INSECT RELATIVES - PANARTHROPODA

Transcription

1 Entomology BIO 3323 INSECT RELATIVES - PANARTHROPODA The Panarthropoda includes today's arthropods and their close relatives the onychophorans and tardigrades. Although they may not look like they are related to each other they share the characteristics of the moulted alpha-chitin cuticle, loss of external cilia, appendages with terminal claws, and the dorsal ostiate heart. Animals in the three phyla are all that remain of the panarthropods that first appeared with the Cambrian explosion; a period when the arthropod body plans was the most diverse. As new fossils from this period, and those like the Burgess Shale fossils are being re-examined, the diversity of types is increasing and transitional forms that link today's Crustacea, Chelicerata, and Uniramia are being identified proving that the group is monophyletic in its origins. ONYCHOPHORA Onychophorans are commonly called velvet worms, something that's a little hard to imagine as you look at a preserved specimen. But when it's alive all those little bumps, tubercles or papillae, on the surface of the animal give it a velvety look, and hence their name. Although originally though to be related to worms it now seems clear that they are members of the panarthropods. Onychophorans live in moist soil and leaf litter and are often found hiding in cracks and crevices. Although their cuticle is composed of the usual alpha-chitin, it's thin and moulted in patches rather than all at once. This is related to the worm-like movements of the animals and the role of underlying sheets of circular and longitudinal muscles and hemocoel is a hydrostatic skeleton. The thin cuticle, and absence of waterproofing in the epicuticle explains why these animals are restricted to moist environments. If your specimen is preserved in liquid you might want to let it dry off a bit before making your observations, if it gets dry enough you may even feel the velvety texture of the body wall! PAGE: 1 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

2 BIO 3323 Entomology Figure 1 External anatomy of the ventral surface of Onycophora. BIODIDAC Oral papilla Lip Claw Antenna Labrum Jaw Prebuccal lobes Leg 1 Excretory pore Head There is very little cephalisation in onychophorans and three pairs of appendages mark the head's location. The most obvious are the paired annulated antenna. These aren't segments! Look on the dorsal surface near the base of the antennae and you'll see a pair of eyes. The innervations of the antennae and eyes by the protocerebrum and deuterocerebrum are typical of the other uniramians that have a brain consisting of three parts. On the sides of the head are the oral papillae that contain the slime glands that shoot sticky threads that entangle prey and in some species harden to form a permanent trap. The mouth opens on the ventral surface of the head and is surrounded by peribuccal lobes. Look inside and you should be able to see the third pair of appendages, the paired chitinous jaws. When onychophorans feed they push their mouth against their meal and lateral movements of the jaws tear off pieces of food that are mixed with saliva liquifying the meal before its sucked back into the digestive tract. The fleshy lobes surrounding the mouth make sure that nothing leaks out. Trunk Just as there is no visible segmentation on the head, the same is true of the trunk. The fleshy lobopods extending from the sides of the animal are all this is visible of an underlying segmental plan that is repeated internally by the metanephridia. The legs move using an internal hydrostatic skeleton and are distinctly different from the jointed legs of other arthropods. Even with this difference the chitinous claw at the tip of the leg is similar to that found in panarthropods and is used to grip the substrate when moving. Look closely under the ventral surface of the legs where they connect with the trunk and you'll see a groove in side of which is located the opening to the metanephridia - you won't be able to see the opening itself. You also won't be able to see the small spiracular openings in the grooves of the rings that encircle the body. Look closely at the body surface and you'll see that there are two types of tubercles and although both are probably sensory some have a cuticular spine at PAGE: 2 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

3 Entomology BIO 3323 their tip. At the posterior end of the animal is the anus, and depending on the species available the gonopore opens between the last two pairs of legs. TARDIGRADA Tardigrades are microscopic in size and only a few get any larger than a millimeter or two in length. When they were first observed crawling up small pieces of vegetation, how they used their four-paired legs to paw the surface was reminiscent of bears pawing at their food. Tardigrdes are all aquatic and found in both freshwater and marine environments. The chitinous cuticle covering a tardigrade's body has an outer epicuticle, formed of cross-linked proteins; middle intracuticle containing lipid; and inner procuticle, a mixture of chitin and proteins. The cuticle is not waterproofed. Tardigrades are capable of withstanding adverse conditions by cryptobiosis. The dormant forms, called tuns, drop to only 3 percent water and load up with trehalose and glycerol to protect them from ice crystal formation. The tuns can survive temperatures from -272 degrees C to 151 degrees C, live for over 100 years, and survive radiation levels 1,000 times what humans can handle. If you're using prepared slides be sure to look at a number of different individuals. Some will be squished making it impossible to see any structures, while others may be on their sides, backs or fronts - take advantage of these different orientations in your observations. Cirrus Eyespot Frontal plate Scapular plate Median plate Leg Paired plates Lateral filament Terminal plate Claw Papilla Figure 2 External anatomy of the dorsal surface of Tardigrada. BIODIDAC Head Cephalisation in these animals is weak, and there are no appendages like those found in other panarthropods. They may have been lost as these unique little animals adapted to their miniature world. The mouth is located at the anterior tip of the animals. In prepared slides PAGE: 3 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

4 BIO 3323 Entomology the stylets of the feeding apparatus should be visible through the transparent cuticle. Tardigrades feed by piercing their food and sucking the fluids out using the muscular pharynx that is also visible. Trunk The trunk consists of four segments covered with sculpted cuticular plates. Four pairs of fleshy lobe-like legs include three that extend from the sides of the tardigrade and the posterior most pair extending behind. The lobe-like legs are extended by hydrostatic pressure and retracted by bands of muscle that extend to the tip of each leg. The legs terminate in a claw, or in some species an adhesive pad. CHELICERATA - MEROSTOMATA Don't let the common name of Limulus fool you. The horseshoe crab isn't a crab it's a chelicerate, but a very ancient one at that. There is one species that comes ashore on the east coast of the North America and two more inhabit Asian oceans. These living fossils are all the remains of an ancient chelicerate lineage and their bodies are covered by a massive tough exoskeleton that protects the underlying structures. They also burrow into the sand for further protection. The body consists of the usual chelicerate prosoma and posterior opisthosoma connected to each other by a hinge-like ligament. The opisthosoma has a posterior spine, or telson, that is used to flip the horseshoe crab over if it happens to end up on its back. External anatomy The body of all chelicerates is divided into two main tagmata, the anterior prosoma (or cephalothorax) and the posterior opisthosoma (or abdomen). The long tail isn't a segment, or tagma. Because it's located behind the anal opening it's a telson. The cuticle of the horseshoe crab is hardened by the calcium salts and in this way it is similar to that of the Crustacea. PAGE: 4 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

5 Entomology BIO 3323 Book gills Operculum Gnatobases Carapace Figure 3 External anatomy of the ventral side of the horse show crab. BIODIDAC Mouth Telson Anus Gill opercula Chelicera Pedipalp Abdomen th 4 leg First walking leg Opisthosoma Prosoma Prosoma The dorsal surface of the prosoma forms an enlarged horseshoe shaped carapace that surrounds the legs underneath. The prosoma is flattened and this helps the animal to shovel its way into the sediments. At the outer edge of the lateral ridge on the dorsal surface is a pair of compound eyes. Merostomata are one of the few chelicerates that still have compound eyes but they may not be homologous to the compound eyes that are found in the rest of the Arthropods. On the anterior tip of the medial ridge are the median eyes. These simple eyes may be hard to see if there is debris or organic matter encrusted on the surface of the carapace. On the ventral surface of the prosoma are the eight pairs of appendages typical of all chelicerates. The chelicera are small, consist of only three joints and are located in front of the mouth. Behind them are the larger pedipalps which have a similar structure as the following four pairs of legs behind them. In mature horseshoe crabs the male pedipalp is modified and the last segment becoming thicker and hook-like. It's used as a clasper to hold onto the carapace of the female during mating. The tips of the legs are chelate and used to manipulate food and pass it to the base of the legs where the basal segments of each leg forms a spiny gnathobase used to grind and tear food before it is passed to the mouth underneath. If your specimen isn't too brittle try spreading the legs to see the mouth. Although similar in segment number and the presence of the gnathobase the last pair of legs have two unique modifications. On the base of the leg, and on the outer surface, is a flattened, spatula-like structure that is used to clean debris from the gill surface. The second modification is found at the tip of the leg where the second to last segment consists of four flattened plates that help to push the horseshoe crab is it burrows. At the base of the last pair of legs is a moveable cuticular PAGE: 5 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

6 BIO 3323 Entomology extension called the chilaria and are all that remains of the pregenital segment of the opisthosoma. Opisthosoma Six flap-like plates are found on the ventral surface of the opisthosoma. Lift up each and look at what is underneath. Under all but the first you'll find the book gills that are used for gas exchange and the flapping motion of the opercula pump water over the respiratory surface. The first pair of plates is the genital opercula and paired genital openings can be seen underneath along the midline and about halfway back from the margin of the plate. In addition to the six opercula the six flexible spines on the margins of the opisthosoma identify six of the segments that form the tagma. The anal opening is located at the base of the telson. CHELICERATA - SCORPION Like the horseshoe crab segmentation is still visible in the tagma of the scorpion and this reveals how ancient these chelicerates are. Scorpions were some of the first animals to invade the terrestrial environment, feeding on soft-bodied invertebrate living in moist locations. Chronologically then, they are the first arthropod on land but it would be the insects that would be the most successful at exploiting the environment and as their numbers grew they became food for the scorpions Figure 4 External anatomy of the dorsal surface of a scorpion. BIODIDAC Median eyes Chelicera Prosoma Pedipalp Coxa Mesosoma Pretarsus Tibia Tarsus Patella Femur Trochanter Metasoma Telson Sting External anatomy Like other chelicerates a scorpion's body is divided into two tagma, that anterior prosoma, or cephalothorax, and posterior opisthosoma, PAGE: 6 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

7 Entomology BIO 3323 or abdomen with the opisthosoma being divided into an anterior mesosoma and distal metasoma with the sting at it's tip. Prosoma A cuticular carapace covers the dorsal surface of the prosoma and at the anterior end are the four pairs of simple eyes, ocelli. One pair forms the larger medial eyes and the other three pairs of ocelli are located on the lateral edge of the prosoma. Six pairs of appendages are attached to the prosoma, the chelicera, pedipalps and four pairs of walking legs. When viewed from the ventral surface the base of the various appendages surround a small sternal plate. The hardened pincers of the chelicera are attached to a large segment that forms the top of the buccal cavity. The pincers, or chelae, are used to tear and rip apart captured food before it is swallowed. The second pair of appendages are the six-segmented pedipalps consisting of a basal coax, trochanter, femur, patella, tibia and tarsus. The last two segments of the pedipalps form the pincer-like chela used to capture prey and the coxal plates form the sides of the buccal cavity. The eight segmented walking legs are attached to the body by the coax and additional segments include the trochanter, femur, patella, tibia, metatarsus, tarsus, pretarsus and claws. Cuticular extension of the coax on the first and second pairs of legs forms the bottom of the buccal cavity. Pedipalp Figure 5 External anatomy of the ventral surface of a scorpion. BIODIDAC Chelicera Genital operculum over genital pore Pectines Book lungs Opithosoma Each segment of the opisthosoma consists of a dorsal cuticular plate, the tergite, connected by a pleural membrane to the ventral sternite. The most conspicuous feature on the ventral surface of the mesosoma are the paired pectines. They resemble a comb, with a series of cuticular teeth that are embedded in a rod attached to the second segment of the mesosoma. The pectines are have a rich nerve supply that extends into each of the teeth suggesting a sensory role for the PAGE: 7 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

8 BIO 3323 Entomology structure, although just what it detects is still not clear to zoologists. In front of where the pectines are attached to the mesosoma, and on the first mesosomal segment, are two cuticular plates, the genital opercula, the cover the openings to the reproductive system. Both the genital opercula and the pectines are modified appendages. The remaining mesosomal segments have no appendages but on the ventral surface of segments three to six are the spiracular openings to the book lung. The five segments of the metasoma consist of the fused cuticle of the tergite and sternite, there is no pleural membrane, with the sting located at its tip. The sting is not a segment and is also referred to as the telson because of its location posterior to the anus. The sting consists of a hallow bulb and barb and the poison located in the bulb release their poison through an opening at the tip of the barb. The anus is located in the membranous region between the sting and the last metasomal segment. Most scorpions feed on insects and small invertebrates and when prey is located they grasp it with the chelae of the pedipalps and the opisthosoma is bent over the head and the captured prey is stung. Food is torn off and passes in to the buccal cavity, mixed with salivary secretions and digestive enzymes to liquefy it, and strained by setal hairs on the gnathobases before it enters the digestive tract. CHELICERATA - SPIDER ARGIOPE Nothing evokes fear and anguish in more people than spiders - arachnophobia. Spiders are aerial predators of insects and have mastered the terrestrial environment. They are covered with an alpha-chitinous exoskeleton with a waxy epicuticle essential for survival on land. The compound eye typical of other arthropods is missing and instead they rely on tactile information from the webs they spin and if vision is important they use their simple eyes. Instead of mandibles they use chelicera as their main feeding appendage. Spiders are variable in their morphology and the common garden spider, Argiope sp., allows us to identify most of the characteristics of animals in the group. External anatomy One of the things you'll immediately notice about your spider is that its legs are positioned above the body, rather than underneath! In a unique way that optimizes the small space inside the leg a spider legs contain a single muscles that works against the elasticity of the leg itself. Instead of having one muscle to raise the leg and another to lower it, muscles move the leg down and the elasticity of the leg moves it back up. When they die the muscles relax and the spring of the leg raises it over the body. The spiders cuticle is leathery and PAGE: 8 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

9 Entomology BIO 3323 their bodies aren't covered with the tough, thick cuticle typical of insects and large Crustacea. The chelicerate body is composed of two tagma, and anterior prosomoa, or cephalothorax, and posterior opisthosoma, or abdomen. Fang Chelicera Figure 6 External anatomy of the ventral side of a spider. BIODIDAC Pedipalp Prosoma Pedicel Book lung Epigastric furrow Opisthosoma Spiracle Anus Spinnerets Prosoma A carapace covers the dorsal surface of the spider prosoma and at the anterior end are four pairs of simple eyes. Although it may look like there are only six, the most lateral "eye" is actually a combination of two. Some of the eyes look forward, others up or down and to the side. Like most chelicerates the compound eyes are missing, as is the deuterocerebrum of the brain that would innervate it. The prosoma has six pairs of appendages attached to it: the chelicerae, pedipalps and four pairs of walking legs. The distal tips of the chelicerae are modified into fangs attached to a large basal segment that is in turn attached to the head. The poison glands, which produce a mixture of digestive enzymes and neurotoxins, are located in the basal segment and a duct carries its secretions to an opening at the tip of the fang. Look at the head from the ventral surface and located the enlarged base of the six-segmented pedipalp which helps to manipulate food before it's ingested. Termed either a maxilla or gnathobase the latter is probably the better term since it prevents confusion with the maxillary appendage found in other arthropods. A labium is located between the gnathobase of the pedipalps forming the lower lip of the buccal cavity. In male spiders the pediplaps are modified for sperm transfer. The upper lip, labrum, is located behind the chelicera and may be hard to see. If your specimen isn't too brittle, open the buccal cavity and try and locate the labrum inside. PAGE: 9 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

10 BIO 3323 Entomology A large sternal plate covers the ventral surface of the prosoma. Around its margins is the pleural membrane that connect the dorsal carapace and ventral sternum. The pleural membrane is hard to see because it surrounds the coxal joints of the four pairs of sevensegmented walking legs consisting of the coax, trochanter, femur, patella, tibia, metatarsus, tarsus and its claws. Opisthosoma Although the membranous cuticle of a spider hides it, the opisthosoma is composed of twelve segments consisting of dorsal tergites connected by pleural membranes to ventral sternites. The first segment of the opisthosoma is modified into the waist-like pedicel that connects the opisthosoma to the prosoma. At the anterior end of the opisthosoma are two triangular cuticular plates that mark the position of the underlying book lungs that open to the outside through the slits located just behind these plates. The book lungs are formed from sheets of thin cuticle, lamellae, arranged like the pages of a book, the origin of the structures name. In female spiders the cuticular extension between the spiracular openings is the epigynum that covers the gonopore and contains the opening to the seminal receptacles. At the posterior end of the opisthosoma are the three pairs of spinnerets and the anal papillae. The two largest spinnerets are composed of two segments and located towards the anterior and posterior and surround the unsegmented middle spinnerets. To see the middle spinneret you'll have to move the anterior and posterior ones out of the way. The spinnerets origins are though to be the appendages that were originally on these segments. Look closely and you'll see a variety of bumps on the surface and each produces a different type of silk from the silk glands underneath. Locate the anal slit on the tip of the anal papillae. In front of the spinnerets is a single medial spiracular opening. Spiders often have two types of respiratory systems including the book lungs that we've already mentioned and a tubular tracheal system. This spiracle is the opening to the tracheal system. UNIRAMIA - CHILOPODA Chilopods are commonly referred to as centipedes, although in reality there are never 100 pairs of legs - its usually somewhere between pairs. Centipedes are live in moist, terrestrial environments and are predacious feeding on insects, worms, and small molluscs. They can range in size from 10 centimetres to 30 centimetres found in some tropical species. Their outer epicuticle of the cuticle lacks the waterproofing waxes found in other uniramians and this, combined with the use of spiracles that can't be closed, explains why these myriapods are restricted to moist environments. PAGE: 10 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

11 Entomology BIO 3323 Tergites Eye Antenna Figure 7 External anatomy of head region of a centipede. BIODIDAC Second maxilla Labrum First maxilla Maxilliped (prehensor) Second leg First leg External Anatomy The body is divided into two tagma with the anterior trunk followed by the posterior trunk. The trunk is dorsoventrally flattened and a pair of legs extends from the sides of the segments. The first and last two segments don't have legs. Head The head consists of the typical six segments that are found in all uniramians and appendages include a pair of antennae, mandibles and two pairs of maxillae. The antennae are formed from twelve or more segments and vary in length depending on the species. As you look at the ventral surface of the head the most obvious feeding appendage is the poison claw, or fang. Technically it's a maxilliped since it is a trunk appendage that is now involved in feeding and in poisonous species it injects poisons from the tip of the claw to subdue prey. In non-poisonous species it is still an affective way of capturing prey. The mandible is also easy to see with the darkened teeth. Between the mandible and the maxilliped are the two pairs of maxillae. The first maxillae are fleshy lobes and the second maxillae are fused along the midline to form the base of the bucal cavity and only the second pair of maxillae has palps. Centipedes are nocturnal and in most species the compound eye has disappeared and a cluster of ocelli remain, although in some species this many include as many as 200 simple eyes that may be confused with a compound eye. Simple eyes don't form images and only respond to light levels. Trunk Each of the trunk segments is dorso-ventrally flattened and the dorsal tergite is connected to the ventral sternite by a pleural membrane. Seven segmented legs extend from the pleural membrane. As we've mentioned already the first trunk segment is the maxilliped consisting of four segments. At the tip is the hardened point of the claw; the basal segments meet along the midline creating a second lower lip to enclose the buccal cavity. At the posterior end of the body the second to last segment has appendages modified as gonopods, or in PAGE: 11 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

12 BIO 3323 Entomology some species these last appendages may be large and important sensory structures. The last segment houses the opening to the reproductive system and the anus. Look closely at the wall of each segment and locate the spiracles. In some species they are located on alternating segments. UNIRAMIA - DIPLOPODA The diplopods are commonly called millipedes, a name derived from what seems to be thousands of legs that really number around 375 pairs. Like centipedes, millipedes are terrestrial and restricted to moist soil, leaf litter and rotting vegetation because their epicuticle lacks the waterproofing waxes and the spiracles can't be closed. The diplopods and myriapods are combined as the Myriapoda. Unlike their centipede cousins, millipedes aren't predators and are herbivores feeding on plants or decomposing plant materials. Figure 8 External anatomy of head region of a centipede. BIODIDAC Tergites Collum Eye Sternites Antenna Labrum Genital pore Gnatochilarium Mandible External Anatomy The body is hard and cyclindrical and composed of two tagma, a head and trunk. While their common name comes from their numbers of legs, Diplopoda refers to what appears to be two legs for each segment of the trunk when seen from above. Well take a look at this a little later when we examine the trunk in more detail. Head The head of a millipede is bent forward and the dorsal cuticle forms a head capsule or epicranium. The anterior most edge of the head capsule is bilobed and it's edge is ridged with teeth. It's assumed that this is homologous with the labrum found in other uniramians and forms the roof of the buccal cavity. Two pairs of segmented antennae are the most prominent appendages on the head and they can be held tight to the head in grooves. At the base of each antennae is a cluster of approximately fourty simple eyes. The ventral surface of the PAGE: 12 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

13 Entomology BIO 3323 head is formed by the gnathochilarium that is the floor of the buccal cavity. The gnathochelarium are homologous with the first maxillae found in other unirmaians but there is either no second maxillary segment that appears to have been lost or the two maxillary segments have fused to form the gnathochelarium. Zoologists still haven't resolved this issue. Between the gnathochilarium and the anterior margin of the head capsule are the mandibles. Use a pair of forceps to pull the gnathochilarium back and expose the inner surface of the mandible composed of two segments. A large, immovable basal segment, and a distal segment modified as a tooth, scoop, bristled plate and grinding molar surface. Trunk First examine the segmentation near the centre of the trunk where each "segment "is an almost perfect circle with the dorsal tergite forming most of the circle. The tergites overlap and the posterior margin of each overlap the anterior margin of the one behind. This overlap is important part of the defensive reactions of millipedes that roll up in a tight ball with out exposing any of the fleshy membranes between the tergites. A second defense is secretions of the repugnatorial glands located at the base of the coax, although the openings of these are too hard to see. On the ventral surface the two pairs of legs for each of the apparent dorsal segments are easy to see and the functional unit is referred to as a diplosegment, a fusion of two ancestral segments. The legs are seven segmented with a basal coax, trochanter, prefemur, femur, tibia, tarsus and the claw shaped last segment. The first five segments of the trunk aren't diplosegments. The first is the enlarged tergite of the collum, and although we count it here as the first trunk segement there are some zoologists that believe that this may be the second post-mandibular appendage of the head. The second and first trunk segments don't extend down the sides as far as those on the rest of the body and a pleural membrane connects the tergite and sternite to each other. The second tergite has a lateral process or spine the projects towards the midline. By the third segment the tergites and sternites meet. In each of the first five segments a single pair of legs correlates to a tergal plate even though two pairs of legs are paired by their proximity. Because there are five pairs of legs here and two in each of the remaining diplosegments millipedes always have an odd number of legs. Look closely at the lateral surface of each tergite and locate the spiracular opening. If you're having trouble finding these on a diplosegment they are often easier to see on the first five trunk segments. The posterior end of the millipede consists of two plates that meet along the midline forming the slit like anus. This last trunk segment lacks appendages. The male genital opening is located between the sternum and tergite of the second segment. The penis is usually retracted and the medial slit from which it everts is difficult to see. The most obvious male PAGE: 13 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

14 BIO 3323 Entomology feature are the gonopods on the second full diplosegment. Here the coax of the legs have been modified and embedded in a fleshy cuticular membrane. The gonopods are probably modified legs and have two segments consisting of an elaborate spoon shaped piece and blade they function like the chela of other arthropods. During mating the male bends its body so that the everted penis contacts the gonopods and either sperm itself, or a spermatophore is formed and passed to the female. The female genital opening is located in the vulva, a almond shaped piece of cuticle located more to the side and front edge of the second tergum - this may be hard to see. The vulva stores sperm from the male, acting as a seminal receptacle, and releases sperm to fertilize the eggs as they are laid. CRUSTACEA The Crustacea are the dominant Arthropod in the marine environment and they range in size from the microscopic to the gigantic. Obviously we can't go over all the different classes and types here so we'll bring out our old friend the crayfish back and look for the typical crustacean characters using that specimen. Examine your specimen, and identify the key arthropod characteristics such as the hardened outer cuticle and jointed appendages and compound eyes, which in Crustacea are always located on the ends of stalks. The body is arranged in a linear series of metameres which have undergone tagmosis into distinct body regions. The two principle regions, or tagma, in the decapods are the cephalothorax and abdomen (pleon). The cephalothorax is covered dorsally by the carapace and the division between the head and thorax (pereon) is represented by the cervical groove in the carapace. Three of the eight thoracopods have fused with the head and are now referred to as maxillipeds. This leaves 5 pairs of appendages on the thorax. Posterior to the cervical groove and running parallel to the body axis are two branchial grooves which identify the underlying branchial or gill chambers. The structural organisation of the abdomen is simple with overlapping curved dorsal tergites and ventral sternites connected by articulating membranes. PAGE: 14 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

15 Entomology BIO 3323 Rostrum Cephalothorax Abdomen Uropods Figure 9 Major external features of a crayfish BIODIDAC Eye Antennule Carapace Swimmerets Telson Antenna Maxilliped Cheliped Walking legs In the ancestral Crustacea all appendages on each segment would have been similar in appearance and when you look at the ventral surface of your specimen you can see the serially homologous appendages.all the appendages are built on a biramous plan consisting of a protopodite, exopodite and endopodite but each of these parts of the basic limb can be modified in a variety of different ways. Pieces may be lost, enlarged, added or fused.as individual pairs of appendages become specialised for the different roles of food gathering, locomotion, respiration and we see the ancestral biramous plan become obscured. Each function requires different modifications and as a consequence the appearance of the appendages changes, although serial homology is maintained. How are the basic elements of the biramous appendage modified? Try and keep track of what is what using diagrams. Examine the specimen and identify appendages starting from the anterior most antennules, antennae, followed by mandibles and the two maxillae. These five cephalic appendages are found in all Crustacea. Three maxillipeds follow and mark the division between the head and thorax (pereon). The first of five walking legs or pereopods is modified as a prehensile organ and referred to as a cheliped and it is followed by the 4 pairs of walking legs. The walking legs are biramous but only the endopodite remains. The tip of the legs are chelate and you should be careful that you don't confuse this chelate condition with being biramous. The difference here is that the second to last segment extends a lateral process forward. It gives a branched appearance but in fact the segments that make up the leg are still in a linear sequence and the apparent branch is at the distal end rather than at the base of the appendage. Behind the walking legs are six pairs of abdominal appendages, five biramous swimmerets and the last set of modified legs the uropod which, when combined with the telson, forms the rudder like posterior end of the PAGE: 15 - INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN

16 BIO 3323 Entomology animal. The first two swimmerets are sexually dimorphic and are modified as copulatory organs in the male. PAGE: 16 INSECT RELATIVES - PANARTHROPODA JON G. HOUSEMAN