numerous nymphal instars (6-14), and the adult, or imago. Blattarians are paurometabolous with nymphs closely resembling small adults except for the absence of wings. Paurometabolous insects have no pupa or metamorphosis. The eggs are laid in a tough, sclerotized protein case, the ootheca, normally containing 16 eggs. Females average one ootheca per month for most of the year but may exceed that average during periods of peak reproductive activity. Nymphs lack functional wings, of course, but wingpads appear in the 4 th instar and gradually increase in size with each molt. The life span is 2-3 years with only the last year being spent as an adult. Adults live for about a year.
This omnivorous, opportunistic, scavenging species feeds on almost anything organic, plant or animal, but prefer starches and sugars. Adults and nymphs have similar feeding niches.
Cockroach population density is controlled naturally by several species of parasitoid wasps including Evania and Aprostocetus. Both are egg parasitoids in which the female wasp oviposits in the roach ootheca where the wasp larvae hatch and feed on roach eggs. Evania is a distinctive, small, black wasp with a laterally flattened triangular abdomen that waves flag-like (hence the common name “ensign" wasp) as the adult walks. Adult Evania are about 1.5 cm in length. The female, with great difficulty, inserts one egg in each ootheca. The larva hatches and consumes all 16 roach eggs before achieving a length of 8 mm, pupating, and emerging from the now empty ootheca. Adult Aprostocetus are tiny, about 2 mm long. Both species are stingless and no threat to humans. Small wasps, especially those waving a flag, seen in roach-infested areas should be protected and encouraged, not killed. Attempts to control roach populations with pesticide sprays can be counterproductive because they indiscriminately kill parasitoids in addition to the roaches. Roach pellets (“roach hotels”), which are selective, are more effective.
Cockroaches are not biological vectors for human disease although they can serve as mechanical vectors simply by harboring infectious organisms such as Ascaris eggs, bacteria, or protozoan cysts on their body surfaces. The American cockroach is the host for the cystacanth stage of the rat intestinal acanthocephalan, Moniliformis moniliformis.
The Major Synanthropic Cockroaches
Seven species of cockroaches enjoy close synanthropic associations with humans and have been spread throughout the world by human migrations and commerce and are now serious pests. All these species are thought to have originated in Africa. Many other species are loosely associated with humans and their activities.
The American cockroach, Periplaneta americana , despite its specific and common names, is an exotic that was introduced to North America from Africa in the early 17 th century. It is the largest of the domiciliary species, reaching 44 mm in length. Both sexes have fully functional wings but rarely fly. The pronotum is bordered by a pale yellowish margin. It is easily confused with P. bunnea. The last article of the cercus of P. americana is over twice as long as wide (Fig 16) whereas that of P. brunnea is less than twice as long as wide (Fig 14). Periplaneta americana is the second most abundant cockroach pest in the United States.
Periplaneta fuliginosa (smoky brown cockroach) is an introduced peridomestic species. It is common in the southern United States mostly found out of doors or in crawl spaces or unheated buildings such as outhouses or garages. It also occurs in buildings in the south and can be found further north in heated buildings. In some localities it may account for almost 80% of cockroaches captured near homes. The color is uniformly dark brownish black and it lacks the pale pronotal margin of other synanthropic Periplaneta. Nor does it have pale areas on the forewings that distinguish P. australasiae. It reaches 35 mm in length, not quite as large as P. americana
Periplaneta brunnea (brown cockroach) is an African native spread by human travel and commerce. It is common in the southeastern United States, sometimes moreso than P. americana, but it is usually found outside. This species has a pale border around the pronotum as do most Periplaneta but it lacks the pale areas on the forewings characteristic of P. australasiae. It reaches 37 mm in length. Both sexes are winged and can fly.
Periplaneta australasiae (Australian cockroach) is almost as serious a domestic pest as is P. americana. It probably arose in Africa and spread throughout the tropics and subtropics and is now circumtropical. It is the most common domiciliary cockroach in Florida and although it occurs farther north it is intolerant of the cooler temperatures tolerated by P. Americana. It is similar to P. americana but slightly smaller, reaching 35 mm in length. It is reddish brown, like P. Americana. Unlike our other Periplaneta species, its forewings have a pale yellowish band on the proximal anterior edge. Both sexes have wings and fly. The specific epithet “australasiae” means “south Asia”, not Australia.
Blatta orientalis (oriental cockroach, black beetle, water bug) probably originated in North Africa and has become a common pest in the southern, midwestern, and northwestern United States but is known as far north as southern Canada, the Netherlands, and Britain. It prefers cool, moist habitats. Males are winged, females have tiny vestigial wings, and neither is capable of flight. Individuals reach 27 mm in length. Its color is dark, almost black, and it prefers moist habitats. Blatta lacks an arolium on its tarsi and cannot climb smooth surfaces. The common name “water bug” derives from its preference for wet areas.
The German cockroach, Blatella germanica , originated in Africa and traveled first to Europe and from there to North America. It is the most abundant domiciliary cockroach in the United States and is an important pest. At 15 mm, it is much smaller than the Periplaneta species. It is pale brown with two dark longitudinal bands bordering the pronotum.
Supella longipalpa (brown banded cockroach) is assumed to have originated in Africa. It is always found in buildings (except in Africa). It was originally introduced into the Florida and spread north hitchhiking on fruit shipments and in the luggage and automobiles of tourists returning north after a vacation. It is now found throughout the contiguous United States. This is a small roach, reaching only 14 mm, and similar in size and general appearance to Blatella germanica. The forewings cover the abdomen of the male but those of females are shorter and the posterior abdomen is left uncovered. Two pale bands extend transversely across the dark abdomen but these may be difficult to see with the wings folded at rest. They are obvious in the wingless nymphs. Supella lacks the longitudinal stripes on the pronotum characteristic of Blatella. The lateral margins of the pronotum of Supella are transparent.
Laboratory Specimens
Periplaneta is available at modest cost, alive or preserved, from biological supply companies. It can also be captured locally in homes or university buildings. Look for it in warm, moist, dark places such as basements, drawers, wood piles, sewers, sunken water meter enclosures, and compost piles. In the home it can be found wherever there is food, including the usual debris on the kitchen floor and under the refrigerator, on unwashed dishes left overnight in the sink, and even in your library where it eats the bindings of your favorite books. Female Periplaneta are distinguished from other genera by having a divided 7 th sternite.
Specimens may be dissected alive and anesthetized, recently sacrificed and unpreserved, or preserved. Recently sacrificed is preferable unless you anticipate making physiological observations, in which case the specimens should be alive and anesthetized. Living specimens may be sacrificed in a covered dish with a cotton ball dampened with ethyl acetate. Living specimens should be anesthetized using chloroform, ether, or carbon dioxide. Sacrificed specimens should be immersed in water or 80% ethanol (or 40% isopropanol) in a small dissecting pan.
If both external and internal anatomy are to be studied it is advantageous to have separate specimens for these two undertakings. The study of external anatomy is likely to damage the specimen to such an extent that it will not be useful for studying internal anatomy. Students should have access to both sexes, either their own specimens or by sharing with a lab partner.
Anatomy
External Anatomy
Species of Periplaneta are elongate, oval in outline and strongly dorso-ventrally depressed, or flattened. The body is divided into the three tagmata characteristic of insects, i.e. head, thorax, and abdomen (Fig 21-1C). The head is inconspicuous in dorsal view but the pronotum of the thorax is very large. The remainder of the thorax and abdomen are hidden by the two pairs of wings. Six pairs of similar, strong, spiny legs are present.
Study a recently sacrificed or preserved cockroach with the dissecting microscope. The specimen may be dry or submersed in liquid.
Figure 1. Dorsal view of a female smoky brown cockroach, Periplaneta fuliginosa, from Greenwood, South Carolina. Blatt37L.gif
Head
Little of the head can be seen in dorsal view (Fig 1). Position the specimen with the ventral surface up and examine the head with the dissecting microscope. The head is flattened dorsoventrally and is more or less pear-shaped in outline with the mouthparts extending posteriorly from the narrow end of the pear. The head is normally carried with its flat front surface, the face, held horizontally facing the substratum (Fig 2). In this position the mouth opens posteriorly, the mandibles project posteriorly, and the palps touch the coxae of the forelegs. When feeding, the flattened face is held vertically with the mouthparts located ventrally, toward the substratum. The flattened face is the anterior aspect of the head even though it is oriented ventrally. Similarly, the thin edge of the broad end of the pear is dorsal, even though it is normally held in an anterior position. When in the feeding position the face is held anteriorly and the thin edge is dorsal.
Cockroaches have typical unspecialized mouthparts and provide a good example of the primitive insect mouthparts adapted for biting and chewing (Fig 21-1A,B). The head can be retracted beneath the prothorax.
Figure 2. Anterior view of the head of Periplaneta fuliginosa. Blatt38L.gif
Head Capsule
The head consists of a complex of fused sclerites, which constitute the epicranium, or head capsule, associated with unfused sclerites and appendages. This hard sclerotized capsule encloses the mouthpart muscles, brain, and anteriormost region of the gut and provides sites of attachment for the mouthparts and antennae.
The head capsule is divided into several regions. Locate the large compound eyes on the dorsal edge of the head (Fig 1, 2). The antennae arise on the face immediately ventral to the eyes. These are useful as convenient, easily recognized landmarks.
The epicranial suture marks the junction of three fused sclerites that form the face. In Periplaneta this suture is usually visible as a faint, yellow, Y-shaped line on the face (Fig 2) but sometimes it cannot be found. The two arms of the "Y" begin at the antennal sockets and converge between the two eyes. From the convergence the single stem of the "Y" extends dorsally between the eyes over the top of the head. It divides the top of the head into a pair of sclerites, the epicranial plates (Fig 2). The part of the capsule between the two arms of the “Y” is the frons, or front, of the capsule (Fig 2). The frons is an unpaired sclerite.
Two white circular areas between the antennae and compound eyes, at the ends of the arms of the epicranial suture, are the fenestrae and are the vestiges of the ocelli of other insects (Fig 21-1A). Two additional pale circles on the frons, ventral and a little medial to the fenestrae, mark the sites of muscle insertions inside the capsule.
The vertex, which is a region, not a sclerite, occupies the dorsum of the capsule between the two eyes. It is divided in two by the stem of the epicranial suture (Fig 2) and includes the two epicranial plates. In some insects the posteriormost region of the vertex may be a distinct sclerite referred to as the occiput.
The clypeus (KLIP ee us) is an unpaired sclerite on the face (Fig 2, 21-1A). It lies ventral to the frons and the two are joined along a transverse, indistinguishable suture. The clypeus is not movable.
A third unpaired sclerite, the labrum, is attached along a transverse, movable articulation with the ventral edge of the clypeus (Fig 2). The labrum is equipped with muscles and is movable. It forms the anterior wall of the preoral cavity into which the mouth opens. The labrum is usually considered to be derived from a sclerite of one of the head segments but some entomologists believe it to be homologous to a pair of fused appendages, similar to the labium in this regard.
The sides, or cheeks, of the epicranium are formed by the paired genae. In the cockroach these form the thin lateral edges of the head ventral and posterior to the eyes. The genae are formed of several indistinguishably fused sclerites.
Posteriorly the head capsule is penetrated by a large circular opening, the foramen magnum through which pass the gut, salivary ducts, and nerve cord. The foramen is bordered by the vertex dorsally and the genae laterally. The soft, cervix, or neck, extends posteriorly from the foramen. Because of the attachment of soft tissue of the cervix to the head capsule, the foramen is not visible externally. Ventrally slender, transverse cervical sclerites are embedded in the otherwise soft integument of the cervix.
Antenna
The anteriormost head appendages are the two antennae. These arise from a socket ventral to the compound eyes. The first article of the antenna is the scape and the second is the pedicel (Fig 2). From the pedicel arises a long, whiplike, sensory flagellum of 75-90 articles. The antennae are equipped with mechanoreceptors but apparently not with chemoreceptors.
Preoral Cavity
Ventrally the mouthparts surround and enclose a chamber, the preoral cavity, from which the mouth opens (Fig 21-7). As its name suggests, the preoral cavity precedes the mouth and is not considered to be part of the gut, although both mechanical and chemical digestion begin there.
With the specimen positioned with its ventral side up, grasp the labrum and mandibles with a pair of fine forceps and pull the head into the feeding position (i.e. with the anterior face anterior, and the mouthparts ventral). Lift the labrum and look beneath it using a second pair of forceps or a fine needle to move structures aside as needed.
The labrum, which you are currently holding aside, is the anterior wall of the cavity. The heavy, toothed mandibles and softer maxillae are its side walls and the labium is its posterior wall. A soft, tonguelike, mostly unsclerotized outgrowth of the ventral body wall, the hypopharynx, extends into the preoral cavity immediately posterior to the mouth. Move the mandibles and maxillae aside and find the mouth opening dorsally from the preoral cavity immediately anterior to the hypopharynx.
Mandible
The two mandibles lie beside the mouth and form the lateral walls of the preoral cavity (Fig 2, 21-1A, B). They are derived from a pair of segmental appendages. Each mandible consists of a single article bearing a sclerotized, distal, cutting surface. The cutting edge of the mandible consists of a lateral shearing incisor with three or four sharp denticles and a more medial grinding molar (Fig 3, 21-1A).
The mandible is largely hidden from view but can be glimpsed on the side of the head ventral to the gena and lateral to the clypeus (Fig 2, 4). With forceps carefully lift the labrum and peer beneath it to get a better view of the mandible. Its dark, heavily sclerotized denticles are visible in this view.
The mandible articulates with the head capsule by two ball and socket-like condyles, one anterior and one posterior (Fig 3, 4). The anterior condyle can be seen at the dorsolateral corner of the clypeus, on the face. The posterior condyle articulates with the ventral corner of the gena and can be seen on the side of the head capsule. Its plane of motion is transverse, perpendicular to the long axis of the body. Movement in this plane brings the two mandibles together on the midline. In contrast, vertebrate mandibles oppose each other by moving in the vertical plane. Observe the mandible with the labrum moved aside and imagine the motion of the two mandibles with respect to each other. Grasp the mandibles with forceps and move them in their preferred plane of motion. If desired, one of the mandibles can be removed for closer examination.
Figure 3. The left mandible. A. Anterior view. B. Posterior view. Blatt39L.gif
Maxilla
Look at the head from the side and find the maxilla (Fig 2, 21-1A,B). Like the mandibles, the two maxillae are paired appendages lying beside the preoral cavity. The maxilla can be recognized by its large 5-articulate palp (the labium also has a palp but it is much smaller and has three articles). Each maxilla consists of a proximal cardo that articulate with the head capsule and a distal stipes from which arise several processes.
Figure 4. Oblique view of the head from the left showing the articulation of the mandible with the head capsule of P. fuliginosa. The labrum has been drawn as if transparent to reveal the left mandible beneath it. The right mandible has been omitted for clarity. Blatt40L.gif
The cardo in a short transverse partly sclerotized article extending laterally from the head capsule (Fig 5). It is inconspicuous because it is hidden by the larger stipes. The stipes is easily seen when looking at the head from the side. It is elongate and articulates with the cardo proximally, beside the ventral edge of the eye. Distally three processes arise from the stipes.
Most obvious of these processes is the maxillary palp consisting of five articles. The palp is sensory with chemo- and mechanorecepotors used to evaluate the suitability of potential food. From the distal end of the stipes arise a lightly sclerotized, lateral galea and median, sclerotized, bladelike lacinia, which bears denticles distally. The lacinia assists the mandible in cutting food into smaller particles. The lacinia is completely hidden by the soft bulbous galea. The galea acts as a sheath to cover the lacinia and because of it, the lacinia is not visible until the galea is moved aside. You may remove one of the maxilla for closer study if desired.
Labium
The insect labium, also known as the second maxilla, is formed by the fusion of the posteriormost pair of head appendages. The labium is the posterior wall of the preoral cavity. It is best viewed by lifting the head to reveal its posterior surface. The labium consists of a large, platelike, proximal submentum (Fig 6, 21-1A,B). Articulated with it is the similarly platelike, but smaller, mentum. Submentum and mentum are the fused regions of the ancestral appendages. Arising from the distal edge of the mentum is a pair of prementa, the homologs of the original two appendages. Each prementum bears a distal median glossa with a lateral paraglossa beside it. The glossae and paraglossae are together known as the ligula and function in the manipulation of food. A 3-articulate labial palp arises laterally from the side of each prementum. The palp is chemo- and mechanosensory. You may remove the labium and examine it with higher magnification if you wish.
Figure 5. Posterior view of the left maxilla of P. fuliginosa with the galea moved slightly to reveal the lacinia. Blatt41L.gif
Hypopharynx
The hypopharynx, or tongue, is a long, process protruding from the ventral wall of the head into the preoral cavity (Fig 7, 21-7). It is not a segmental appendage being instead a fold of the body wall. It can be seen by looking into the preoral cavity, either ventrally by spreading the mouthparts aside, or posteriorly by lifting (or removing) the labium. The hypopharynx divides the preoral cavity into an anterior cibarium and a posterior salivarium.
Thorax
The thorax is the tagma specialized for locomotion and as such bears three pairs of legs and, in adults, two pairs of wings and houses the muscles to operate them (Fig 16-2). It is the middle tagma of the body and consists of the anterior prothorax, middle mesothorax, and posterior metathorax, of which the prothorax is by far the largest and the only one visible in dorsal view. In ventral view the prothorax, mesothorax, and metathorax are all visible and easily recognized by virtue of the pair of walking legs carried by each segment.
Figure 6. Posterior view of the labium. Blatt42L.gif
Figure 7. Anterior view of the hypopharynx of P. fuliginosa. The clypeus, labrum, left mandible, and labium have been removed for clarity. Blatt43L.gif
Legs
The large powerful legs are responsible for the cursorial competency for which cockroaches are renown. Three pairs are present, of course, one on each thoracic segment (Fig 8). The three pairs are similar but increase in size from anterior to posterior. Each consists of a large, flattened, proximal coxa, a small trochanter, a long femur, a tibia, and a long tarsus (Fig 9, 21-1E). The femur and tibia bear strong spines. The tarsus is a series of 5-articulate tarsomeres. Tarsomeres 1-4 each bear a posterior pad-like adhesive pulvillus. Tarsomere 5 ends with a pair of tarsal claws beside a pad-like arolium. This distal arrangement of claws and arolium is sometimes referred to as the pretarsus. The arolium is an adaptation for clinging to smooth surfaces and makes it possible for Periplaneta to climb smooth walls.
Figure 8. Ventral view of a female P. fuliginosa. The distal articles of the legs have been removed for clarity. S = sternite, T = tergite. Blatt44L.gif
When flexed, the femur fits neatly into a recess in the surface of the coxa (Fig 8). The articulations between coxa and trochanter and between femur and tibia are dicondylic joints which restrict movement to a single plane (as, you may recall, does the dicondylic joint between the mandible and head capsule). The coxa-body articulation is monocondlyic and permits movement in a variety of planes as is the tibia-tarsus articulation. Dicondylic joints are functionally similar to the hinge joints in your knee and elbow whereas monocondylic joints function like ball and socket joints in your shoulder and hip.
Wings
The meso- and metathorax of adults of both sexes of Periplaneta each bear a pair of large wings. The anterior mesothoracic wings, or forewings (= wing covers, often known as tegmina in orthopterans and blattarians), are obvious in dorsal view whereas the equally large metathoracic wings (hindwings) are almost entirely hidden by the forewings (Fig 10). The heavier, parchment-like forewings protect the more delicate hindwings. The hindwings are membranous and fold in pleats when at rest under the forewings. The folded wings completely cover the dorsal surface except for the head and pronotum.
Fig 9. Ventral view of the right midleg of P. fuliginosa. Blatt45L.gif
Use forceps and your fingers to extend the wings and examine their shape and texture. Note the way in which the hindwing folds when at rest.
Cockroach nymphs, like immatures of other insects, lack wings, but like other pauro- and hemimetabolous insects, develop wing pads in older instars (Fig 11). The wing pads are transformed to functional wings by the last molt.
Thoracic Segments
The thorax consists of three segments but your view is obscured dorsally by the wings and ventrally by the walking legs, especially their coxae. Each segment has the expected complement of sclerites consisting of dorsal tergite (or notum), lateral pleurites, and ventral sternites.
The ventral surface of the thorax consists of large expanses of unsclerotized, flexible, white exoskeleton in which are embedded myriad hard, golden brown, sclerites. Anchor the specimen on its back with a #1 stainless steel insect pin through the margin of the pronotum. Move the legs aside as needed to reveal the ventral surface of the thorax. The proximal end of the coxa of each thoracic limb articulates with a complex of sclerotized pleurites (Fig 12, 21-1D). On the midline, between the right and left pleurites, lies a series of thoracic sternites. The prosternum is a small, triangular, median, unpaired sclerite in the center of the prothorax, between the clusters of prothoracic pleurites. The mesosternum is more complicated than the prosternum and consists of several sclerites including a large bilobed anterior plate followed by two slender posterior sclerites. The metasternum is similar to the mesosternum except that the bilobed plate is completely divided into a pair of plates.
Figure 10. Dorsal view of a female P. fuliginosa with the wings removed or extended to reveal the dorsal surface of the thorax and abdomen. The abdomen has been stretched slightly to reveal tergites 8 and 9 which are normally hidden under tergite 7. blatt46L.gif
The thorax is equipped with two pairs of spiracles which open into the tracheal respiratory system. The spiracles are lateral and belong to the mesothorax and metathorax although they have migrated anteriorly (Fig 12). Consequently, the mesothoracic spiracle is on the posterior prothorax and the metathoracic spiracle is in the membrane between the mesothorax and metathorax.
Study the dorsal thorax by moving the wings aside. The pronotum is not obscured by wings and is always easily seen. It forms a large shield behind the head and may be mistaken for the head by the uninitiated. It is heavily sclerotized and dark in color with a pale border. The mesonotum (Fig 10) is a transverse, lightly sclerotized plate covering the dorsal surface of the mesothorax. The metanotum is similar and covers the mesothorax.
Abdomen
The abdomen is the largest of the three tagmata and contains most of the viscera. Its segmentation is conspicuous both dorsally and ventrally once the wings have been removed. The abdomen consists of 11 segments of which the 11 th is reduced and fused with the 10 th so that a maximum of ten segments are distinct and visible (Fig 21-1F). Further, some of the segments, especially in females, are highly modified or in unexpected locations and are not immediately visible and countable.
Figure 11. A late instar, 24 mm Periplaneta nymph. Blatt47L.gif
The first seven segments, known as the pregenital, or visceral, segments are similar, unspecialized, visible and countable. Segments 9 in males and 8-9 in females are the genital segments and are modified for reproductive functions. Segment 10 (and the vestigial 11) are postgenital and are also modified.
Dorsally each segment is covered by a sclerotized tergite and, similarly, is covered ventrally by a sclerotized sternite. Consecutive sternites articulate with each other by unsclerotized articular membranes (Fig 15). Laterally, tergites and sternites are connected by unsclerotized flexible pleura.
Examine the dorsum of the abdomen and find the abdominal tergites (Fig 10). Those of segments 1-7 are similar in size and easily distinguished but tergite 8 is smaller and tergite 9 smaller still. These two tergites are usually telescoped under tergite 7 and may be difficult to see until you lift tergite 7. Tergite 10 is a large posteriorly bilobed plate overhanging the anus (Fig 10). Tergite 11 is fused with tergite 10. The bilobed posterior extension of tergite 10 is sometimes referred to as the epiproct, which is homologous to the tergite of segment 11 (Fig 10, 21-1F).
Study the ventral surface of the abdomen (Fig 12, H). Sternite 1 is a small sclerite lying on the midline between the hindleg coxae. Sternites 2-6 are elongate, more or less rectangular, sclerotized plates covering the ventral surface of the abdomen. Sternites 7-9 of males resemble sternites 2-6 but the posterior sternites of females are very different. In females the posteriormost visible sternite is the large, highly modified sternite 7 (Fig 12, 13). Sternites 8, 9 are internalized under tergite 7 and cannot be seen without dissection.
Figure 12. Ventral view of a female P. fuliginosa with the right legs removed to reveal the ventral body surface and its sclerites. The position of the abdominal spiracles under the overhang of the preceding sternite is indicated. Blatt48L.gif
The anus is on segment 11 and lies ventral to the epiproct (tergite 11). It is flanked by a pair of triangular sclerites, the paraprocts (Fig 14, 21-11B, 21-12B,C) which are modifications of sternite 11. Sternite 10 is vestigial.
Most abdominal segments lack appendages but segment 10 bears a pair of large, segmented cerci, which are visible in both dorsal and ventral views (Fig 8, 13, 21-1F). Embryologically the cerci arise as the appendages of the 11 segment but in adults are attached to segment 10 since segment 11 has fused with segment 10. The cercus, although segmented, does not contain intrinsic muscles. Extrinsic muscles, however, extend from the cercus to the abdomen so it is capable of motion. Cerci bear vibration-sensitive hair sensillae sensitive to air movements, including sound, and perhaps to ground vibrations.
Eight pairs of spiracles are present in the abdomen and are much smaller than the thoracic spiracles. The abdominal spiracles are located laterally on the anterior-dorsal corner of the pleura of the first eight abdominal segments (Fig 12, 14, 21-1D,F). Lift the free posterior edge of a sternite and look at the antero-lateral corner of the following tergite to see the spiracle. It will be a small white oval. Abdominal spiracle 1 is on the lateral edge of tergite 1 rather than at the corner (Fig 12).
Genital Segments
Female
Study the dorsum of the posterior abdomen of a female. The tergites of the genital segments (abdominal segments 8, 9) are reduced and their tergites are largely hidden by the overhang of tergite 7 (Fig 10). Tergite 10 (fused with tergite 11) is a large, thin, fan-like plate extending posterior to the genital segments. It overhangs the anus and genital pouch, which will be described shortly. In P. americana tergite 10 is cleft posteriorly (Fig 10).
Figure 13. Ventral view of the posterior end of the abdomen of a female of P. fuliginosa. Blatt49L.gif
Look at the ventral surface of the abdomen (Fig 13). Sternites 8 and 9 cannot be seen externally. Sternite 7 (= hypogynum) is much larger and more heavily sclerotized than other sternites and posses a pair of clamshell-like posterior valves, the subgenital plates (also referred to variously as sternites 7 ¢ or 7a). Although at first the plates may appear to be independent of sternite 7, they are in fact continuous with it and connected by a short isthmus (Fig 13, 14). The isthmus is flexible and the plates can move apart during copulation or to permit extrusion of the ootheca as it is formed.
Hold the specimen upright and focus on the posterior end. This is much easier if you first separate the posterior abdomen from the remainder of the body (Do not remove the posterior abdomen if you plan to use this specimen later for study of internal anatomy). Make the separation between segment 6 and 7. You can now stand the posterior abdomen upright on the wax of the dissecting pan and you will not have to hold it in position. Focus on the posterior end of the abdomen (Fig 14).
Figure 14. Posterior view of the abdomen of a female P. fuliginosa. Arrows indicate movement of the subgenital plates. Blatt50L.gif
Tergites 7-10 dorsally and sternite 7 ventrally enclose a large space. The space is divided into dorsal and ventral regions by two lateral sclerites, the paraprocts (Fig 14, 21-11B). The anus opens into the dorsal chamber whereas the oviduct and seminal receptacle open into the ventral chamber, which is the genital pouch (Fig 15). With forceps pull the paraprocts aside to reveal the anus on segment 10-11 under the epiproct.
The genital pouch is formed by invagination of the exoskeleton of the sternum which brings sternites 8 and 9 deep into the pouch (Fig 15). This is the reason you could not see these sternites externally. With forceps and needles as needed, pull the subgenital plates of sternite 7 laterally and ventrally to reveal the genital pouch (= gynatrium). The genital pouch receives the spermatophore from the male during copulation and later the ootheca is molded in it.
The most conspicuous feature in the genital pouch is the ovipositor (Fig 15). It is a median process arising from the roof of the genital pouch and consists of three pairs of cuticularized valvulae (= gonapophyses). The valvulae are derived from the paired appendages of segments 8 and 9. The ovipositor guides eggs from the oviduct to the forming ootheca. The male external genitalia grip the ovipositor during copulation.
Sternite 8 can be seen by lifting the ovipositor dorsally and looking under it Fig 15, 23). Sternite 8 is a large sclerotized plate penetrated by the female gonopore opening from the common gonoduct. Sternite 9 is at the base of the ovipositor.
Figure 15. Sagittal section of the posterior abdomen of a generalized female cockroach. S = sternite, T = tergite. Redrawn from Cornwell (1968) after McKittrick (1964). Blatt51L.gif
Male
The dorsum of the male posterior abdomen is similar to that of the female. Tergite 7 is large and tergites 8 and 9 are much smaller and obscured by the overhang of 7 (Fig 16). Tergite 10 is a large, posteriorly cleft plate fused with tergite 11 to form the epiproct.
Figure 16. Dorsal view of the posterior abdomen of P. americana. T = tergite. Blatt52L.gif
The male venter is very different from that of females. Sternites 8, and 9 are unremarkable and unmodified from the condition of the more anterior sternites and visible externally (Fig 17). Sternite 9 is the posteriormost sternite and its posterior border bears a slender stylus on each side. Styli are absent in mature females but are present in nymphs of both sexes (Fig 21-12B).
A pair of triangular paraprocts is located immediately ventral to tergite 10-11 and lateral to the anus (Fig 14). Between the paraprocts and sternite 9 is a large genital pouch in which are housed the male external genitalia.
Figure 17. Ventral view of the posterior abdomen of a male P. americana. Blatt53L.gif
Anchor the posterior abdomen, ventral side up, with a pair of pins passing through the lateral tergites of segment 7. Lift or remove sternites 8 and 9 to reveal the genital pouch and its contents (Fig 18, 24). The male external genitalia comprise the several complex sclerites and soft tissues of three phallomeres (= phallic lobes, = gonapophyses), surrounding the male gonopore. The phallomeres are responsible for moving the subgenital plates aside to open the female’s genital pouch and for holding the female genitalia during copulation. Muscles arising on segment 9 extend to the phallomeres and operate them. Phallomeres are the appendages of segment 9.
Viewed from the venter, the ventral phallomere (Fig 18) is the most obvious of the three and can be seen immediately under sternite 9 (which has been removed or lifted). It is ventral to the gonopore and is by far the simplest of the three lobes. Its ventral surface is a slightly cupped sclerotized plate but it is membranous dorsally. The membranous ejaculatory duct ends at the gonopore on the dorsal surface of its base. Because it bears the gonopore, it is sometimes known as the penis.
The left phallomere is left of the ventral lobe and a little dorsal to it (Fig 18). It is a complex structure consisting of many sclerotized pieces including a hooked lobe (= grumolobus, titillator), prickly lobe (= acantholobus), a sharp stylet (= acutolobus, asperate lobe), and a pseudopenis. These are used during copulation to hold the valvulae of the female ovipositor so the spermatophore can be deposited in the correct location (at the opening of the seminal receptacle).
Figure 18. Ventral view of the posterior abdominal segments of a male P. americana. Sternites 8-9 have been removed and the phallomeres rearranged slightly for clarity. Some membranes and connective tissue have been removed. S = sternite, T = tergite. Blatt54L.gif
The right phallomere is to the right of the ventral phallomere and gonopore and is farther dorsal than either of the other two lobes. It is less complicated than the left lobe but also has easily recognized sclerotized parts. These include a piece that looks like a crayfish cheliped (= dikella, serrate lobe) and slender hooked piece (= falx).
Internal Anatomy
The cricket, Acheta, is recommended as an alternative for the study of internal anatomy in preference to the cockroach. Even though it is smaller, it is easier to dissect and its internal structures are easier to demonstrate.
Begin the study of cockroach internal anatomy with a fresh, undamaged specimen if possible. The dissection will be made from the dorsal side so you must first remove the wings by cutting their attachments with a pair of fine scissors. The dissection is facilitated if the legs are removed by cutting across their trochanters. Organ systems will be considered in order of their appearance in dorsal dissection.
" Place the specimen in a small dissecting pan of water or alcohol so it is completely immersed. Insert the blade of a pair of fine scissors under the posterior overhang of the right side of tergite 7 about 1 mm from the lateral margin of the tergite. Insert the blade only as deep as is necessary to penetrate the exoskeleton and be careful you do not damage internal organs with deep cuts. Cut anteriorly along the right side of the tergites all the way to the anterior end of the pronotum (Fig 19). Cut transversely across the anterior margin of the pronotum, just posterior to the head, and upon reaching the left side, change directions and cut posteriorly along the left side all the way back to tergite 7. Make a transverse cut through the exoskeleton across the posterior border of tergite 7. You have now cut all of the way around the dorsum.
Figure 19. Dorsal view of a male P. americana. Thoracic nota and abdominal terga 1-7 have been removed. T = tergite. Blatt55L.gif
Anchor the specimen to the wax of the dissecting pan with a #1 stainless steel insect pin through the left side of one of the thoracic segments, lateral to your incision. Insert the pin at a 45 ° angle. Remove the abdominal and thoracic tergites beginning with tergite 7. Lift each tergite and remove it without removing any of the underlying soft tissues. The tergites are transparent and thin whereas the underlying tissues are opaque. These tissues include muscles, heart, and tracheae. Do not remove the opaque tissue at this time.
In the abdomen the tergal muscles form a broad thin sheet of longitudinal muscle fibers (Fig 19). In the thorax the tergal muscles are smaller and confined by other muscles, chiefly those operating the legs. Running along the midline of this muscle layer is the heart. If you accidentally remove the muscle layer, the heart will be destroyed, so be careful.
Hemal System
The hemal system is the first organ system uncovered in dorsal dissection (Fig 19). It consists of the hemocoel, blood, and heart. The hemocoel is divided by perforated horizontal membranes, known as diaphragms, into a shallow dorsal pericardial sinus, a spacious perivisceral sinus, and a small ventral perineural sinus (Fig 16-7). Together the sinuses form a large partitioned blood space, the hemocoel. The hemocoel is the functional body cavity of these acoelomate animals.
The heart, surrounded by the pericardial sinus (= pericardial hemocoel) lies immediately under the tergites. Removal of the tergites opens the shallow pericardial sinus, although it is not at all obvious that you are looking at a blood space. It is the space between the exoskeleton and the dorsal diaphragm. The perforated, horizontal dorsal diaphragm is the sheet of connective tissue with longitudinal tergal muscles on top of it (Fig -16-7). This diaphragm separates the pericardial sinus from the much larger perivisceral sinus. These two sinuses are part of the hemocoel. Blood on its way back to the heart flows through the perforations in the diaphragm.
The heart is a longitudinal middorsal tube extending the length of the body in the pericardial sinus and resting on the dorsal diaphragm (Fig 19). The heart has segmental swellings, paired segmental ostia, paired segmental arteries (unusual in insects), and paired segmental alary muscles. The ostia, segmental vessels, and muscles will not be seen and most of the swellings are inconspicuous. The heart opens anteriorly into the cephalic hemocoel of the head and posteriorly bifurcates to form two arteries.
In arthropods blood is pumped anteriorly, during systole, by contractions of circular muscles in the heart wall. Blood leaves the heart, enters the cephalic hemocoel and passes posteriorly through the three sinuses of the hemocoel, eventually passing through the perforations of the diaphragms to enter the pericardial sinus. During diastole contractions of radiating alary muscles cause the heart to dilate and draw blood into its lumen through the ostia. The heart is now refilled with blood and ready to enter another systole. Heartbeat reversal has been observed in cockroaches.
Respiratory System
The respiratory system consists of 10 pairs of spiracles which open into a complex system of distributory tracheae which deliver oxygen to the tissues. In insects the blood is not involved in oxygen transport.
Removal of the tergites exposes the network of branching, tubular, white or silvery tracheae (Fig 19). Use fine forceps to remove muscles and connective tissue covering some of the tracheae and spiracles to improve your view of them.
The tracheae arise at spiracles (Fig 12, 19) in the pleura near the posterolateral corners of the tergites and join with other tracheae to form a single network of air tubes for the delivery of oxygen to the tissues (Fig 21-10B, C). Two pairs of spiracles are present in the thorax and eight pairs in the abdomen. No spiracles are present in the head. The spiracles were seen earlier in your study of the external anatomy.
On each side the respiratory system includes three longitudinal trunks and segmental tracheae to the three hemocoel sinuses and their viscera. The portion of the system you see at present consists, on each side, of a lateral longitudinal trunk extending from spiracle to spiracle (Fig 19), a series of dorsal segmental tracheae extending from the spiracles to the heart and pericardial sinus, and a dorsal longitudinal trunk connecting the dorsal segmental tracheae medially. The dorsal longitudinal trunks lie beside the heart. Other segmental vessels, which can