The ommatidia in the compound eyes of insects absorbs incidental light to prevent it from reaching the lens via "scattering pigment."

   
  "Each ommatidium…consists of several basic parts. There is a layer of transparent cuticle on the outside, which allows light into a lens beneath it. This is usually surrounded by cells containing 'scattering pigment' which absorbs scattered or incidental light rays, so that the only light entering the ommatidium is directly parallel to its axis. This beam of light is directed by the lens down the narrow visual centre or rhabdom where it reacts with pigment, stimulating the nerve cells that surround the rhabdom. The nerve cells pass the message to the optical centre in the insect's 'brain' where it is interpreted…The ommatidia of different insects are varied. They may even be of different sizes within a single compound eye. The scattering pigment reduces the total amount of light entering the eye, so insects active by day may find themselves blind at dusk when the light is lower and more diffused. Nocturnal insects, however, often have the ability to withdraw the scattering pigment from their eyes at night in order to absorb every scrap of available light and to allow light from many of the lens facets to focus on a single light-sensitive rhabdom, thus increasing the effective aperture of the lens system. Many moths go even further, possessing (like cats and some other animals) a kind of mirror - the tapetum - at the back of the eye: this reflects light back through the retinal cells, so every beam of light is used twice over." (Foy and Oxford Scientific Films 1982:122-123)
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The exoskeleton of insects detects strain and load via sensilla organs.

   
  "In their rigid state exoskeletons are stiff laminated composite structures made of chitin fibres embedded in a highly crossed matrix. The exoskeleton acts as a detector of displacement, strain or load via special organs called sensilla, which are partly intergraded into local sections of exoskeleton. These organs amplify the information for the main detector organ, which is connected to the nerve stem. The local information obtained is used to modify the exoskeleton by changing thickness, stiffness and fibre orientation depending on the situation." (The University of Bath 2008)
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The mandibles of many herbivorous insects have exceptional cutting abilities due to the presence of zinc or managese salts.

       
  "Many invertebrates use crystals of metal salts to harden their cutting, rasping, and grinding equipment…The mandibles of herbivorous insects contain zinc or manganese salts (Vincent 1990)." (Vogel 2003:333)
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Socketed hairs of insects detect environmental stimuli through vibration.

           
  "Most insects have socketed hairs (sensory setae) scattered over much of the body which vibrate in response to sounds and may also be sensitive to touch, humidity and light. Nocturnal insects, such as cockroaches, are particularly sensitive to sounds via their setae and have been known to shy away from vibrations issued at 3000 cycles per second--way beyond human hearing capabilities. The setae may also play other roles. Locusts use those on the head, between the antennae, to judge the direction and humidity of the breeze, and climb some eminence for this purpose. Subsequently, they may use the information thus gained to fly to areas of low pressure where rain is likely to induce lusher feeding pasture." (Wootton 1984:48)
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Insects interpret sensory input from antennae using Johnston's organs.

       
  "Some insects' antennae do in fact act as sound-wave receivers. Those of male midges and mosquitoes are quite as feather-like as moths' but are geared to respond to the sound of the females' wing beats, the whine of other males' flight, as well as that of other species, being ignored. While the antennae receive the sounds, interpretation of the latter is made by special structures at their base called Johnston's organs. These organs are found on most adult winged insects, as well as in aquatic insects and larvae, although they may have varying sensory roles, such as assessing air velocity, water current and, notably in subterranean insects, the effects of gravity." (Wootton 1984:46-47)
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Insects can stop dead without falling over because three legs are always on the ground while moving.

     
  "Extra legs do not help an animal to move faster. The millipede is slow for all its legs - in fact, if it hurries it is liable to trip over its own feet! Insects have six legs and tend to have three of them on land at any given moment while moving; they can therefore stop dead without falling over." (Foy and Oxford Scientific Films 1982:46)
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Exoskeleton of insects detects strain and load change via campaniform sensilla.

   
  "In insects, the campaniform sensillum is a hole extending through the cuticle arranged such that its shape changes in response to loads. The shape change is rotated through 90° by the suspension of a bell-shaped cap whose deflection is detected by a cell beneath the cuticle. It can be sensitive to displacements of the order of 1 nm. The essential morphology [is] a hole formed in a plate of fibrous composite material." (Vincent et al. 2007:63)
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The ocelli of insects sense day length via a small lens and pigmented retinal cells.

   
  "Each ocellus usually consists of a small lens backed up by several pigmented retinal cells, which can determine the quality and source of light and usually perceive something moving nearby. Ocelli usually look like small dark dots, and are often grouped in a triangle on the back of an insect's head. They enable the insect to judge the length of daylight, for example, by which it may regulate its whole life cycle. Spiders' eyes form extremely good images and have, for their size, excellent resolution." (Foy and Oxford Scientific Films 1982:122)
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Eyes of some birds, insects, and fish see better than humans because they can detect ultraviolet and/or infrared light.

         
  "The eyes of some birds, insects, and fish respond to ultraviolet wavelengths. Other animals have a spectral response that includes red or near-infrared. This response is helpful in penetrating cloudy or murky conditions." (Courtesy of the Biomimicry Guild)
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The wings of insects combine structural support and material economy because they are flat, braced surfaces.

   
  "Insect wings provide yet another example of braced, flat surfaces--cylindrical cantilever beams (veins) support a thin membrane. A pound of fruit-fly wings laid end to end would stretch about 500 miles, a very low mass per unit length--a steel wire to go so far would have about the same diameter as a red blood cell. Yet in each second of flight the tip of a wing moves several meters and reverses direction four hundred times. Other paddles and fins are fairly flat as well, as are some feathers, the book gills of horseshoe crabs, and a scattering of other stiff structures. In all these cases, though, flatness suits functions other than support. From a mechanical viewpoint the flatness of these systems, however impressive, is perhaps best regarded as a necessary evil--and their designs incorporate features that offset their intrinsically low flexural stiffness." (Vogel 2003:439)
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Feet of insects adjust to rough or smooth surfaces by engaging either claws or adhesive foot-pads.

     
  "Researchers Bert Holldobler and Walter Federle have studied how insects can adhere to both rough and smooth surfaces. They discovered that when an insect walks, two claws at the front of each foot grip the surface and then begin to retract. If the surface is rough, the claws engage and the insect scrabbles along. If the surface is smooth, the hinged claws retract further and adhesive foot-pads protrude between the claws. A miniature hydraulic system helps deploy the footpads." (Courtesy of the Biomimicry Guild)
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The reproductive or growth cycles of many insects are suspended until conditions are favorable via diapause, a hibernation-like mechanism.

     
  "Juvenile insects often undergo a period of suspended development and growth which may be accompanied by a decrease in their metabolic rate. This is known as diapause. It also occurs in adult insects that survive the winter (often referred to as overwintering), such as various species of butterfly and beetle. In these cases the diapause can be thought of as a hibernation mechanism…During overwintering diapause, fertilized eggs that were produced during the fall by the females are retained internally, and their development is halted, while still at an early stage, until the spring. Then, once the adult insects have emerged from this torpid state, their eggs ripen and are laid." (Shuker 2001:109)
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The mouthparts of insects hold food steady during mastication with accessory jaw-like structures, called maxillae.

       
  "Behind the mandibles is another pair of jaw-like structures, the maxillae. These may be simple in shape but often they bear soft lip-like appendages, and projections like tiny antennae, called palps. These bear many sensilla…sensitive cells for tasting, smelling, and touching the food. The maxillae are not usually designed for cutting or chewing food, but they may be used to hold it steady and pass it forwards through the chopping mandibles." (Foy and Oxford Scientific Films 1982:159)
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Insects with two pairs of wings have them work in unison by attaching the wings in various ways, with hooks, folds, or catches.

           
  "[I]n those insects with two pairs of fully operative wings, both are commonly linked together so that they work in unison. Linking devices vary widely. In butterflies and some moths, the upper and lower wings perform as one because of an overlapping fold on the hind edge of the forewing, which thus pushes the hindwing with it on the down stroke. In others there is a more elaborate coupling device consisting of a spine, or frenulum, on one wing which is held by a catch or a group of bristles (retinaculum) on the other. Bees and wasps have an even more elaborate series of hooks and catches on their wing margins." (Wootton 1984:36)
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The wings of many insects can flap rapidly because the wing muscles are attached to the chest with a joint that functions as a pivot.

   
  "There is a system in flies, honeybees, and wasps that transforms wing flaps into 'automatic' movements. The muscles that enable flight in these insects are not directly tied to the bones of the body. The wings are attached to the chest with a joint that functions like a pivot.

"The muscles that move the wings are connected at the bottom and top surfaces of the chest. When these muscles contract, the chest moves in the opposite direction, which, in turn, creates a downward pull." (Yahya 2002:29)
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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.

 

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There is no general agreement on the details of how different groups of insects are related. Our classification (see the Classification tab) has not been fully edited yet, and has errors. We're working on this, but with so many groups, it's a big job. See the Tree of Life page listed below for more information.

 

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Other Web Resources:

 

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    Tree of Life Web Project page on Insecta 

 

References:

 

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• Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
 
• Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
 
• Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. 1987. Living Invertebrates. Blackwell Scientific Publications, Palo Alto, Ca.
 

 

The exoskeleton of insects detects strain and load via sensilla organs.

Process information > Sense signals/environmental cues > Touch and mechanical forces

"In their rigid state exoskeletons are stiff laminated composite structures made of chitin fibres embedded in a highly crossed matrix. The exoskeleton acts as a detector of displacement, strain or load via special organs called sensilla, which are partly intergraded into local sections of exoskeleton. These organs amplify the information for the main detector organ, which is connected to the nerve stem. The local information obtained is used to modify the exoskeleton by changing thickness, stiffness and fibre orientation depending on the situation." (The University of Bath 2008)