By: Zinnia Sidhwa The Diagnostic CharacteristicsArthropoda insectas class is generally characterized by its hard exoskeleton and segmentation. Insect exoskeletons have three basic layers: the epidermis, procuticle, and the epicuticle. The epidermis is the innermost layer and is responsible for creating the upper cuticle layers. The procuticle lies directly above the epidermis. It is composed of a matrix of chitin and other proteins. The thickness and exact structure of the procuticle varies from insect to insect and on different areas of the body on the same insect. The outer most layer is the epicuticle. It is responsible for water retention and protection. (KL)(2)Insect bodies have three regions: a head, a thorax, and an abdomen. Also Insects are mainly terrestrial but some can be found in aquatic places. The head contains the antennae, eyes, brain, and mouth. The thorax contains muscles, fore wing, hind wing, and the legs. The abdomen contains the "guts," and reproductive organs. (SM)(7)
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General insect characteristics and anatomy (ZS)(22)


Oviparous insects lay eggs which hatch after a non-consistent amount of time into young. However Ovoviviparous’ eggs, seem to remain within the parent’s body after hatching and are produced with the ability to move. Eggs extremely small, laid singly or in clustered, and vary greatly from species to species. Covered by a Chitinous shell- the chirion- marked with ridges reticulations and other markings as well as color. One spot on the shell has one or several openings for sperm entrance called micropyle. In hatching the shell is broken and the young insect crawls out. The young insects are often nothing like the adults they will become, they must grow and often change physically to mature- many go through a metamorphosis. Insects that do not go through metamorphosis are Ametabola insects that do are Hemimetabola or Paurometabola. (MC) “For ametabolous insects like grasshoppers, the exoskeleton would limit the size of the insect, so it will molt its exoskeleton when it becomes large enough, and then there will be a new soft exoskeleton formed under which will take a period of time to harden (ranges from a few minutes to a few days). During the younger stages of an ametabolous insects life, it is referred to as a nymph and often has underdeveloped wings, sexual organs, and coloring compared to their adult form.” (SD) (18)
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The diagram to the left shows the stage of the metamorphosis of a caterpillar (AC)(23)Acquiring and Digesting foodInsects have a wide range of mouth parts specialized in acquiring the foods they eat. The two basic differences between the types of mouth parts is whether they are meant for sucking or chewing. A proboscis, or tube shaped mouth part, is used for sucking. An insect's mandibles on the other hand can be used for cutting and chewing food. (RL) (15)Insects’ internal anatomy includes several complex organ systems such a complete digestive system. Insects digestive system breakdown food and absorbs nutrients like human digestive track and their digestive track has two openings, a mouth and an anus, opposed to one opening making the digestive tract incomplete. Insects excrete their nitrogenous wastes from metabolic processes from a unique excretory organ, Malpighian tubules.Insects' extracellular digestive track, which starts at the mouth and ends at the anus, has three main sections: the foregut, the midgut, and the hindgut. The foregut is then divided into three sections: the gullet, the crop, and the proventriculus. Food moves from the mouth through the gullet, then to the crop. Food is stored in the crop for eventual digestion. Salivary glands empty into the gullet to help digest food. Most digestion then occurs in the midgut. Nutrients are absorbed in both the midgut and hindgut. The remaining waste is then excreted through the anus. (SI) (6).The midgut runs from the gastric caeca (digestive caeca) to just before the Malpighian tubes. Between these is the ventriculus (the stomach) where digestion is most active. The gastric caeca increases the surface area of the midgut, which in turn increases its ability to secrete digestive enzymes and to extract useful proteins, vitamins, and fats from the partially digested food. These useful products pass across the wall of the midgut into the body cavity. This is possible because the midgut is lined by a semi-permeable membrane. The membrane is composed of protein and chitin and allows for the passage of liquids/dissolved substances while blocking the passage of solid food particles. (AK) (20) Picture - The digestive system of an insect. (DB) (25)
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Picture - The digestive system of an insect. (DB) (25) Sensing the Environment
Insects have well-developed sensory organs, including eyes, and antennae for touch and smell. Compound eyes are found in insects along with other species. Compound eyes consist of up to several thousand light detectors called ommatidia (the lens segment of the eye), each with its own light-focusing lens. Rhabdom, a stack of pigmented plates on the inside of a circle of receptor cells, traps light and guides it to the receptor cells. The image formed by a compound eye is a mosaic of dots formed by the different intensities of light entering the many ommatidia from different angles. Compound eyes are very acute at detecting movement; important adaptions for flying insects constantly threatened by predation, but are not very good seeing long distances.
Insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia. The two cords meet in the head, where the ganglia of several anterior segments are fused into a cerebral ganglion (brain) close to the antennae, eyes, and other sense organs concentrated on the head.
An insect's sense of smell is very important for survival- it helps find food, identify mates, and to avoid predators. Others use it for navigation, by spraying a chemical by their nest. Insects produce such chemicals, called semiochemicals, to act as odor signal. Since plants also emit pheromones, insects need advanced olfactory sensilla- sense organs to detect these signals. Usually they lie on the antennae, but they also might be on an insects mouth or genitalia. The sense organs produce sensations by using the insect nervous system. Within the sensilla, special cells produce odor-binding proteins. These proteins bind to the chemical molecule signals and transport them through the lymph to a dendrite, where the chemical interacts with the receptor molecule on the dendrite membrane. This causes the nerve impulse that tells the insect brain what certain smells are. (TM)(13)
Insects were the first animals to produce and sense sounds. Generally, insects create sounds by mechanical actions like rubbing and clapping. Many insects communicate through vibrations traveling through the ground. These sounds are used for communication in mating calls or warning cries. (JS) (24)
LocomotionInsects have many means of movement and transportation such as walking or flying. Many insects have one or two pairs of wings coming out of the thorax aside from their walking legs which also branch out from the thorax. Flying allows insects to escape many predators, find food and mates, and disperse to new habitats quickly. Insects have 6 legs, which serve as both struts to support the body's weight, and levers to increase ease of movement. Hardbodied insects,to balance their large and cumbersome exoskeletons, use a tripod gait of movement, meaning that when they move quickly they do so with three legs moving at a time(the outside legs of one side and the middle leg of the other) and the other three on the ground. Softbodied insects, however, move by peristaltic contractions, pulling the hind legs up to meet to forelegs then pushing the forelegs to gain ground. (ZXU) (3)
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Insect respiratory system. (LW) (14) Respiration
In insects by a tracheal system gas exchange is accomplished. In a tracheal system, chitin-lined tubes infiltrate the body and carry oxygen directly to the cell. The system opens to the outside of the body through spiracles, pores that can open or close to regulate air flow and limit water loss.
Air enters the body through valve-like openings in the exoskeleton. These openings are called spiracles. They are located laterally along the thorax and abdomen of most insects- usually one pair of spiracles per body segment. Air flow is regulated by small muscles that operate one or two flap-like valves in each spiracle- contracting to close and relaxing to open. After passing through a spiracle, air enters a longitudinal tracheal trunk. Eventually, it diffuses throughout a complex, branching network of tracheal tubes that subdivides into smaller diameters and reaches every part of the body. At the end of each tracheal branch, a special cell called a tracheole provides a thin, moist interface for the exchange of gasses between air and the living cell. Each tracheal tube develops an invagination of the ectoderm during embryonic development. To prevent its collapse under pressure, a thin wire of cuticle called taenidia winds spirally throughout the membranous wall.
The lack of taenidia allows for the formation of collapsable air sacs that store a reserve of air. (LW) (3)
Metabolic waste removal
Insects excrete their nitrogenous wastes from metabolic processes from a unique excretory organ, Malpighian tubules. Malpighian tubules are outpocketings of the digestive tract.The number of malpighian tubules throughout the abdomen varies greatly by species, with some insects having hundreds of malpighian tubules. The metabolic processes result in a nitrogenous waste product, and the malpighian tubule helps with the excretion of these wastes. Nitrogenous waste often begins in the form NH4+, which is very toxic unless extremely diluted. Because insects live in dry environments, they do not have the necessary water to dilute NH4+. In the malpighian tubules the NH4+ is quickly converted to a less toxic form, uric acid, which is a paste-like solid. The uric acid gets combined with other wastes in a fecal pellet. (ORS)(4) Malpighian tubules branch from the alimentary canal which absorbs water, wastes and other products from the hemolymph. The malpighian tubules are mainly used in excretion and osmoregulation, but in some insects they have other functions as well. Larvae of the New Zealand glowworm have special Malpighian tubules that give off a blue-green light in order to attract prey towards traps.(CW)(21)
external image insectexcret_1.gifThis diagram is of the excretory system of the ant. (2) Circulation
Insects have an open circulatory system like other arthropods. They have a heart pumping hemolymph, general internal body fluid, through the sinuses of the hemocoel. Throughout the insect’s body, a type of greenish blood flows known as hemolymph, which makes direct contact with organs and tissues. Insect blood is only about 10% hemocytes (blood cells), and most of the hemolymph is watery plasma. It is important to note that the insect circulation system does NOT carry oxygen, and thus does not have red blood cells like many other organism have. Hemolymoh is confined to vessels during only a portion of its circuit through the body. Throughout the rest of its circulation, the hemolymph takes place within the body cavity –hemocoel, which is divided into chambers called sinuses, where exchanges of materials takes place. The order of blood flow: (1)A single blood vessel runs along the dorsal side of the insect, from the head to the abdomen and carries the hemolymph. (2) In the abdomen, the vessel divides into chambers(sinuses) and functions as the insect heart. (3) Perforations in the heart wall, called ostia, allow hemolymph to enter the chambers from the body cavity. Muscle contractions push the hemolymph from one chamber to the next, moving it forward toward the thorax and head. The coordinated movements of the body muscles gradually bring the hemolymph back to the dorsal sinus. Between contractions, tiny valves in the wall of the hearts open and allow hemolymph to enter. Insects are able to use this type of circulatory system because they have numerous openings in their bodies (spiracles) that allow the "blood" to come into contact with air. (YA)(5)

Self Protection
Insects have exoskeleton made of chitin and protein which is used for protection. Camouflage is another device that animals use to blend into their environment for protection by imitating the colors and textures of the surrounding environment in order to avoid being seen by predators or prey. Certain insects use Müllerian mimicry for protection from predators. Müllerian mimicry is where two or more unpalatable species resemble each other. Both species in this scenario gains an additional advantage because both are dangerous, but both species collectively cause the predator to learn more quickly to avoid any prey with the similar appearance.
Insects also developed a sort of hibernation period in order to survive harsh winters. A lot of insects will go into what is called diapauses, and produce a glycerol shell for protection. They also use up only one tenth of the energy they normally do because they are inactive. during this period, females do not lay eggs and the development of larvae ceases until warmer temperatures arrive.(GR)(10)Since some insects developed wings, (ability to fly) they could easily escape from predators and travel large distances without any danger in the form of other animals in the air. The more primitive insects, most likely the first insects are wingless, thus this suggests that flying was a natural selective advantage at the time and has continued to be for many insect species. In addition, their small stature also enable insects to occupy smalls spaces and only require a small amount of food for survival. (SR)(12) Insects also use chemical signals to protect themselves. Many insects have developed system to protect themselves against predators. The Darkling Beetle, for example, releases a pungent odor when it feels threatened, which makes predators (and humans) instantly stop persuing the beetle. We are also familiar with stink bugs (Family Pentatomidae) which release a foul stench when they feel threatened. Insects also use stingers to protect themselves. (RG) (19)Osmotic Balance
Osmotic balance or the balance of the amount of water in an organism in insects is done in order to maintain homeostasis, or the stable physiological condition of the body. In insects the spiracles that are used in the tracheal system by opening and closing to regulate air flow also limit water loss. Also, the cuticle covering of insects keep water from entering the body and leaving the body, retain water that is necessary for insects to survive. Malpighian tubules are also used in insects for osmotic balance. As described above (see: metabolic waste removal), malpighian tubules lie open in the digestive tract of insects, with their tips immersed in hemolymph. They are made up of a single layer of epithelial cells, and range in number from 4-200 per organism, depending on the species. Malpighian tubules secrete fluids into the alimentary canal, located between the midgut and the hindgut, to maintain osmotic balance. Urine of insects is only formed in secretion, due to the low pressure of the circulatory system. In herbivores, the malpighian tubules secrete K+ and Na+ in blood-feeders. (CSR, 8)

Temperature Balance
Temperature balance is important in maintaining homeostasis in an organism. Many insects of flying species are endothermic, the smallest of all endotherms. Such endothermic insects to elevate their body temperature depend on their powerful flight muscles which generate large amounts of heat when in use. Shivering is a process by contracting the flight muscles in synchrony, so that only slight wing movements occur but considerable heat is produced. Many endothermic insects use shivering to warm up before taking off. Chemical reactions, and hence cellular respiration, speed up in the warmed-up flight “motors” enabling these insects to fly even on cold days or at night. Many endothermic insects (bumblebees, honeybees, and some moths) have a counter-current heat exchanger that helps maintain a high temperature in the thorax, where the flight muscles are located. In contrast, insects flying in hot weather run the risk of overheating because of the large amount of heat produced by working flights muscles. In some species the counter-current mechanism can be shut down to allow muscle-produced heat to be lost from the thorax to the abdomen and from there to the environment. Bumblebee queens use this means to incubate their eggs: They generate heat by shivering their flight muscles and then transfer the heat to the abdomen, which the bee presses against her eggs.
Mating Signals
Species of insects use different mechanisms to find a mate. Some insects use visual signals. Animals that rely on visual signals include butterflies, luminous beetles, flies, and odonates. For example, when fireflies mate, the signal is initiated by the female, who flashes her light to alert the male. The light translates to a specific code that indicates her sex, species, and interest in mating. A male then replies with his own signal, and they continue flashing lights until they find each other. Other insects use auditory signals, including crickets, Coleopterans, Hemipterans, and Orthopternas that sing to find a partner. In addition, some insects use chemical cues, called pheromones, to attract their mate. These insects include bumble bees, moths, Japanese beetles, etc. (MT)
Review Questions:
1. Insects are very small creatures that seem to be eaten by many other larger creatures such as birds, rats, etc. What kind of behavior helps insects to protect themselves from predators?
2. What are the defining characteristics of insects? How do theses characteristics further benefit the organism and do they help in other functions other than defining characteristics? (MP)
3. Describe the Locomotion of Insects. How do the features of the insects locomotion help the insect? (MLK)
4. How does sexual selection lead to evolution and speciation in insects? Provide a hypothetical example? (LJ)
Main source: Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings
1. (KL)
2. (ZXU)
3. LW
4. (ORS)
5. (YA)
8. (CSR)
9. (MT)
10. (GR)
11. (MC)
13. (TM)
14. (LW)
15. (RL)
16. Picture 2 (NG)
17. (MF)
20. (AK)
22. (ZS)
23. (AC)(picture)
24. (JS)