Category Zoology

            Insects have had 340 million years of experience in flying, which they now do very efficiently. Insects have two pairs of wings, which are flapped using a different kind of mechanism from birds. There are several ways in which the wings can beat, but usually they are raised by the contraction of a muscle. The wing then snaps down powerfully due to the spring-like properties of the insect’s external skeleton. This mechanism allows the wings to beat very fast – 250 times per second for bees, and as many as 1,000 times per second for some midges (which accounts for the high-pitched whine these insects make).

            Insects such as grasshoppers and locusts emerge from eggs as miniature adults. Other insects go through a complicated development during which they completely change their shape and structure. Houseflies, bees, wasps, butterflies and moths go through various stages of development. First they produce a larva, such as a grub or caterpillar, which lives an independent existence for some time. Next the larva goes through a process which is called metamorphosis. This is when the animal rests in a protective cover called a pupa. Inside the pupa, the insect’s internal structure is gradually rearranged into the adult form. Finally, the pupa splits open and the adult insect breaks out – it looks totally different to its larval stage. At first its body is very soft and vulnerable to predators, but the hard exterior skeleton soon forms. The adult is now ready to breed.

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            Together with scorpions, ticks and mites, spiders are part of a group called arachnids. They are not insects. The body of an arachnid is divided into two parts, while the body of an insect is in three sections. Arachnids have four pairs of walking legs, while insects have three pairs. Spiders are the most common members of this group of arthropods, and they are all carnivores. They capture their prey in webs or by running after them, paralyzing them by injecting poison from their jaws. Spiders cannot swallow solid food; instead they inject digestive juices into their prey, which liquefy it. They are then able to suck in the digested food through their jaws.

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            Spiders spin their webs from silk. It is pumped out from tiny nozzles at the back of the abdomen, which are called spinnerets. As the silk is stretched by the spider into a thread, it hardens. For its thickness, the thread becomes proportionately stronger than steel. Some of the threads are sticky, while others simply support the web. The spider is able to feel the vibrations of the web when an insect flies into it, and it runs quickly across the web to capture and subdue the prey. Usually, the insect is wrapped in silk before being eaten by the spider.

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            The rigid shell of arthropods cannot stretch to allow the animal to grow, so the shell must be discarded. When the shell splits open, the animal emerges and withdraws its legs and antennae. The newly emerged animal is soft and can now increase in size. A new hard shell forms within a few days. The animal is vulnerable to predators during this period, and must hideaway while the new shell develops.

            Arthropods are animals with a hard external skeleton like a suit of armour. The skeleton is jointed to allow movement. Arthropods have evolved in a different way to vertebrates and even their blood is chemically different and so it is not red. They do not have a brain and spinal cord like vertebrates. Instead, they have a nerve cord running along the underside of their body, and small thickenings of this nerve cord instead of a brain. Arthropods have efficient eyes, but these work in a different way from those of vertebrates. Arthropods such as spiders may have many eyes.

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