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All these species are divided up into about 32 orders, depending on whose taxonomic system you use, of which, the largest is the Beetles, or Coleoptera, with 125 different families and around 500,000 species they are an incredibly diverse group of animals. In fact, one in every four animal species on this planet is a beetle. When Everywhere is Home Well, everywhere on land anyway, very few insects have colonised the sea, though some like the Marine Flies (Halobates sp.) and the Seashore Collembolan, Anurida maritma, live on the surface. Also the larva of a small number of True Flies (Diptera) On the land however there isn't anyhere you can go that you can't find some insects, even in the frozen extremes of Arctica and Antartica you will find some insects alive and active during the warmer months. The Biggest
Another competitor for the title is the extemely rare South American Longhorn Beetle Titanus giganteus, these giants can have a body length (not including antennae) of over 16cms (6.5 ins), other longhorn beetles are nearly as large and may look even bigger because of their longer legs i.e. Xixuthrus heros from Fiji. Another beetle, Dynastes hercules is also well known for reaching 16cms in length, though it is not nearly a heavy. However other insects are larger in other ways, the longest insect in the world is the Stick-Insect Pharnacia kirbyi, the females of which can be over 36cm long. Some living lepidoptera have wingspans as great as 32cm and an area of over 300 square cms. The Smallest
There are also many small Hymenoptera, especially in the Superfamily Chalcidoidea, such as the Fairy Flies, of the family Myrmaridae, of which Alaptus magnanimus, at 0.21mm long, was once thought to be the smallest insects in the world. However another Hymenopteran parasite now holds the record. Megaphragma caribea from Guadeloupe, measuring out at a huge 0.17 mm long, is now probably the smallest known insect in the world. Simply The Most The incredible size of individual species of insects is only dwarfed by the incredible numbers they sometimes occur in. In 1943 Profeesor Salt found that an acre of British pastureland near Cambridge supported over 1,000,000,000 Arthropods of which nearly 400,000,000 were Insects and 666,000,000 were Mites the remaining 38,000,000 were Myriapods (Centipedes and Millipedes). Some Scientist have recorded the otherwise inconspicous Springtails at densities as high 100,000,000 per square metre in the ordinary farm soil of Iowa U.S.A. In Africa swarms of Orthoptera ( Desert Locusts Schistocerca gregaria) may contain as many as 28,000,000,000 individuals. Although each Locust only weighs about 2.5grams when they are all added up together this comes to 70,000 tons of locust.
With around one million named species and perhaps several times that number unnamed, insects account for a great majority of the species of animals on earth. They are a tremendously successful group. Insects can be found in almost all terrestrial and freshwater habitats, from the driest deserts to freshwater ponds, from the canopy of a tropical rainforest (where their diversity is unbelievably great) to the arctic wastes. A few species are even marine. Their feeding habits are similarly varied; almost any substance that has nutritive value is eaten by some group of insects. Insects also show huge variety in shape and form. Almost the only condition their group does not attain is very large body size. A number of features, however, are shared by most kinds of living insects. In addition to the general characteristics of uniramians, these include a body composed of three tagmata, a head, thorax, and abodmen; a pair of relatively large compound eyes and usually three ocelli located on the head; a pair of antennae, also on the head; mouthparts consisting of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx; two pairs of wings, derived from outgrowths of the body wall (unlike any vertebrate wings); and three pairs of walking legs. Insects have a complete and complex digestive tract. Their mouthparts are especially variable, often complexly related to their feeding habits. Insects "breathe" through a tracheal system, with external openings called spiracles and increasingly finely branched tubules that carry gases right to the metabolizing tissues. Aquatic forms may exchange gases through the body wall or they may have various kinds of gills. Excretion of nitrogenous waste takes place through Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. The ganglia are largely independent in their functioning; for example, an isolated thorax is capable of walking. Yet ganglia also use sensory output. A grasshopper with one wing removed can correct for this loss and maintain flight, using sensory input from its brain. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities are astounding. Insects are dioecious and fertilization is internal in most. The ways in which mating is accomplished, however, are incredibly variable; study of this variability by evolutionary biologists has greatly advanced our understanding of the evolution of behavior, social evolution, and traits such as number, size of young and patterns of investment in them. Reproduction by insects often involves a male locating a receptive female through chemicals (pheromones) released by the female. In most species, females store the sperm in a special receptacle in their abdomens; even species that lay huge numbers of eggs (in honeybees, for example, the number may be over one million), females mate only once and rely on sperm stored during that mating for the rest of their lives. The manner in which growth is accomplished is an especially important characteristic of insects. In some, hatching eggs produce miniature adults, which to grow must shed their exoskeleton in a process called ecdyisis. In almost 90% of insect species, however, newly hatched young are completely different in appearance from adults. These larval forms usually live in different habitats, eat different foods, and assume a body form completely different from that of their parents. The larva feeds and grows, molting its skin periodically. At some point larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. In this nonfeeding condition it is called a pupa or chrysalis. While so encased, the larva undergoes a complete transformation or "metamorphosis" of its body form, and a fully-formed adult emerges. Insects that experience this sort of complete change are called "holometabolous." Other species undergo a more gradual process, in which the newly hatched young are more similar to the adult but are small in size, lack wings, are sexually immature, and may differ in other, relatively minor ways as well. The young in these insects are called nymphs, and the lifestyle is referred to as "hemimetabolous." Insects are incalculably valuable to man. Usually, we think of them in a negative context. Insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit horrible diseases. But without them, we could not exist. They are a fundamental part of our ecosystem. A brief and incomplete list of their positive roles would include the pollination of many, perhaps most higher plants; the decomposition of organic materials, facilitating the recycling of carbon, nitrogen, and other essential nutrients; the control of populations of harmful invertebrate species (including other insects); the direct production of certain foods (honey, for example); and the manufacture of useful products such as silk and shellac. The Insect Abdomen The insect abdomen is built up of a series of concave upper integumental plates known as 'tergites' and convex lower integumental plates known as 'sternites', the whole being held together by a tough yet stretchable membrane. It contains the insects digestive tract and reproductive organs, it consists of eleven segments in most orders of insects though the 11th segment is absent in the adult of most the higher orders. In many of the Hymenoptera, and several other orders, the number of abdominal segments is reduced and in the Honey Bee only seven segments are visible. In the 'Collembola' (Springtails) the abdomen has only six segments. In the Hymenoptera there is a constriction where the 1st and 2nd abdominal segments meet, this is called the waste, and the remaining portion of the abdomen is called the gaster. Unlike other Arthropods the the insects possess no legs on the abdomen in the adult form, though the 'Protura' do have rudimentary leg-like appendages on the first three abdominal segments. Many larval insects particularly the 'Lepidoptera' and the 'Symphyta' (Sawflies) have appendages called 'pseudo' or prolegs on their posterior abdominal segments as well as their more familiar thoracic legs these allow them to grip onto the edges of plant leaves as they walk around. The Muscles Like us Insects need muscles in order to move the various different bits of their bodies around, however insects have their muscles attached to the inside of their skeleton because like all the arthropods they have their skeletons on the outside of their body. The inside of an insect's exoskeleton has special contours and bits and bobs on it which project inwards and allow for muscles to be attached and to help give them leverage, these projections are called 'apodemes'. The musculature of even the smallest insect can be as complicated as our own and makes for a fascinating study of design in miniature. The muscles of insects are generally light grey or translucent, unlike ours which appear red. This is because insects lack both the blood system that we have and the haemoglobin that makes our blood and hence our muscles red. The Heart and the Blood The haemolymph (blood) of insects flows freely around the inside of their bodies. Because an insect's haemolymph is not responsible for the transmission of oxygen to its cells, this is the job of the Trachaea, and therefore does not contain haemoglobin, it is not red. Normally it is a watery green colour, though it is pigmented (coloured) in some species. This haemolymph is a sort of soup rich in nutrients that flows around the inside of the insects body allowing the various organs to get at whatever resources they need and into which they dump their waste products. These waste products are later removed from the haemolymph by the Malpighian tubules. Because insects do not have veins and arteries like us and the rest of the vertebrates, (they do not need to), they do not have a complicated heart like ours either. The insect heart it is basically a tube, sealed at one end, which runs along their back. It beats regularly thus swishing the blood in and out and around the body. In some cases it has inlets, with one way valves, and in others it has outlets as well. The Aorta is simple a tube that runs from the heart towards the brain, it is open at the end near the brain. You can get some idea how the heart of an insect works by cutting the nozzle off the top of an empty washing-up-liquid bottle and taking it into the bath with you. Hold it under the water and squeeze it a few times and watch how the water swirls around. Here is a lovely drawing of the internal organs of an ant made by a talented Russian artist and physiologist called Nasonov before the great revolution in the late 1800s, it is very large (190K jpg) and if you have a slow link you might like to see the smaller sectioned drawings below it instead though they have lost some clarity in resizing. Digestion
The Digestive system of an insect is usually a long straight tube running from the mouth to the anus, it is often divided into the 'fore gut', the 'mid gut' and the 'hind gut'. Immediately behind the mouth are the Salivary Glands, in most species these secrete saliva, generally a watery fluid that lubricates the food and contains a few enzymes to begin the processes of digestion. However in some carnivorous insects the saliva is composed entirely of digestive enzymes, this applies particularly to those with external digestion of the food. In other insects the salivary glands have become modified for purposes that have nothing to do with digestion. In Lepidopteran caterpillars and Caddisfly larvae they have been converted to the production of silk, while in the Queen Honey Bee they are called the mandibular glands and secrete hormones. The Fore Gut The fore gut is generally considered to consist of four sections, the Pharynx, the Oesophagus, the Crop and the Proventriculus. It is also known as the Stomodaeum. The pharynx is the first part of the fore gut and apart from being a tube that connects the interior of the mouth area (sometimes known as the 'Buccal Cavity) with the more inward parts of the gut it sometimes serves as a pump to suck up the liquefied food of those insects which feed by means external digestion. The Oesophagus is basically a tube leading to the mid gut via the crop and the proventriculus or gizzard. The crop is simply a storage area and the proventriculus, or gizzard, is a muscular extension of the crop. In those insects which feed on solid foods it is used to grind the food up into smaller particles, it can also serve as a filter to keep oversized particles out of the main digestive tract and as a valve controlling the flow of food into the midgut. The fore gut and the mid gut are separated by the 'stomodeal or cardiac valve'. The Mid Gut The midgut (Mesenteron in some books) runs from the 'digestive or gastric caeca', a series of stubby pointed tubes leading off from the stomach to just before the Malpighian tubules, a series of long thin tubes. In between the two of these is the stomach, or ventriculus, which is the area of most active digestion. The gastric caeca serve to increase the surface area of the midgut, thus increasing both its ability to secrete digestive enzymes and its ability to extract useful products from the partially digested food. The useful proteins, vitamins and fats that are released by the digestive processes pass across the wall of the midgut into the body cavity. The mid gut is lined by a semipermeable membrane composed of protein and chitin, like the cuticle, which allows the passage of liquids and dissolved substances to the midgut wall while preventing the passage of solid food particles, it is continually worn away by the passage of food through the gut and replaced by the epithelial cells of the mid gut wall. The mid gut and the hindgut are separated by the 'proctodeal valve'. The Hind Gut From the mid gut food passes to the hind gut (Proctodaeum in some books). The hind gut comprises the 'intestines' which is where much of the diffusion into the the insects body occurs. The 'rectum' which compresses the undigested food and waste products, extracts more water from this if necessary before it is passed out through the 'anus' as faeces. Arising from and the foremost part of the hind gut are the Malpighian tubules (named after Malpighi who discovered them) are not really to do with digestion at all but with elimination. They act like our kidneys and extract metabolic waste products (mostly nitrogenous ones such as urea, and uric acid) from the circulating body fluid called the haemolymph and excrete them into the intestines which is the first part of the hind gut. Though insects possess a large number of digestive enzymes, they are often helped by the presence of symbiotic micro-organisms, such as protozoa in the case of the termites and some primitive cockroaches which feed on wood, and bacteria in the wax moth Galleria mellonella which feeds on the wax that honey bees Apis mellifera uses to make the combs in its hives. Reproductive Structures Most insect species are bisexual, i.e. there are males and females in most species, these often look very different and have even been mistaken for different species in the past, some species are capable of reproduction without males, the eggs are unfertilised but develop and hatch into nymphs or larvae that are always female themselves, this is called 'parthenogenesis'. Externally the sexual organs, called genitalia, of a female insect generally consist of an 'ovipositor' which is often encased in a pair of filaments called a 'sheath' and is which is used to by the female to put her eggs where she wants. Its form very greatly throughout the Insecta (i.e. the whole order of insects). The ovipositor of the Diptera (True Flies) is functionally similar i.e. it is used to lay eggs, but is morphologically distinct i.e. it arises or is made from different parts of the insects anatomy and should be called a 'pseudovipositor'. The median part of the oviduct which receives the aedeagus during mating is called the 'vagina'. Externally the sexual organs of the male, also called genitalia, consist of a pair of 'claspers' which the male uses to hold onto the females genitalia and an intromittant organ called the 'aedeagus' which is the means by which the male passes the sperm onto the female. Internally the female reproductive organs consist of a pair of ovaries which contain the ovarioles which is where the eggs or ova are formed, the bursa copulatrix which is where the sperm is first received )in those insects which have it) and a spermatheca which is where the sperm is stored. There are also various tubes down which the ova travel on their way from the ovaries to the outside world, fertilisation occurs in the common oviduct after the the egg has received its shell or the 'chorion'. To facilitate this the shell contains a very small opening at one end called the micropyle which allows the sperm to enter. As well as tubes there are several important glands some of which (spermathecal glands) allow the female to keep the sperm alive and viable for a long time, as much as 20 years in some social insects (Ants and Bees); and some of which (collaterial glands) secrete the substances which allow the female to stick the eggs where she wants then to stay i.e. underneath a leaf, or to protect the eggs as in the ootheca produced by the Cockroaches and Mantids Internally the male reproductive organs consist of a pair of 'testes' containing the 'testicular follicles' where the spermatozoa are made, the 'vas deferens' which is the tube down which the sperm travels, a 'seminal vesicle' which is where the sperm is stored prior to mating, and accessory glands which supply seminal fluid for additional volume and to nourish the sperm before and during their journey. For more information about Insects refer to : |
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and Beetles (Coleoptera) live beneath the surface, mostly in rockpools.
Some of the most adventurous insect are the Brine Flies (Ephydra), you can find them living in the strangest places including, the larva of Ephydra hyans in Mono Lake California which is nearly as salty as the Dead Sea, the larva of Psilopa petrolei in pools of crude oil also in California, and the adults and larva of Scatella thermarum in the hot springs of Iceland, the adults live on the mats of algae which float on the water's surface, and the larva live beneath the mats and in water, which is as hot as 48 degrees Celsius, which, for most people, is too hot to put your hand into.
There are several different ways of measuring the size of an insect, most people would consider the largest insect to be the bulkiest, in this case the champion insect is the Acteon Beetle (Megasoma acteon) from South America the males of which can be 9cms long by 5cms wide by 4cms thick, however there is a serious challange for heaviest insect in the world in the form of the True Wetas from New Zealand. For instance a gravid female Deinacrida heteracantha can weigh as much as 70 grams. 
There are an incredible number of very small insects in the world, far more than there are giants. Many beetles are less than one millimetre in length, and the North American Feather-winged Beetle Nanosella fungi, at 0.25mm, is a serious contender for the title of smallest insect in the world. 
