Insects include butterflies, beetles, bees, ants, spring-tails, silverfish, cockroaches, earwigs, termites, flies, aphids, lice, and fleas. More than one million different species are known out of a global diversity estimated at 10 million insect species. Of the 32 orders into which insects are classified, the largest is Coleoptera, or the beetles, with 125 different families and around 350,000 known species. Another diverse and large order is Hemiptera, the bugs.
ThoraxThe thorax is divided into three parts: prothorax, mesothorax, and metathorax. Each part carries a pair of legs. Most insects have wings: one pair or, more usually two. They are attached to the mesothorax and metathorax.
ExoskeletonThe hard outer "shell" of an insect, called the exoskeleton, provides an anchor for the muscles. It is made of a tough waterproof substance, chitin, and entirely covers the insect's body.
LegsAll insects have three pairs, which are modified according to the use they serve, such as digging, jumping, striking, and swimming. They are also jointed rather like our own. Although their structure is very different, some of the parts have been given the same name.
The compound eyes of insects are very complicated. They consist of a large number of small facets, each of which is itself a tiny eye having a lens and a retina, just as our eyes have, though their structure is very different.
The number of these facets varies in different species from less than 10 to 4,000 in the house-fly, while some dragon-flies have up to 30,000.
Although they are complicated, these eyes are not as efficient as ours in producing a distinct image, and are not able to focus, but are very sensitive to movement, so that an insect can quickly see the approach of an enemy. This is why a fly is so hard to catch.
The ocelli are much simpler; although they are obviously light-sensitive organs of some kind, their real function is not known with certainty.
AntennaeIn addition to its eyes an insect has another pair of sense organs on the head – the feelers or antennae. The most important function of the antennae is as organs of smell. A male moth finds the female by his sense of smell; if his antennae are cut off he cannot find her.
The form of the antennae varies greatly in different kinds of insects. They may be thread-like, thickened like clubs, or elaborately branched.
MouthInsects eat in one of two ways, either by chewing or by sucking. Their mouthparts are modified according to the way in which they eat. A grasshopper (left illustration below) chew up leaves; a butterfly (right) sucks nectar from flowers.
How an insect breathes
MetamorphosisMany insects, including some of the most familiar ones, develop in a remarkable way. The illustration below shows the four successive stages in the life if a butterfly – a progression known as metamorphosis. Among other insects with a life-history of this kind are moths, beetles, bees, and ants.
EntomologyThe scientific study of insects is called entomology. The foundations of entomology were laid by the Dutch naturalist, Jan Swammerdam (1637–1680). His researches and his book General History of Insects provided a system for classifying insects. Swammerdam also discovered (1658) the existence of red blood corpuscles.
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