EXERCISE 6 ARTHROPODA: OBSERVATION AND CLASSIFICATION OF SPECIMENS Structure 6.1 Introduction Objectives 6.2 Materials Required 6.3 General Characters and Classification of Arthropoda 6.4 Subphylum Chelicerata General Characters Class Arachnida Class Merostomata 6.5 Subphylum Crustacea General Characters Class Malacostraca 6.6 Subphylum Uniramia General Characters Class Chilopoda Class Pauropoda Class Insecta 6.7 Terminal Questions 6.1 INTRODUCTION Before you go through this exercise you are advised to refer to Unit 8, Block 2, of the Animal Diversity course and try to recapitulate the salient features and characters of Phylum Arthropoda and its classification. In this exercise you will examine and identify some of the common representatives of Arthropoda. The phylum Arthropoda as the name signifies (arthros = joined + podos = foot) are the animals which possess jointed appendages. Arthropoda constitutes the largest phylum of animal kingdom comprising animals like crabs, lobsters, scorpions, spiders, millipedes, centipedes and insects such as cockroaches, bedbugs, houseflies, mosquitoes, butterflies and honeybees. In this exercise you will study the specimens of Palamnaeus, Limulus, Palaemon, Cancer, Scolopendra, Julus, Periplaneta, and Apis. Objectives After performing this exercise you should be able to: • identify Palamnaeus (scorpion), Limulus (king crab), Penaeus/Palaemon (prawn), Scolopendra (centipede ) , Julus (Millipede ), Periplaneta (cockroach) and Apis (honeybee) and give their scientific and common names, Animal Diversity: • classify the identified arthropods up to the level of the class Laboratory • list characters justifying their classification and mention special features, if any, • mention the habit, habitat and geographical location of the identified genera, • draw labelled diagrams of the identified specimens and • mention economic importance, if any, of each of the identified genera. 6.2 MATERIALS REQUIRED 1. Preserved specimens/permanent slides of the following animals: Palamnaeus (Scorpion) Limulus (King crab) Cancer (Crab) Scolopendra (Centipede) Julus (Millipeda) Palaemon (Prawn) Periplaneta (Cockroach) Apis (Honey bee) 2. Compound Microscope 3. Hand Lens 4. Magnifying glass 5. Practical Record Book 6. Pencil 7. Rubber 6.3 GENERAL CHARACTERS AND CLASSIFICATION OF ARTHROPODA These are metamerically segmented animals with an exoskeleton of cuticle. Schizocoelic coelom is much reduced and is replaced by perivisceral haemocoel, which is filled with blood. Circulatory system is of open type. True nephridia are absent, but coelomoducts act as gonoducts and often as excretory organs. Body of the animal is bilaterally symmetrical. They also bear paired appendages. You will examine, identify and draw labelled line diagrams of the preserved specimens/ permanent slides of the whole mounts of the animals. You are supposed to write the comments on the observations you have made. 68 Arthropoda: 6.4 SUBPHYLUM CHELICERATA Observation and Classification of 6.4.1 General Characters Specimens The subphylum Chelicerata includes familiar horse shoe crab, spiders, scorpions, ticks and mites. i) Body consists of an anterior prosoma and a posterior opisthosoma. ii) Prosoma bears the appendages involved in feeding and locomotion. Opisthosoma may have exterior segmentation but appendages are either absent or considerably reduced. iii) Their main appendages are chelicerae and pedipalpi. iv) Antennae and mandibles are absent. v) These are the first land animals in evolution of arthropoda which have successfully colonised the terrestrial environment. 6.4.2 Class – Arachnida The class Arachnida ( arachne = spider) has terrestrial and few aquatic animals. It includes spider, scorpions and mites. i) Prosoma bears six pair of appendages: one pair of chelicerae, one pair of pedipalpi and four pairs of walking legs. ii) Abdomen is generally devoid of appendages. iii) Respiratory organs are tracheae, book lungs and book gills. iv) Excretion is performed by malpighian tubules and coxal glands. v) Sexes are separate but sexual dimorphism is not conspicuous. So, you have refreshed your background knowledge of the characters of subphylum Chelicerata and class Arachnida. Let us now study individual examples under the class Arachnida. Example – Palamnaeus (Scorpion) Examine the specimen both from dorsal and ventral sides. Observe the following features:- i) It has dark coloured body to match with the surroundings. The underside is pale yellow. ii) Body is divided into prosoma or cephalothorax and opisthosoma or abdomen. Prosoma is covered by square shaped shield of dorsal carapace. iii) Dorsal carapace has right and left frontal lobes. It also bears a pair of median eyes with 2-13 pairs of smaller lateral eyes on the antero- lateral margins. Eyes are simple in structure. On the ventral surface of the prosoma there is a single, median, small, triangular plate, the sternum . It lies between the coxa of the 3 rd and 4 th pair of legs. iv) Opisthosoma is distinguished as anterior mesosoma and posterior metasoma. Mesosoma consists of seven segments. Each segment is 69 Animal Diversity: covered by tergal plates dorsally and sternal plates ventrally. Both are Laboratory joined laterally by pleural membrane. v) Sternum of the first mesosoma bears genital aperture which is covered by a genital operculum. Sternum of second segment bears comb-like appendages called pectines. Pectine has small comb-like 24-26 bristles. vi) Metasoma or post-abdomen is slender and consists of five cylindrical segments with a telson or sting . The sting comprises an ampulla and a distal spine or aculeus. Ampulla contains poison glands with an opening at the tip of the distal spine. (a) (b) Fig. 6.1: Palamnaeus , A -Dorsal view; B – Ventral view. vii) Scorpion has six pairs of appendages. These are a pair of chelicerae , a pair of pedipalp and four pairs of walking legs . Chelicerae are three-segmented with two distal segments forming chela. Pedipalps have six joints, walking legs are composed of coxa, trochanter, femur, tibia and three tarsi. viii) Respiratory system is highly characteristic consisting of 4 pairs of book lungs or pulmonary sacs. Paired book lungs are situated on the ventro- lateral sides of the 3 rd , 4 th , 5 th and 6 th mesosomal or preabdominal segments. ix) Sexes are separate. Habit and Habitat Scorpion is nocturnal found under stones or bark of trees or in burrows. Geographical Distribution: Most tropical and subtropical countries. Very common in India, most prevalent in Maharashtra, Assam, Rajasthan, Saurashtra and Uttar Pradesh. Classification and its Justification Kingdom Animalia Animals, multi-cellular organisms with cells that lack a cell wall, many capable of Arthropoda: Observation and movement or movement of some of their body Classification of parts or capable of movement at some time of Specimens their life cycle; heterotrophic nutrition. Phylum Arthropoda Jointed appendages, segmented body. Subphylum Chelicerata Main appendages are chelicerae and pedipalpi, body divided into prosoma and opisthosoma. Class Arachnida Four pairs of walking legs Genus Palamnaeus Species bengalensis Common Scorpion Name 6.4.3 Class Merostomata The Class Merostomata are aquatic chelicerates in which five or six pairs of abdominal appendages are modified into gills. At the end of body there is a sword like telson. Example Limulus (king crab) King crab (also called horse shoe crab) is not really a crab, true crabs are included in Malacostraca. King crab is the largest living merostome. It comes under the category of living fossils. Examine the specimen from both dorsal and ventral sides and look for the following features: i) Limulus is horse-shoe shaped crab with dorsally convex carapace covering prosoma. The posterior side of the body is fitted into the slots of carapace. ii) A long caudal spine extends from the rear of the opisthosoma. Spine helps in locomotion. It is neither offensive nor defensive in function. iii) Ventral side bears segmental appendages. Flanking labrum are chelicerae. Three segmented chelicerae form the pincers, forming a chelate appendage. iv) Opisthosoma is hexagonal broadly joined to prosoma and consists of mesosomatic segments, an unsegmented vestigial metasoma. v) The next four pairs are chelated legs and the 6 th pair is non-chelate legs. Just behind the mouth an oral plate-like appendage called chilaria is present. vi) The mesosomatic appendages are flattened and plate-like. The first pair is united in the middle to form a genital operculum. Exopodite of the remaining five pairs bears respiratory book gills. 71 Animal Diversity: Laboratory (a) (b) Fig. 6.2: Limulus . A) Dorsal view; B) Ventral view. vii) Horse shoe crab eats polychaetes, small molluscs and soft invertebrates of the ocean floor. viii) The foregut of Limulus includes an oesophagus and a gizzard, the midgut comprises a stomach and paired hepatic caecae and an intestine. Habit and Habitat Marine animal found in shallow waters, burrowing in sand, sluggish and mostly buried in sand. Feeds on soft mollusca and other small animals. Geographical Distribution: North Western Atlantic coast, Gulf of Mexico, West Indies, also South East Asian Pacific coasts. Classification and its Justification Kingdom Animalia Animals, multi-cellular organisms with cells that lack a cell wall, many capable of movement or movement of some of their body parts or capable of movement at some time of their life cycle; heterotrophic nutrition. Phylum Arthropoda Jointed appendages, segmented body.