Question

1. Describe these arthropods in detail:  crustaceans, insects and chelicerates.  Include the major organization of their bodies, their skeletons, their appendages, how they grow (ecdysis) and how they develop (metamorphosis).  Also describe for each their major habitats, tagmata, numbers of antennae and limbs, and ecological roles.

Answer 1

Arthropoda is the largest phylum of animal kingdom. It comprises the following subphyla;

• trilobites - they are extincted marine arthropods
• chelicerates- they comprises arachinids
• myriapods- it includes millipedes centipedes
• crustaceans - aquatic arthropods, have biramous appendages
• hexapods- include insects

CRUSTACEANS

They are generally aquatic insects.

ORGANIZATION OF BODY

• Body is formed of number of segments called somites
• In some crustaceans the somites get fused, in some they are free
• Number of somites varies in different crustaceans
• Each omite can bear a pair of appendages
• On the anterior end of the body a structure called acron is present, which is an unsegmented region.
• In most of the crustacean the head is formed by the fusion of atleast 4 segments with the acron.
• At the tain end of the body, there is a unsegmented region called telson is present.
• Telson may have two processes or rami.
• Telson together with rami forms a structure called furca.
• The size of the rami varies in different crustaceans, sometimes they are longer than the rest of the body.

SKELTON

• The outer covering of crustaceans are called exoskelton, cuticle or integument.
• The main function of exosleton is to protect the body.
• They seve as a site for the attachment of muscles.
• The cuticle may be a thin structure or hard as a shell of crabs.
• Epidermis secretes the cuticle.
• The outermost layer of cuticle is called, epicuticle which lacks chitin.
• The innermost layer, procuticle have chitin.

APPENDAGES

• Antenna is the only appendage all crustacean have in the common. All other appendages varies in different groups.
• Variation in the appendages are used in the classification of crustacean groups.
• In simplest condition, each segment of crustacea have a pair of biramous appendages.
• A biramous appendage has two branches. The basal portion is called protopod.
• The outsides branches is called exopod
• The inner branches are called enopod.
• Sometimes the protopod have additional outgrowths.
• The additional outgrowth are called epipods and endodites based on their location.
• 

NAME AND FUNCTIONS OF DIFFERENT APPENDAGES

Name	Region	Function	Comments	Other names used
Antennule	head	sensory		1st antennae
Antennae	head	sensory		2nd antennae
Mandible	head	crushing or grinding food items		jaw
Maxillae	head	chewing and shredding food items	1 or 2 pairs	acessory jaws
Maxilliped	thorax	manipulating food items	0-3 pairs
Pereopod	thorax	walking, clinging	as many as 5 pairs	walking legs
Pleopod	abdomen	respiration, swimming, holding offspring		natapods, swimmerettes
Uropod	telson (last abd. segment)	protection, escape		tail fan

ECDYSIS AND METAMORPHOSIS

The life cycle of different creustacean groups varies.

Commonly the females lays eggs in water, which is then fertilized by sperm. The zygote undergo cleavage to form a larva called nauplius. Nauplius larva has unsegmented body, simple naupliar eyes and 3 pair of appendages. The larva undergoes molts to form copepodid, which ressembles adult. Copepod undergo molts to form adult.

INSECTS

ORGANIZATION OF BODY

Segmented body

The body is divided into head,thorax and abdomen.

The head is enclosed in epicranium.Head contains antenna, mouth parts and eyes.

Thorax is divided into pro thorax meso thorax and meta thorax.

Abdomen is the largest part of the body, it is multisegmented. It contains digestive, excretory and reproductive organs.

ECDYSIS

METAMORPHOSIS

Insects undergo metamorphosis during their devolepment. There are two types of metamorphosis, complete metamorphosis or holometabolism and incomplete metabolism or hemimetabolism.

SKELTON

The exoskelton, the cuticle is made up of two layers, the outer epicuticle without chitin and the inner procuticle, which is thicker and contain chitin.

CHELICERATES

The body is divided in to two segments. The fused head and thorax called opisthosoma and abdomen called as prosoma. Prosoma bears six pairs of appendages namely, pair of checerae, a pair of pedipalpi and four pairs of walking legs. The opisthosoma bears three pairs of short conical projections called spinnerets.

LIFECYCLE

METAMORPHOSIS

During the larval and nymph stages, the young arachnids look like their parents, although they are much smaller in size, and sometimes have different coloration and patterns. Molting is a process in which the larvae and nymphs grow and shed their exoskeletons when their bodies become too big for them. Spiders molt between four and 12 times during their lives, while scorpions molt five or six times before they mature.

SKELTON

The skelton is formed of chitin. Provide protection to body.