File: <insect internal anatomy

Embryological Development. -- During embryological development the material that lines the hind and fore guts is the same as the integument that forms the insect body. It is usually called cuticula also frequently intima.

When molting, the cuticula is cast off including that which occurs in the fore and hind guts. The principal absorptive area for digestible materials occurs in the midgut. This is also the main site of secretion.

Morphology (Digestive Tract). -- The digestive tract is divided into the Stomodaeum, Mesenteron and Protodaeum. A simple diagram of the tract may be seen in Fig. ent24.

The Stomodaeum (or foregut) contains the esophagus, which is an undifferentiated tube for simple conduction of food. The crop follows this, which is a storage organ. It may serve for partial digestion, especially sugar and starch, as fluids from the mesenteron pass forward into the crop. In some insects, e.g., mosquito, the crop may appear in the form of a diverticulum.

The Salivary Gland empties from the anterior portion of the insect near the mouth. It is a very active structure, and the saliva is used in digestion and as an anticoagulant in some groups (e.g., aphids). It also may be modified as in the silk-secreting organ of Lepidoptera.

The proventriculus follows the crop and may be a secondary masticating organ, as in the cockroach. It actually serves as a food strainer.

The cardiac valve keeps the food moving in a posterior direction.

The Mesenteron (or midgut) is the site where nearly all digestion occurs, which is primarily fat and protein. Most nutrients are absorbed here. The cells are extremely active and there is a continual replacement of cells in the lining.

The gastric caecae are expansions of the midgut (Fig. ent24).

The peritrophic membrane is secreted by epithelial cells, which are either isolated groups of cells or by all the cells of the midgut. It is permeable to liquids and also may serve as a protective device for tender tissue of the stomach.

The Proctodaeum (or hindgut) is the principal site for water absorption. It contains a pyloric valve, and the Malpighian tubules that serve for excretion enter the hind gut at the junction of this valve. The rectal sac at the end of the digestive tract is very important in water absorption for most insects. However, some groups such as clothes moths utilize metabolic water.

Excretion. -- Hypodermal cells of the integument absorb waste products and secrete them into the cuticle, which is shed at the molt. However, the Malpighian tubules serve as the principal excretory structure.

Fat Body. -- Many insects cannot feed in the adult stage; hence their energy as adults comes from the fat body, which is built up during immaturity.

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The Circulatory System

A simple diaphragm shows the principal parts of the Insect circulatory system (ent29):

Diaphragms. -- These consist of sheets of muscle that are connected together by membranes. The dorsal diaphragm is usually present, but the ventral diaphragm is variable in occurrence.

Dorsal Vessel. -- There are several hearts, which are provided with circular muscles for contracting and expanding. They serve as pulsating organs. The blood is propelled forward usually, but sometimes it may go backwards. Auxiliary pumping membranes sometimes occur at the bases of the extremities, and they pump blood to those areas (e.g., legs, antennae, etc.).

Blood. -- Insect blood is a viscous liquid that consists of about 75 percent water. Pigmentation varies from clear to red, brown or yellow. The oxygen concentration is never very high in insect blood, so the blood does not serve as an efficient oxygen carrier. Amoeboid cells (phagocytes) occur, which engulf foreign materials and assist in clotting. Clotting is mainly a coagulation of phagocytes and there is no fibrinin. In most insects this clotting mechanism is very efficient.

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The Respiratory System

As noted the insect blood does not serve as a principal vehicle for the transmission of Oxygen. There are other ways that this is accomplished.

Diffusion Through The Integument. -- For this to occur the integument must be thin and wet at all times. Gill filaments or plates may be involved and parasitic forms obtain their oxygen directly through the integument.

Tracheal System. -- This is a complicated system of tubes, which extends all through the insect body. The individual tubes are called tracheae. Entrance with the external environment is via the spiracles.

Internections have occurred between the metameres to form a continuous system. The tracheal system originates from invaginations of the body wall, and at the molt the tracheae are also cast off.

Air sacs may appear at any point on the trunks. Bracing of the tracheae is accomplished by taenidia, which occur in a spiral within.

Tracheoles are a series of single-celled tubes that are located at the ends of the system. These cells lie on the surface of the muscles and other body tissue. They are not lined with intima and they are filled with a fluid, which serves as a medium for gas exchange. Molting includes a portion of the tracheoles. A diagram of trachea and tracheoles in the system may be viewed in Fig. ent68.

Body movements augment the respiratory function. Figure ent69 shows how trachea transfer oxygen to the muscles:

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The Central Nervous System

Ventral Nerve Cord. -- Primitively there was a pair of ganglia for each segment. Modern insects have these ganglia fused and the total number reduced.

There are never more than eight ganglia in the abdomen, and then thee is a tendency for further reduction.

The Insect Brain. -- This is primarily a sensory area. Most body functions can continue for a long time even when the head is removed (See Ent35 and Ent36 for diagrams)

The insect brain is divided into three regions. (1) The Supraesophageal Ganglion consists of a protocerebrum, a deutocerebrum and a tritocerebrum; (2) the Subesophageal Ganglion is a fusion of three pairs of ganglia. Each pair enervates a portion of the mouthparts; and (3) the Frontal Ganglion is connected to the tritocerebrum and forms a part of the stomodael system.

Insect Ocelli. -- The lens is simply a clear space in the integument. Light can enter at any angle, and hence thee is no clear image produced. They are sensitive to light intensity and may serve to detect motion.

The Compound Eye. -- Each lens is also a clear space in the integument. A crystalline cone lies beneath the cornea. There are pigment cells that contain granules of pigment, which can migrate up or down, depending on the light intensity. Light sensitive cells are located in an octagon at the bottom of the cone.

There are two opinions about what the insect actually sees: (1) each ommatidium sees a whole image, and (2) each ommatidium seas only a segment of the image, thereby producing "mosaic vision."

Auditory Organs. -- Some insects, such as the grasshopper, possess a tympanum that allows hearing. But most insects hear through their antennae.

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The Reproductive System

All insects are unisexual, but when gynandromorphs occur they are nonfunctional. Functioning wise, many insects, such as worker bees, are asexual. In bees there is a suppression of the sexes and only one female is singled out for reproduction.

Secondary sexual characteristics are often present. This is illustrated by the frenulum in moths, and the eyes of male flies are closer together than those of the female.

Diagrams of the male and female reproductive systems may be seen in Figure Ent40.

The Female Reproductive System. -- There are two ovaries, each consists of ovarioles and an ovary wall; a filament attaches the ovary to the body wall, and nurse cells lie between the ovules of the ovariole at varied sites.

An Egg chamber receives a mature egg from an ovariole.

Additional structures are a lateral oviduct, median oviduct, genital chamber, spermatheca with an accessory gland to nurse the spermatozoa and accessory glands. The latter serve for gluing the eggs to a surface and for pod formation.

The genital opening may occur on the primitive 8th segment or it may have been shifted to the 9th segment.

The Male Reproductive System. -- The morphology of the male system consists of two testes, a seminal vesicle, a vas deferens, accessory glands that furnish a matrix for passage of the sperm, an ejaculatory organ and an intromittant organ.

Fertilization. -- Spermatozoa enter the egg by way of minute pores in the chorion called micropyles. A tube may then extend down to the interior of the egg.

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The Musculatory System

The muscular system is quite complex, consisting of from hundreds to several thousand individual muscles. They are all striated muscle cells, even those that surround the alimentary canal and the heart.

Insect muscles are able to contract very rapidly and because of this they are very efficient, as such action requires and adequate oxygen supply. This is supplied by the tracheal system.

The antennae of insects have muscles only in the proximal end, with a few exceptions among the beetles (Coleoptera). There is no internal musculature in the wings, mandibles and some other structures. However, the legs and thorax have a well-developed musculature.

Thorax Musculature. -- The various structures associated with muscles that occur in the thorax are discussed as follows:

(1) Phragma are apodemes that extend down from the dorsal surface of the integument. They are used for muscle attachment. (2) Dorsal Longitudinal Muscles arch the tergum and make down strokes of the wing. In the prothorax they extend to the head. (3) Dorsal Ventral Muscles depress the tergum and are for producing upstrokes of the wing. (4) Furcae are ventral apodemes for muscle attachment. (5) Lateral or oblique muscles are present. (6) Ventral longitudinal muscles are also present. (7) Additional smaller muscles occur whose function is debatable.

The Insect Head. -- Figure ent65 shows a generalized diagram of the insect head.

The Mandible. -- The two mandibles are attached at two points on the head (See Figure Ent41: A & B). There are no muscles within the mandible itself. The adductor muscle in the head pulls the mandible inward while the abductor muscle extends the mandible.

The Insect Leg. -- Muscles extend from the coxa down through the tibia, but there are none in the tarsus. The femur has a protractor muscle, which thrusts out the tibia and a retractor muscle that pulls it back.

The tibia has some small and varied muscles and a tendon that extends down to the tip of the tarsus. (See Figure Ent42)

The Insect Abdomen. -- Figure ent66 gives a diagram of the dorsal part of the abdomen, showing various parts and muscles.

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References

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