Metamorphosis in Arthropods

What are arthropods?

An arthropod (arthro-, joint + podos, foot) is an invertebrate animal having an exoskeleton, a-segmented body, and jointed appendages (paired appendages).

Arthropods form the phylum Arthropoda, which includes the insects, arachnids, myriapods, and crustaceans. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species, some of which, unlike most animals, are very successful in dry environments.

What is metamorphosis?

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.

Some insects, fishes, amphibians, mollusks,crustaceans, cnidarians, echinoderms and tunicates               undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").

Types of Metamorphosis in Arthropods

Ø  simple metamorphosis

Ø  complete metamorphosis

ü Simple Metamorphosis

The changes in form that occur as an insect approaches adulthood. When the immature insects and the adults are similar in appearance, the process is called simple metamorphosis, and the juvenile insects are called nymphs.

 

  {Thysanura (Silverfish)}

 

 {Heteroptera (True Bugs)}      

Ø Complete Metamorphosis

When the immature insects and the adults have different forms, the process is called complete metamorphosis, and the worm, or grub, like juvenile insects are called larvae. After the last larval instar, the insect changes into a pupa. In this stage, the insect does not feed or move around much. It may be covered by a protective cocoon. Eventually the insect molts for the last time and emerges as an adult. 

 {Lepidoptera (Butterflies and Moths)}

Tagmatization in Annelids

What are annelids?

The annelids (anellus, "little ring"), also known as the ringed worms or segmented worms, are a large phylum, with over 17,000 extant species including rag worms, earthworms, and leeches.

 The species exist in and have adapted to various ecologies - some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments. The annelids are bilaterally symmetrical, triploblastic, coelomate, invertebrate organisms. They also have Para podia for locomotion. Most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species. Cladistics research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes mollusks, brachiopods, flatworms and nemertean.

What is meant by TAGMATIZATION?

Tagmatization. An annelid's body is made up of a series of repeating units sequentially arranged down the length of the animal. Metamerism describes the process that resulted in the segments, metameres, each of which is identical to the one on either side of it. Arthropods took metamerisation a step farther.

Tagmatization in annelids

Earthworm bodies are divided into series of ring like segments. The body is also divided internally. Metamerism influences every aspect of annelid structure and function. It includes anatomical arrangement of organs. This arrangement is associated with Metamerism.

There is compartmentalization of the body. Therefore, each segment has its own excretory, nervous and circulatory structures. There are two related functions Metamerism: flexible support, efficient locomotion.

These two functions depend on the metameric arrangement of the coelom.

Origin of coelom: 

The body cavity of annelids arises by segmental splitting of mesoderm during embryonic stage. Mesoderm occupies the region between ectodermic and endoderm on both side of the embryonic gut. Each cavity is enlarged and it forms a double-membrane septum. This septum separates the anterior and posterior margin of each coelomic space. It also separate the dorsal and ventral mesenteries associated with the digestive tract.

Origin of muscles

 Muscles also develop from mesodermal layers of each segment. A layer of circular muscles lies below the epidermis. A layer of longitudinal muscles is present just below^– the circular muscles. These muscles run between the septa that separate each segment. Some polychaetes also have oblique muscles. The leeches have dorsoventral muscles.

Advantages of segmentally arranged coelom and muscles

1. The coelomic spaces and muscles produce hydrostatic compartments. Coelomic hydro skeleton has many advantages. It has locomotory and supportive functions. These functions are not possible in non-metameric animals that have a hydrostatic skeleton. Each segment is controlled independently of other segments. The muscles can act as antagonistic pairs within a segment. The coelomic fluid provides a hydrostatic skeleton. The muscles operate against it. These arrangement causes swimming, crawling, and burrowing.

2. A second advantage is that it lessens the impact of injury. If one or a few segments lured, adjacent segments are separated from injured segments by septa. They are o maintain nearly normal functions. It increases the chance of survival.

3. Metamerism causes modification in certain regions of the body. They become lazed for feeding, locomotion, and reproduction. The specialization of body regions in a metameric animal is called Tagmatization.