by sara waugh

external image intro_insect-2.jpginsecta.jpg
external image moz-screenshot-3.pngexternal image moz-screenshot-4.png(jjf)

Insects are classified in the phylum Arthropoda, and they outnumber all other forms of life combined in species diversity, living on almost every terrestrial region of the earth and in fresh water. Insects are invertebrates, as they have no backbone. It is estimated that the arthropod population of the world numbers about a billion billion individuals, mostly made up of insects. In terms of species diversity, distribution, and sheer numbers, arthropods are regarded as the most successful of all animal phyla. Class Insecta is divided into twenty-six orders, and the study of insects is referred to as entomology. [Insects include among their number parasites, parasitoids, carnivores, herbivores, scavengers, and deritivores. RS]
Diagnostic Characteristics
The phylum Arthropoda is generally characterized by the organism's segmentation, hard exoskeleton, and jointed appendates. The class Insecta are generally characterized by a body divided into a head, thorax, and abdomen; antannae (as shown below); mouthparts modified for chewing, sucking, or lapping; and two pairs of wings and three pairs of legs. Another main characteristic of insects is that they are mostly terrestrial, although they can be found in fresh water.
Anatomy of a Grasshopper (insect)
Anatomy of a Grasshopper (insect)

Acquiring and Digesting Food
Althrough insects eat a variety of kinds of food, a fossil record of several different insect mouthparts indiciate that insects fed on gymnosperms and other Carboniferous plants. Insects compete with humans for food, as they eat many of the crops that humans grow. Insects have a complete digestive system, meaning that the digestive system runs through the body from the mouth to the anus (two openings, as opposed to one, which would be an incomplete digestive system). This digestive system is regionally specialized with discrete organs functioning in the breakdown of food and the absoption of nutrients. Insects excrete their nitrogenous wastes from metabolic processes via Malpighian tubules. (APS)

Sensing the Environment
Insects have several means of sensing the environment around them. Insects have well-developed sensory organs, including eyes, olfactory receptors for smell, and antennae for touch and smell. Each of the compound eyes of an insect contains a large number of small lenses (for example, about 4,000 in the common fly). This is very useful for detecting movement, but not for seeing long distances. Most insects have three simple eyes between the compound eyes called ocelli (JM). The insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia. These ganglia are fused into a cerebral ganglion, which serves as the insect's brain. This cerebral ganglion is close to the antennae, eyes, and other sense organs concentrated on the head.
Specialized hairs cover the entire body of an insect. These hairs provide the insect with information about the environment around it and its position in the environment. Contact of these hairs with the ground or other object may inhibit the insect's movement. (SF)
The insect's main mean of transportation is flight. Many insects have one or two pairs of wings coming out of the thorax. Insects are able to fly without sacrificing their walking legs because their wings are flaps of the exoskeleton, unlike other flying vertebrates that have legs modified for wings and are generally clumsy on the ground. Some insects beat their wings at speeds of several hundred cycles per second by using muscles to warp the shape of the entire cuticle covering the thorax. As the wings flap, they change angles, producing lift on both the up and down strokes of the wings. Flying allows insects to escape many predators, find food and mates, and disperse to new habitats quickly. In addition, there are three pairs of walking legs on the thorax, which allows insects to have an alternative means of locomotion. Insects have a hip and knee joint which allow for better movement on land than some other flying organisms. (LD)

added by (AS).
added by (AS).

Insects have a number of specialized organs for gas exchange that allow for diffusion of oxygen and carbon dioxide in spite of their exoskeleton. Insects exchange these respiratory gases between the air and their tissues through a tracheal system, or a system of branched, chitin-lined tubes that infiltrate the body. These fine tubes carry oxygen directly to the cells. This system opens to the outside of the body through spiracles, which are pores that can open or close to regulate air flow and limit water loss.
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Metabolic Waste Remval
Unique excretory organs called Malpighian tubules, which are outpocketings of the digestive tract, remove metabolic wastes from the hemolymph in insects. Malpighian tubules open into the digestive tract and dead-end at tips that are immersed in hemolymph (circulatory fluid). The tract secretes certain solutes that help the nitrogenous wastes to be eliminated. The main nitrogenous waste that exists in insects is insoluble uric acid, and this is eliminated as nearly dry matter along with the feces. Insects then dispose of their waste through the anus.

Insects have an open circulatory system, similar to other antropods. In this open circulatory system, fluid called hemolymph is propelled by a heart through short arteries then into sinuses surrounding the tissues and organs. These sinuses of the insect's body are collectively referred to as the hemocoel. Hemolymph reenters the arthropod heart through pores that are usually equipped with valves after circulating through the body. Gas exchange occurs through direct contact between the organs and tissues with the hemolymph. Hemolymph is a watery (usually green) fluid that is full of enzymes and other nutrients. It does not contain hemoglobin (which is why it's not red) because hemolymph isn't responsible for oxygen exchange. It bathes the organs with the nutrients they need and collects waste products that go to the malphigian tubules. (DRM)
The heart is located in the insects abdomen and multipul chambers run from the heart towards the head of the organisms. Lead by the flow of a single blood vessel stretching from the heart to the head, muscle contractions move the blood through the chambers. (JAF)
The Circulatory System of Arthropoda Insecta. (Walker K.)
The Circulatory System of Arthropoda Insecta. (Walker K.)

Self Protection
The body of an insect, like all arthropods, is completely covered by a cuticle, or an exoskeleton constructed from layers of protein and chitin. Not only does this exoskeleton protect the animal, but it also provides points of attachment for the muscles that move the appendages, such as the wings. This rigid exoskeleton must be shed in order for an insect to grow, and the insect must secrete a larger one. This shedding of an old exoskeleton and gaining of a new one is referred to as molting, a process that requires a lot of energy and leaves the insect temporarily vulnerable to predators and other dangers. Arthropods also protect themselves through use of camouflage. The coloration and pattern of a butterfly's wings enable it to camouflage with its environment in order to hide from its predators.(KTD)

Osmotic Balance
Osmotic balance, or the balance of the amount of water in an organism, must be regulated in order to maintain homeostasis, or the steady-state physiological condition of the body. The spiracles that are utilised in the tracheal system by opening and closing to regulate air flow also limit water loss in an insect. Also, the cuticle covering of insects keep water from entering the body, as well as contains the water that is necessary for the insect to survive. Finally, the Malpighian tubules that removed nitrogenous wastes also function in osmoregulation, or the control of water balance in organisms living in hypertonic, hypotonic, or terrestrial environments. These tubules open into the digestive tract and secrete nitrogenous wastes and salts from the hemolymph into the tubule lumen, and water follows these solutes by osmosis. Most of the salts and water are reabsorbed across the epithelium of the rectum. The insect excretory system is highly effective in conserving water.
Malpighian tubules of insects

Temperature Balance​
Temperature balance is also important in maintaining homeostasis in an organism. Many species of flying insects are endothermic, which means that insects' high metabolic rate generates enough heat to keep its body substantially warmer than the environment. Insects are the smallest of all endotherms, and they are able to generate such heat because of their powerful flight muscles, which creates lage amounts of heat when in use. Also, many endothermic insects use shivering to warm up before taking off for flight. This shivering consists of the contraction of the flight muscles in synchrony so that it is not enough wing movement to fly but it is enough to generate some heat. Many endothermic insects have a countercurrent heat exchanger that helps maintain a high temperature where the flight muscles are located (the thorax). Insects that live and fly in hot weather can shut the countercurrent mechanism down to allow the muscle-produced heat to be released into the environment,

Insects go through some level of metamorphosis as they mature, either gradual (egg->nymph->adult) or complete (egg->larvae->pupa->adult). In gradual metamorphosis, the nymph often resembles the adult, living in the same habitat and eating the same types of food. One example of gradual metamorphosis would be in grasshoppers, which look like smaller wingless adults as nymphs. Complete metamorphosis can be found in the familiar example of the caterpillar and butterfly. Larvae and adults are generally very different. The pupa is the stage in which all the transformations between larvae and adult take place. (AZ)

Review Questions:
1) What is the fluid hemoplymph used for in insects? How does it work? [KS]
2) How do these insects respire? What specialized organs do they have? [Rabya Saraf]
3) How can we differentiate Arthropoda Insecta from Arthropoda Crustacea when talking about diagnostic characteristics? (Jake Schwartz)
4) What is the exoskeleton composed of? What are the functions of the exoskeleton? (Leo Schwartz)
5) What type of digestive system does an Arthropoda Insecta have? And why is that so important to the Insecta?(CP)
6) What about flight distinguishes Arthropoda from other flying vertebrates? (DG)
7) Other than the hard exoskeleton in what way might insects protect themselves? (msr)

Sources: (KTD) (JAF) (Jackson Murphy) (Sarah Fleming) (Liz Daley) (JJF) (Alyssa Zisk) (Walker K.) [BY] (Donna McDermott)
(APS) (Jesse Landy)