Basic External Insect Anatomy
Insects are all related, they share a common ancestor at the base of their family tree. From this ancestor all insects inherited a basic anatomy and body plan. The diversity in form, and ultimately function, found in insects is a result of changes made to basic anatomical elements such as legs or mouthparts. A consequence of this is that the basic anatomy of one insect is the same as the basic anatomy of all insects (there are exceptions when parts are occasionally lost or added). Below is a very brief overview of external insect anatomy. For a more exhaustive overview visit Invertebrate Anatomy OnLine .
Overall Body Plan
Insects have three major body regions: head, thorax, and abdomen (see Insect Body Regions, right).
The head is made of 5-7 fused segments and bears the eyes, antennae, and mouthparts.
The thorax consists of three segments called the pro-, meso-, and metathorax. Appendages used for movement are attached to the thorax. Each of the segments of the thorax bears one pair of legs and if wings are present they are found on the meso- and metathorax only. The top of the prothorax is called the pronotum.
An insect's abdomen consists of 11 or fewer segments that generally do not bear any appendages, except for segments near the rear which may have appendages associated with reproduction.
Head and Mouthparts
The head can be divided into general regions (see General Insect Head Regions and Mouthparts, left): the top of the head is the vertex, the side or cheeks are gena, the front of the face is the frons, and below the frons is the clypeus. These regions may be highly modified or lost in some groups of insects. Adult insects may have two types of eyes, larger compound eyes that consist of many facets (ommatidia), and eyes that occur as a single facet, ocelli. The number and placement of ocelli can be important for identification.
The mouthparts of humans consist of five layers or horizons; upper lip, upper jaw, tongue, lower jaw, lower lip. Insect mouthparts also consist of five horizons and are made of appendages that have been modified for food handling (see General Mouthparts, right). The labrum is similar to an upper lip. It is not divided but may have a notch on the outer (distal) edge. Below the labrum are the mandibles which are paired structures generally made of strong material (heavily sclerotized) and used for cutting or grinding. The specific shape and various features found on the mandibles may be very important for understanding what and how an organism eats. The hypopharynx is an internal structure located below the mandibles and has a tongue-like function. Below the mandibles (externally) are paired appendages called the maxillae. Generally each maxilla bears an appendage, the maxillary palpus that is used for food handling and may contain taste or smell organs called sensillae. The bottom horizon of insect mouthparts is the labium which is made of two fused maxilla-like structures and bears labial palps.
All insect mouthparts are modifications of this basic plan. A mosquito's proboscis contains all five mouthpart types, see the cross section in Mosquito Mouthparts, B., right. In cases of extreme modification some mouthparts may become fused, reduced, or lost. Mouthpart arrangement can be very important when studying the potential an insect has to vector a disease, access a portion of a plant, etc.
Insect Legs
Insect have three pairs of legs, one pair on each of the three segments of the thorax and are generally called the fore-, mid-, and hind legs. Any of the pairs of legs may be heavily modified and are important for locomotion, prey capture, mating, etc. Thankfully, just like mouthparts, all insect legs contain the same basic parts. From proximal (toward or against the body) to distal (away from the body) the parts of an insect leg are: coxa, trochanter, femur, tibia, and tarsus. The tarsus almost always has one or two claws at the type used to grasp the substrate. The figure Insect Legs, right, shows legs modified for numerous purposes: A, running; B, jumping; C, digging; D, grasping; E, catching; F, walking and digging; G, reduced leg used for walking and digging; H, male leg modified for grasping females during mating.
Basic Internal Anatomy
The internal anatomy of insects is amazingly complex. A good sized caterpillar has more muscles than a human. The internal anatomy of insects differs from vertebrates (including humans) in several major ways.
Digestive/excretory system: Insects have a complete digestive system just like vertebrates (tube from the mouth to the anus) but it differs in a very important way (see Digestive System, left). The insect digestive system has three major regions, foregut, midgut, and hindgut.[1] The foregut and the hindgut are lined with chitin, the same stuff that makes up much of the exoskeleton of the insect. When an insect molts (sheds it's "skin", see below) it also sheds the internal lining of the fore- and hindguts. Loss of the gut contents is a problem if the insect relies on gut microorganisms (gut fauna) to help with digestion. The gut fauna often lives in the hind gut (termites, for example). Suddenly the gut fauna is lost and must be replenished with every molt.
Insects do not have kidneys. Instead, metabolic wastes are removed with the Malpighian tubules.
Respiratory (ventilation) system: Insects don't have lungs. They obtain oxygen and dispel carbon dioxide through a series of tubes called tracheae (see Respiratory System, right). The tracheae are attached to openings on the body called spiracles. The number and placement of spiracles varies and smaller insects may not have any. Traditionally, the view has been held that respiration in insects is passive, but recent evidence has demonstrated that some insects actively expand and contract trachea to ventilate their bodies.
Circulatory system: Insects do not have blood, or blood vessels that are part of a closed circulatory system. Instead insects have an open circulatory system where a substance called hemolymph bathes the organs directly. Some insects have a long heart-like organ along the dorsal side of the internal organs that helps circulate the hemolymph through the body. It comprises a single sheath of tissue and a series of muscles, and in many insects includes a tubular portion that functions as a dorsal aorta. Hemolymph also circulates through the legs, wings, and antennae via a series of simple one-way valves.
Life Cycle
Three general lifecycles occur in insects, but some insects (e.g. aphids, blister beetles, telephone-pole beetles, etc.) may have additional steps or variations. Most insects have direct internal fertilization, like mammals. This means they do not need to return to water to mate, nor do they need to worry with spermatophores like the arachnids. Most insects lay eggs, although some retain the egg inside the body until it hatches and then give "birth". Immature insect growth occurs through shedding of the skin called molting. Immature phases between molting are called instars and the growth sequence is denoted first instar, second instar, etc. Most insects grow through a specific number of instars between hatching from an egg and becoming an adult, but some insects have an indeterminate number of instars that depend on environmental temperature and food availability. In some cases appendages that were lost can be re-grown in immatures. Once an insect molts to adulthood it cannot molt again (except in some cases, such as silverfish). Adults mate (or not, many insects are parthenogenic), lay eggs (or not) and the cycle starts again.
Ametabolous
Ametabolous Life Cycle: egg, multiple instars, and adult. Immatures look very similar to adults, but tend to be smaller, and lack fully formed reproductive structures. The primitively wingless orders have this life cycle: Protura, Collembola, Diplura, Microcoryphia, and Thysanura.
Hemimetabolous
Hemimetabolous and/or Paurometabolous: egg, multiple instars, adult. "Simple metamorphosis" is the common term used to describe this life cycle. The immatures tend to look like miniature versions of the adults, except in the immature the head is larger in proportion to the body, wings are not fully formed and appear as wing buds, and reproductive structures are not developed. The Grasshopper Life Cycle and Squash Bug Life Cycle (right) are good examples. Insect orders with this life cycle are grouped under the term Exopterygota because of visible wing buds on the immatures. Major orders of insects in this group include Orthoptera (grasshoppers and crickets), Blattodea (cockroaches), Isoptera (termites), Plecoptera (stoneflies), Thysanoptera (thrips), Hemiptera (true bugs), and Phthiraptera (lice).
Holometabolous
Holometabolous: egg, multiple instars, pupa, adult. "Complete metamorphosis" is the common term for this life cycle. The immatures look very different from the adults (e.g., caterpillar vs. butterfly, maggot vs. fly, grub vs. beetle) and never have wing buds. Insect orders with this life cycle are grouped under the term Endopterygota because immatures never had visible wing buds. The pupal stage is only found in the Endopterygota. This is typically a resting stage (e.g., the chrysalis of butterflies) where the insect undergoes a final metamorphosis from immature to adult. Major orders in the group include Coleoptera (beetles), Hymenoptera (bees, ants, and wasps), Lepidoptera (butterflies and moths), Siphonaptera (fleas), and Diptera (flies).
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