The Structure of Insects

A knowledge of insect structure is essential to understanding descriptions and the characters distinguishing different groups. The following account is rather brief; more information is given in the accounts of some of the insect orders.

General Structure. The body of an insect is segmented and more or less elongated. The segments are grouped in 3 body regions, the head, thorax, and abdomen. The head bears the eyes, antennae, and mouth parts; the thorax bears the legs and wings; the abdomen usually bears no locomotor appendages but often has some appendages at its posterior end.

An insect's body wall is somewhat hardened (sclerotized). It serves as a shell to protect the internal organs and acts as a skeleton or, more properly, exoskeleton (muscles are attached to its inner surface). The surface of the body is divided by intersegmental and other lines into a number of platelike areas, or sclerites.

The Head. The head is the anterior capsulelike body region that bears the eyes, antennae, and mouth parts. It is usually quite hard. The surface is divided by sutures into a number of areas, each with a name; the same names are used in different insect groups — where the areas can be homologized — but special terms are used in some groups.

Insects generally have 2 kinds of eyes, simple and compound. For the majority there are 3 simple eyes (ocelli), located on the upper front part of the head. Some insects lack ocelli and others have only 2. The compound eyes are situated dorsolaterally on the head, each composed of many facets. In some insects they occupy most of the head and contain hundreds of facets.

The Antennae are usually located on the front of the head below the ocelli; they vary greatly in form and in the number of segments contained, and are often used to distinguish different insect groups. Various terms that describe the antennae are explained in the accounts of the groups in which antennal characters are used for identification.

Mouth Parts. The mouth parts of an insect generally are located on the ventral or anterior part of the head, and they vary a great deal in different insect groups. The mouth part structures typically present are a labrum (upper lip), a pair of jawlike mandibles, a pair of jawlike maxillae, a labium (lower lip), and a tonguelike structure called the hypopharynx.

Insect mouth parts are of 2 general types, chewing and sucking. Insects with chewing mouth parts have laterally moving mandi-

Head eye

Thorax

Abdomen nota (3)

terga (10)

epiproct eye nota (3)

terga (10)

epiproct

ovipositor

3 thoracic segments (see p. 34)

Diagram of an insect Each thoracic segment typically has 2 lateral sclerites separated by a pleural suture (a)—an anterior episternum (b) and a posterior epimeron (c)

ovipositor ocellus.

occiput postoccipital suture ocellus.

occiput postoccipital suture

ijrz palps

Front view of head ijrz palps

Front view of head bles (see opp.) and generally chew their food; those with sucking mouth parts have the parts modified into a beak (proboscis) through which liquid food is sucked. The mandibles are either lacking in sucking mouth parts or are styletlike and form part of the beak; they do not move laterally. A few insects (such as bees) have laterally moving mandibles and a beaklike tongue, and suck liquid through the tongue; a few, like the larvae of dytiscid beetles (p. 154), have well-developed mandibles that move laterally but they suck their food in liquid form through channels in the mandibles.

Chewing mouth parts normally can be recognized by the laterally moving mandibles, visible on the lower front part of the head; most chewing insects do not have a beak. Scorpionflies have the head prolonged ventrally into a beaklike structure (see illus., p. 209), with mandibles that move sideways, and these insects chew. Snout beetles (see illus., p. 203) have the front of the head prolonged into a snout, but there are tiny laterally moving mandibles at the tip of the snout.

Mouth parts of sucking insects (see p. 32) vary in appearance and in how they operate. Many sucking insects (such as bug, froghopper, mosquito, and stable fly) pierce the tissue fed upon. What you see of the beak is usually (bug, mosquito, froghopper) not the part that does the piercing but a sheath enclosing the piercing structures. The piercing is done by a group of hairlike or swordlike stylets. When these stylets pierce something, the sheath folds up or back out of the way. If when about to be bitten by a mosquito you will let it alone and watch it in action, you will see that the beak bends in the middle as the 6 hairlike stylets inside it go into the skin. The proboscis of a butterfly or moth does no piercing; generally it is coiled like a watch spring on the ventral side of the head when not in use, and is uncoiled when the insect feeds. Some flies (like the House Fly) have a rather fleshy proboscis that is incapable of piercing.

The stylets of an insect with sucking mouth parts usually enclose the food and salivary channels (the proboscis of a butterfly or moth contains only a food channel). When you are bitten by a mosquito, for example, saliva is first injected through the salivary channel (this is what causes the irritation), then blood is sucked up through the food channel.

The maxillae and labium of most insects bear small feelerlike structures called palps; each maxilla bears 1, and the labium bears 2. Some insects lack 1 or both pairs of palps: the flies (Díptera) have only the maxillary palps, most butterflies and moths (Lepidoptera) have only the labial palps, and the bugs (Hemiptera) have no palps at all.

The Thorax (see opp. and p. 34). This, the middle section of the body (between head and abdomen), is divided into 3 segments: (1) prothorax (2), mesothorax, and (3) metathorax. Each segment

Sucking mouth parts.

typically bears a pair of legs lateroventrally, and the mesothorax and metathorax usually bear a pair of wings dorsolaterally. Some insects have only 1 pair of wings (generally borne by the mesothorax), and some are wingless.

Each thoracic segment bears 4 groups of sclerites (platelike areas), the notum (plural, nota) dorsally, a pleuron (plural, pleura) on each side, and the sternum (plural, sterna) ventrally. Any thoracic sclerite can be indicated as belonging to a particular segment by using the proper prefix. For example, the notum of the prothorax is the pronotum, the pleura of the mesothorax are the mesopleura, and so on. The pronotum in many insects is a large and conspicuous sclerite forming the dorsal surface of the body between the head and the base of the wings. The pleura of the mesothorax and metathorax are usually larger than the propleura.

The pronotum is a single sclerite, but may contain various grooves or ridges. Each of the other nota is usually divided into 3 sclerites — the scutum, scutellum, and postnotum. The meso-scutellum in bugs and most beetles is a conspicuous and more or less triangular sclerite between the pronotum and the base of the wings, and is often simply called the scutellum. Each pleuron is usually separated into 2 sclerites by the pleural suture, a line of division which extends from the base of the leg to the base of the wing; the anterior sclerite is the episternum, and the posterior one is the epimeron. The pleura sometimes contain sutures setting off additional sclerites, and frequently there are 1 or more small sclerites in the membranous area between the pleura and the base of the wings. The sternum is often divided by sutures into 2 or 3 sclerites.

The Legs (see p. 34). An insect leg typically contains the following segments: coxa (basal segment), trochanter (generally small, just beyond the coxa), femur, tibia, and tarsus. The tarsus usually bears at its apex a pair of claws and 1 or more padlike structures. Since the legs vary considerably in different insects, leg characters are often used in identification. The legs may differ in the relative size and shape of the various segments, the number of subdivisions of the tarsus (usually called tarsal "segments," though the entire tarsus technically constitutes a single leg segment), and the trochanter, the character of the claws, pads, and other structures at the apex of the tarsus, and the spines or hairs on the legs.

The Wings. Insect wings also vary considerably, and much use is made of this variation in classification and identification. Many insect order names end in ptera (from the Greek, meaning "wing"). The wings are located dorsolaterally on the mesothorax and/or the metathorax. Most of the muscles that move the wings are attached to the walls of the thorax rather than to the base of the wings, and the wing movements are produced largely by changes in the shape of the thorax.

Insect wings vary in number, size, shape, texture, venation, notum scutelium notum scutelium

and in the position held at rest. Most insects have 2 pairs of wings but many have only 1 pair and some are wingless. Although most insect wings are membranous (thin, like cellophane) some are thickened or leathery; many are covered with hair and some bear scales. Typically insects fold the wings over the abdomen at rest, but a few hold them vertically above the body and some hold them outstretched.

Certain terms are used in referring to different edges, regions, or angles of the wing: the anterior edge is the costal margin, and the base of the costal margin is the humeral angle; the posterior edge is the anal margin, and the posterior basal part of the wing is the anal area; an angle of the wing in the anal area is the anal angle, and an angle at the tip of the wing is the apical angle, or apex. There are 1 or 2 lobes in the anal area of the wing in some insects.

Wing Venation. Wing venation — the system of thickened lines in the wing — is frequently used in classification and identification. Although it is generally possible to homologize the veins of different insects and use a standard system of names for them, there are differences of opinion as to how these names should be applied for some insects. A special venational terminology is used in a few groups.

The most widely used terminology of wing venation is illustrated opposite. Longitudinal veins are indicated by capitalized abbreviations, and cross veins by small-letter abbreviations. Branches of the longitudinal veins are indicated by subscript numerals.

Cubitus (Cu) Anal veins (A

Cubitus (Cu) Anal veins (A

2A IA Cu2Cu,bCu'*

Generalized wing venation (explanation in text, below).

2A IA Cu2Cu,bCu'*

Generalized wing venation (explanation in text, below).

The vein forming the costal wing margin is the costa (C). The 1st vein behind the costa is the subcosta (Sc), which typically has 2 branches, Sci and Sc2. The next vein is the radius (R), which gives off basally a posterior branch, the radial sector (Rs); the radial sector usually forks twice. The anterior branch of R is Rx; the anterior branch of Rs is R2+3, and its terminal branches are R2 and R3; the posterior branch of Rs is R4+5, and its terminal branches are R4 and R5. The next vein is the media (M), which generally forks twice; the anterior branch beyond the first fork is Mi+2, and its terminal branches are Mi and M2; the posterior branch is M3 +4, and its terminal branches are M3 and M4. The next vein is the cubitus (Cu), which has 2 branches, Cui and Cu2; Cux often has 2 terminal branches, Cuia and Cuib. The remaining veins are anal veins (A), and are numbered from anterior to posterior (1A, 2A, 3A). The usual cross veins are: the humeral (h) between C and Sc near the base of the wing; the radial (r) between Ri and the anterior branch of Rs; the sectorial (s) between 2 branches of Rs; the radio-medial (r-m) between the radius and media; the medial (m) between 2 branches of M; and the medio-cubital (m-cu) between M and Cu.

Cells in the wing (spaces between veins) are named from the vein on the anterior side of the cell. Cells at the base of the wing are usually named without a subscript numeral. A cell behind a 2-parted or fused vein is named from the posterior component of that vein. Two or more cells with the same name are individually designated by numbering from the base of the wing (1st M2 cell, 2nd M2 cell). An open cell extends to the wing margin; a closed cell does not reach the margin.

A given insect may have more veins than shown in the figure above as a result of additional cross veins or additional branches of the longitudinal veins; it may have fewer veins because some fail to branch, fuse together, or are lost. Special terms are used for particular features of the venation in some orders and are discussed in this book under the accounts of those orders.

The Abdomen (see p. 30). The insect abdomen typically consists of 11 segments, but the last segment is usually represented by appendages only; so the maximum number of complete segments in most insects is 10. Many insects have fewer abdominal segments because of fusing or telescoping of some segments.

Each abdominal segment generally contains 2 sclerites, a dorsal tergum and a ventral sternum. The terga usually extend down the sides of the segments and overlap the sterna.

Most insects lack appendages on the abdomen except at the posterior end. The terminal appendages may be lacking or drawn into the body and hidden. When terminal appendages are present (top figure, p. 30) they usually consist of a pair of dorsolateral cerci (singular, cercus), a median dorsal epiproct, a pair of lateroventral paraprocts, and the genitalia. Cerci when present may be feelerlike or clasperlike; the epiproct, if present, may be short, or elongate and threadlike. The anal opening is at the posterior end of the abdomen, just below the epiproct if the latter is present.

The genitalia are structures associated with the genital openings — those of the male transfer sperm to the female and those of the female lay the eggs. Male genitalia are extremely variable and often quite complex; they provide valuable taxonomic characters in many groups. Some or all of the male genitalia may be withdrawn into the body and not be apparent without dissection. An ovipositor (egg-laying organ) is present in many insects. It is formed by structures on the 8th and 9th abdominal segments and extends beyond the body in some insects and in others is withdrawn into the body when not in use.

The sexes in many groups can be distinguished by the genital structures at the end of the abdomen. In groups having internal genitalia the sexes may differ in other ways (size or color), or they may be indistinguishable without dissection.

Internal Anatomy. Although space does not permit a detailed account here, we believe that some features of the internal anatomy of insects are of sufficient interest to warrant at least brief mention.

The breathing system of insects is very different from that of man. Insects have a system of tubes {tracheae) that open externally at the spiracles and branch internally to supply all parts of the body. There are usually 2 pairs of spiracles on the thorax and several pairs on the abdomen; abdominal spiracles are generally located on the lateral edges of the terga. Oxygen goes from the outside directly to the tissues by way of the tracheal tubes, and is not transported in the hemoglobin of the blood as in vertebrates.

Insects' blood is ordinarily not red, and it does not reach all parts of the body in blood vessels. The heart is a tube located in the abdomen above the digestive tract. Blood is pumped anteriorly from the heart through a dorsally situated vessel called the aorta, and in the neck region empties into the body cavity. It flows through the body cavity, and reenters the heart through lateral openings called ostia.

The excretory system of insects consists of a number of tubes (malpighian tubules) that empty into the alimentary tract. Wastes from the blood enter these tubes and pass into the alimentary track and to the outside by way of the anus.

The nervous system of insects consists of a ganglion called the brain, located dorsally in the head, a pair of connectives passing around the alimentary canal, and a ventral nerve cord; nerves extend from the brain and nerve cord to various parts of the body. There are many sense organs, located mainly in the body wall. Some of these respond to tactile stimuli, some to chemical stimuli, some to sound, some to light, and some to other stimuli. Chemical receptors (organs of taste and smell) are located principally on the mouth parts, antennae, and feet. Special auditory organs, when present, generally consist of drumlike structures (tympana) or special hairs sensitive to sound waves. Many insects can detect sounds pitched far above the hearing range of man.

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