The structure of an Insects
The fundamental difference in the structure of an insect and that of a bird or mammal is that an insect is an invertebrate—that is, it has no backbone or bone structure of any kind. The strength of the body lies in the exoskeleton which is soft in a few cases, especially in the larva, but in most cases it is very hard and proof against any form of damage. The outer casing of an insect is made from chitin which is rather similar in its nature and texture to your own finger nails. The chitin plates of armour are joined together with softer membrane to form segments, one of which forms the head, the next three form the thorax (to which the legs and wings are attached) and the remainder of the segments form the abdomen and contain the digestive system and vital organs.
The digestive system of insects includes the intestine and gut, a system of fine tubes (the Malpighian tubes) replaces the kidney, there is no liver and the heart is quite unlike the heart of a mammal or bird. It is a long tube. Inside the back of the abdomen, which is muscular and able to expand and contract, driving blood around the inside of the insect’s body. Here we have another fundamental difference in insects , the blood is not circulated inside blood vessels but is free flowing inside the exoskeleton so that all the internal organs are constantly bathed in blood. The blood is not red, nor does it contain corpuscles or oxygen for the body. Animals with a backbone and bone structure, there are lungs which absorb air into millions of tiny cavities where it passes into the blood and its circulated by the heart round the entire body. Insects have a rather simpler, less well developed system of breathing holes, a pair on each abdominal segment, down the sides. Little tubes lead from the breathing holes (spiracles . The tubes are called trachea and lead into the main body walls where they divide and branch into minute trachaeoles , dispersing air to every part.
They work almost like lungs but in reverse, distributing air through minute tubules rather than absorbing air into the little tubes, as happens in the lung. Insects are thought not to have clear vision as we know it but they are very sensitive to light and in fact they see certain lights and colours differently from us. They are particularly sensitive to ultra-violet light, and flowers which emit ultra-violet radiation will attract bees for this rather than what we would call their “‘true’’ colour. Moths will be: attracted to ultra-violet lamps which are black and not at all bright to our eyes. Insects have compound eyes, made up of thousands of hexagonal facets which form a compound, almost mosaic image.. The image is almost certainly rather blurred but sufficient for their purpose as they have well developed ways of sensing vibration and movement. Additionally, insects have single cell eyes, known as ocelli, usually situated between the compound eyes. These appear to be capable only of detecting light. A few insects only have ocelli.
The nervous system of insects is primitive compared with our own .There are many parts of an insect which are not served with a nerve ending and are therefore without feeling. Each nerve ending leads to a simple central cord of nerve centers , called ganglia, and the brain is nothing more than a conglomeration of two or three of these ganglia, situated in the head. The nerve usually ends in a sensitive hair on the outer cuticle of the insect and this hair passes the stimulus of vibration, air movement, pressure etc. to the central column of ganglia. It seems very likely hat insects do not experience the sensation we know as pain: their nerve ganglia more likely interpret the sensation by analyzing it computer-fashion as vibration, tickling, pressure, heat etc. which automatically causes a motive reaction designed to get the insect out of trouble.
To insects hearing is a question of their ability to receive and interpret vibration . In many this is confined to the nerve endings and sensory hairs which are often situated in greatest numbers on the antennae and in some insects there is a definite ear, known as a tympanum. The tympanum is a drum-like membrane served with many hairless nerve endings, which receive and interpret sound more accurately than simply as vibrations. These “ears” oddly enough are often situated on the legs, or between the thorax and abdomen and not on the head as might be expected. Insects hear noise of high frequency very well.
Apart from the noise made by their wings in flight many insects are silent and have no means, audible to humans, of calling each other. Certain moths, notably the Death’s Head Hawk Moth, can squeak and this noise is produced by blowing air through its tubular tongue, like a whistle. Grasshoppers and crickets produce their stridulating noise by rubbing their legs against the wing cases with unbelievable frequency. Cicadas have a concave tympanum which is pressed in and out at the rate of 4,000 cycles or more per second, rather like pressing a tin lid in and out, and at that speed the result is a very rapid and loud trilling noise.
