Jointed - Legged Animals - Arthropods

the group of animals which has the largest number of species is the philm aropa arthropods include over three4 of all the million or more known species of animalsles of the five major classes of ARS the centipedes the millipedes the arachnids which include mostly spiders and the largest class of all the insects arthropods are the most successful of all animal F that is they occupy the greatest variety of habitats with the greatest number of species what are the reasons for this success story to help answer this question let us begin with the Crustaceans and use this crayfish as our first example the most striking thing about it is that the body is enclosed in a hard kitous covering or exoskeleton it is jointed to allow movement there are advantages to this jointed armor it provides protection while allowing for motion but it also has some drawbacks the exoskeleton is dead excreted material that does not grow like the rest of the body as the animal outgrows its hard covering it sheds it from time in a process called molting during molting the animal is helpless and vulnerable to attack first the skin separates from the exoskeleton and finally the soft animal withdraws from its shell then it secretes a new and slightly larger covering which gradually hardens if damage is done to an appendage of a lobster's body as in this conflict the Animal by itself is likely to sever that appendage between molts the creature will probably regenerate the Lost appendage so that eventually a wholly new claw will appear another feature of the body is segmentation each body segment typically Bears a pair of jointed kitous appendages these appendages are highly specialized the first pair is the eyes two pairs of antennas function as sensory organs then there are the mouth Parts adapted for eating next come the large crushing claws then the walking legs the action of the legs moves the gills back and forth creating a current of water here we have removed a part of the see the gills as blood through the gills oxygen is absorbed carbon dioxide eliminated in this crayfish we have removed par dorsal exoskeleton over the heart so we can see it pumping the blood crayfish reproduced from eggs after the eggs are laid they are carried about on the female's abdominal appendages the eggs hatch into tiny crayfish eventually these leave the mother and continue a separate exist if they are able to escape enemies certain lobsters are almost identical to crayfish in body structure but like many other crustations that live in the sea their eggs hatch into tiny larvas with three pairs of legs that feed and grow and eventually change through a series of molts into tiny lobsters crabs have a foreshortened but laterally expanded body structure most crabs live in salt water but there are a few fresh water and land species this is a fairy shrimp A Primitive type of crustation its appendages are less specialized than those of the lobster or crayfish the fairy shrimp reminds the scientist of an extinct primitive crustation the trilobit because its appendages are unspecialized this fossil trilobit has been extinct for 500 million years another kind of crustation includes the water fleas such as dhia and copap pods there are aquatic and land form all have bodies that are to bottom cont representing of crustaceans the amphipods are side to side what appear to be jaw glands these assist centipedes in their carnivorous feeding habits the bite of this large centipede is dangerous here is a creature that suggests an interesting problem it looks something like a centipede but isn't one it it is called a peripitus most zoologists do not consider it to be an arthropod at all in that it does not have a kitous exoskeleton or jointed kitous appendages as the arthropods dobles the arthropods in several details such as Tiny kitous claws at the ends of its feet like ner it might belong to the next lower film The segmented worm where fits on the tree of life helps us undiff as the earthworm and the lobster are related to of the body actually the abdominal segments are fused in pairs so the general rule of one pair of appendages per segment still applies the arachnids as a class include the spiders scorpions mites ticks harvestmen the king crab of the sea and others the spiders are the largest and most familiar group of arachnids they are easily distinguished from insects by by the fact that the spider body has only two main regions while insects have three the spider has four pairs of legs insects have three pairs spiders lack feelers or antennas they depend for their sense of touch upon sensitive bristles scattered over the body and appendages unlike insects spiders have pairs of tiny simple eyes the usual number is four pairs the spider's web is almost unique among animal adaptations for capturing prey strands of the web are secreted by special glands at the rear of the animal's body once the prey is entangled the spider May attack at its Leisure the animal injects a poison from its fangs into its prey it pours digestive fluid from the mouth into the wound made by the bite and sucks up the liquefied tissues of the prey not all spiders use a web as a means of securing food the trapdoor spider uses a different kind of device it hides under the door and captures passing prey specialization is the key to arthropod success for it allows different members of the philm to adapt to almost any situation the grasshopper is a member of the largest most varied and most successful class of arthropods the insects the name insect refers to the fact that the body is divided into region three in all though insects follow almost every possible way of life each insect manifests the same basic arthropod plan a body and appendages enclosed in a jointed kitous armor usually though not in all orders there are two pairs of wings wings carry insects over physical barriers and out of the reach of most enemies particularly welldeveloped are the sensory appendages of the head the antennas and the eyes there are two sets of eyes simple and compound the simple eyes distinguish light from Darkness the compound eyes are image forming even though the eyes are not movable the grasshopper has a large field of vision most most insects hatch from eggs from the eggs of the housefly come slender wormlike larvas in five or six days the larvae skin thickens and the pupa or resting stage begins in another five or six days a full grown fly bursts from the pupa case the fly is as large as it will ever be arthropods generally have a short life cycle therefore a rapid Evolution this is one reason why arthropods have reached the peak of invertebrate complexity and predominance although the arthropod body plan remains fairly evident in all the insects there are variations which allow different orders to fill many different niches within an environment some insects are herbivorous like these potato beans some like these aphids suck plant juices some like this mosquito suck blood and some are carnivorous like this aquatic larvae others are parasitic and lay their eggs in the body of a host when the young wasp larvas hatch they feed upon their host and emerge to pupate on the surface many insects have different specializations during different parts of their life cycles the butterfly goes through a process of change called metamorphosis the Egg of the butterfly hatches into a soft bodied wormlike larvae that moves about feeds steadily and grows rapidly the larvae of the butterfly or moth has a familiar name of its own the caterpillar when the caterpillar has fed well grown considerably and stored away a large amount of food it changes its body form finally there emerges the mature stage we call the adult insect the butterfly in this case the cycle is now complete complete metamorphosis or complete change is not Universal among insects but all go through at least a partial metamorphosis some species of insects such as ants wasps termites and bees are social and are organized into colonies each colony has a single queen whose sole function is to to reproduce the species a specialization which no other insect in the colony possesses communal life in a colony of bees functions efficiently because of a division of labor among individuals this is also true in the agricultural societies of leaf cutting ants their colonies May number up to 600,000 individuals each Each of which performs certain definite functions necessary for survival of the colony some workers cut the leaves into transportable size for movement to the nest they or other workers carry the leaves along wellestablished trails to the underground home of the colony inside the nest other workers cultivate a fungus on the leaves which becomes the colony's basic food insect colonies here represented by army ants also have soldiers equipped to protect the others the social organization of insects seems to resemble human organization but it would be unscientific to draw any human implications because the behavior of social insects is instinctive not learned their brain capacity is small the brain is limited by the size of the body and the insect body is likely to remain small because of limitations of the body plan but within the limits of their body structure and brain capacity the arthropods have evolved into the dominant group of invertebrates some arthropods are of benefit to man as a source of food or some valuable service others especially the in do Untold damage to clothing they destroy many crops and they act as carriers of disease the success of the arthropods challenges our efforts to control them the outcome of this competition is still uncertain