Why are some insects attracted towards light?

Generally animals and plants are attracted towards light. This tendency is termed phototropism or phototaxis. Animals which towards the source of light are known as positively phototropic and others that shun light are called negatively phototropic. Most of the insects are positively phototropic but the degree of attraction differs. And some are negatively phototropic. Bed bug shows negative phototropism. Mosquitoes shun intense light, but in dim light they display positive phototropism. This behaviour differs in different species of insects with the exhibition of the following traits.

Insects without eyes also exhibit phototropism. The photosensitivity is distributed or diffused throughout the dorsal surface of insects so photo stimulation can occur even if the insect does not possess any eyes. Some insects are more sensitive to light rays. Their surface cells and eyes are more refined to perceive and follow light sources.

Some are attracted towards yellow light and some towards mercury light etc. Well illuminated areas are used as mating grounds by male insects, full of matured sperms and females with matured eggs.

Why don't we have hair on our palms?

In humans hair is present in the skin of nearly every part of the body excepting the palms of hands, the soles of the feet, the flexor surface of the digits. The structural components of the skin alone decide the generation of the appendages of the skin. Structurally the skin has two layers: the Epidermis and Dermis. Among these two layers, the epidermis has a high capacity for regeneration after damage. It continually replaces the outer dead cells and also generated the appendages of the skin, like hairs, nails, sweat and sebaceous glands.

In the two parts of the hair, namely the root and shaft, the root is the structure which emerges first during development and is called the hair follicle. It is set in between the epidermis and the superficial part of the dermis. Each hair follicle commences on the surface of the skin with a funnel shaped opening. From this opening the follicle passes inwards in an oblique or curved direction.

As the deep end of each hair follicle there is a small conical vascular eminence called papilla, which is continuous with the dermal layer of the skin. The capillaries of the papilla provide nutrients to the hair.

When any one of the layers of epidermis and dermis gets abnormal development it affects the formation of the hair follicle and also becomes an unfit layer to support the hair.

For example, in the skin of palm and soles the stratum cornium of the keratinization zone of epidermis and reticular layer of dermis are comparatively thicker than in the skin of other parts of our body.

Such a thick keratinization zone will not allow the formation of hair follicles and the thick dermis is not the ideal structure to support the germinal matrix of the hair follicles. That is why hairs do not grow on our palm of the hands and the soles of the feet.

Ants do not get hurt when they fall. Why?

The fall of a body is controlled mainly by gravitational attraction of the Earth. The gravitational force depends on the mass of the falling object-a heavier object is attracted more than a lighter object. This attractive force is opposed by an upward thrust (resistance) offered by air on the body. This resistance also depends on the surface area of the object. That is, if the surface area is more, the resistance is also more. Thus in any falling object, these two forces compete with each other.

In the case of an ant, the force of gravity is almost balanced by air resistance and so it is able to land safely. If there is a wind blowing, ants also float away. However, if a cluster of ants or a big ant is forcibly hurled to the ground, they will get hurt. Anyway, it will be difficult to know whether the ant gets hurt or not.

Why does our hair change its colour as we age?

Hairs are the appendages of the skin generated from the epidermal layer. Hair is made up of Keratin, a highly insoluble and mechanically stable fibrous protein. This Keratin is not only found in hairs but also in the skin. Actually Keratin is produced from the Keratinisation zone of the epidermis, which is the outer most layer of the skin. In the skin it provides water proofing quality.

The Keratin is generally pigmented. It is intensively pigmented in the hair. The dark black colour of the hair is due to the presence of high concentration of melanin pigments in it. The skin colour is also due to the presence of this pigment in the keratinocytes.

The Keratin gets its melanin pigments from melanocytes, which are found in the inner layer of the epidermis, which is found just beneath the keratinizing layer. The melanocytes have lond processes which extent between and under the cells of the epidermis. The melanin granules formed in the melanocytes pass along their branches and are secreted at their tips. The granules are subsequently engulfed by the keratinocytes, which make up 90 percent of the epidermal cells.

Melanin is a protein like polymer of the amino acid tyrocin. In its biosynthesis tyrocin is converted in to dihydroxy phenyl alanine (DHPA) by oxidative enzymes amongst which tyrocin is particularly important. Then a series of reactions take place during which polymerisaton occurs to form the final melanoprotein.

The hair grows only from the keratinocytes of the germinal matrix of the hair follicle. This germinal matrix lies in the proximal enlargement of the root hair, called the hair bulb. The hair shaft, which projects from the surface, consists of an inner medulla, an intermediate cortex and an outer cuticle. All these parts are made up of cornified cells.

The medulla is composed of polyhedral cells; the cortex consists of elongated cells with inner lumen. These cells are united to form flattened fusiform fibers. The lumens of these cells contain pigmented granules in dark hair and air space in white hair.

The development of white hairs because of the absence of melanin pigments, may be due to the absence of one or more enzymes, necessary for the DHPA path way. It will lead to the failure of melanin accumulation in the keratinocytes, found in the hair bulb, from which hair is growing.

Usually such physiological disorder occurs in the old age, which results in the growing of gray and white hairs in the body.

Why do our arms move to and fro when we walk?

To keep our torsos stable and conserve energy, we swing our arms backwards and forwards while walking. When you swing, say, your right leg forward to take a step, you provide a rotational moment about the central vertical axis of your torso. By the principle of conservation of angular momentum, an opposite reactionary moment is felt by your torso. By swinging your right arm backwards and your left arm forwards, you counterbalance this moment. Just try running without swinging your arms at all. Or worst still, try running while swinging your arms in the opposite directions to normal: that is: swing your left arm forward when you swing your left leg forward and so on. You will find that your torso rotates from side to side in an uncomfortable and unnatural manner.

Of course, legs are heavier than arms, so as to ensure that the moments are the same, evolution has ensured that our arms are further from the central axis of our bodies than our legs are. This allows the moments from our legs and our arms to be roughly equal.

Two-legged walking animals balance themselves by synchronising the movement of the backbone to the side of the leg that stays in contact with the ground. This keeps their gravitational centre close to the standing leg. It is seen in chicken and, to better effect, in penguins.

Why do runners run in anti-clockwise direction?

As the heart is on the left side, for humans and animals, running anticlockwise makes the centrifugal force in the body to act from left to right. Whereas it is from right to left for clockwise running.

Superior venecava ( the principal vein carrying blood to the heart) takes blood to the heart aided by heart suction. This vein carries blood from left to right.

Centrifugal force due to anticlockwise running helps this suction. If we run clockwise, the centrifugal force impedes suction. That is why, in olden days, health officers ensured that all carnival merry-go-rounds were run only in the anti-clockwise direction.

Racing tracks, animal shows in circuses, bullock-drawn pelton wheels, all mostly have only left turns. Stairways in temple towers have only left turns for going up. Clockwise running tires people, especially, children, easily.

Why do we blink our eyes?

The objective of blinking of our eyelids is to keep the front of the eyeball clean. Blinking is done by means of muscles in the eye lids and the cleansing by tears. The tears are secreted in a little gland and carried along to the eye and when our eyelids open and close, the tears are poured over the front of the eye and they wash away any particles of dust or any other harmful substances.

Some animals like the snake for example, do not have eyelids and hence cannot blink. But there is a hard film or scale over the eyes to protect them from dust and injury.