User:Paul venter/morphology

Head
The head of an animal makes up the foremost part of the body and usually houses the sensing organs for sound, light, touch, taste and smell.

Ears
Ears are the organs used for detecting pressure variation or sound waves through air, water and solids - in the known animal world the frequencies stretch from infrasound to ultrasound, although from the small size of bacteria it is clear that they would be sensitive to sound in the range 109Hz - sound of an appropriate frequency and amplitude will cause the entire body to resonate. Elephants, whales, hippo, rhino, giraffe, crocodiles and other large animals may produce and receive infrasound in the range 5–35 Hz - Humpback Whales down to 30Hz, Blue Whales to 5Hz, Atlantic cod to 2Hz. The lateral lines of fish and amphibians must also be included in this category. The hair cells found in the lateral line are similar to those of the vertebrate inner ear. Lateral lines help fish navigate and locate prey as well as receiving advance warning of approaching storms, enabling them to seek the protection of deeper water.

Bats use ultrasonic sound for communication, navigation and food location. Their external ears have evolved shapes that sharply focus sounds of interest.

Insects hear in a number of ways. The tympanum is a thin membrane stretched across an air space and connected to the nervous system. Siting of tympana varies according to the insect type, common locations being on a thoracic segment or front legs. In some insect species the covering tympanum may be absent with the underlying neural structure still present. Tympana are commonly found in the Orthoptera and Cicadidae. Another insect hearing organ is Johnston's organ in which the movement of hairs on the antenna is detected. Sound sensitive hairs are also found on some Lepidoptera larvae and on some Orthoptera. Labial palps and labral pilifers are found on the head of certain Hawk Moth subtribes, Choerocampina and Acherontiina, and respond to frequencies between 30-70 kHz, allowing the moth to hear and avoid many insectivorous bats.

Eyes
Eyes, in a broad sense, range from simple light-sensitive skin to complex structures yielding high-resolution images, and are the organs for detecting electro-magnetic radiation over a wide portion of the spectrum, from ultraviolet to infrared. Ultraviolet light (UV) is visible to birds, reptiles, and insects, and is used for gender recognition (butterfly), navigation (Monarch butterfly) and food location (honey bee), many flowers having clearly marked petals to indicate the position of nectar and pollen. Raptors such as kestrels can track their prey by the UV-reflecting urine of middens and territory markers. Reptiles absorb UV light through the skin, producing essential parts of their immune system. Reptile lateral eyes are sensitive to UV-A, while the parietal eye, sensitive to light 600-750nm, is active in hormone production and thermoregulation. Some snakes such as the pitviper, python and boa have evolved infrared sensing pits for the detection of predator and prey, and for thermoregulation. The bare patches below the throat on some vultures are thought to be heat-sensitive and to be used for finding thermals or updrafts.

Skin
Skin forms the first protective layer of any organism, and is used in ornamentation and communication (sailfish, cuttlefish, geladas, butterflies), disguise through patterning and colour (chameleon and a host of creatures), It may be covered with scales as in the reptiles and fish, with hair as in the mammals, or with feathers as in the birds. Besides physical protection, hair and feathers aid animals such as otters and penguins in being hydrodynamically streamlined while feathers aerodynamically streamline most birds.

Alkaloid poisons occur on the skin of poison dart frogs such as the Golden Poison Frog. The skin of various animals harbours colonies of bacteria which produce tetrodotoxin, a potent neurotoxin not destroyed by cooking - such hosts are pufferfish, porcupinefish, ocean sunfish, triggerfish, angelfish, blue-ringed octopus, rough-skinned newt, toads of the genus Atelopus, several sea stars, a polyclad flatworm, Chaetognatha (arrow worms), Nemertea (ribbonworms), several species of Xanthidae crabs and many more. The porcupinefish is also covered in spines which, together with its ability to inflate its body, makes predation difficult.

Skin, in many mammals, plays an important role in temperature control through sweat glands. Mammary glands are considered to have evolved from sweat glands. Discus fish produce a milk-like substance from their skin, nursing larvae during their first few days.

The flexibility of skin has led to the developing of marsupial and sea horse pouches, gliding membranes of flying squirrels and the brood pouch of penguins and other birds.