ENTM 340 Insect Pests of Trees Turf and Ornamentals 

C. Sadof  Purdue University

Lecture 4:    Internal Insect Anatomy

  1. In this lecture we will cover the following insect systems to give you an appreciation for how insects perform critical body functions.  As you study this material pay particular reference to how this information can be used to find a weak link that can be targeted by insecticides. Use the handouts and the laboratory exercises to learn the system.   
    • Digestive- Consumption and processing of food
    • Circulatory system- movement of body fluids.
    • Respiratory system - breathing
    • Nervous system – enervation of muscles, and production of neurohoromones
    • Reproductive system

 

  1. Digestive System- (Figure 3.13 Gullon and Cranston)

 

  • Lined by epithelial wall of cells that is one cell thick and surrounded by muscles that help move food through gut.  Foregut and Hindgut are lined with cuticle.  Midgut, due its role in nutrient absorption is not lined.  Gut pH will vary with insects. The effect of pH on the chemistry food and tends to influence kind of food insects by altering the effects of the poisons that plants produce to defend themselves against insects.
  • Foregut- Ingestion, Storage, grinding and transport of food to midgut
    • Food goes from mouth through pharynx and oesophagus where it is stored in the crop until it is ground by the proventriculus. Paired salivary glands at the anterior of the foregut function to produce saliva that aids digestion, prevents clotting in animal and plant tissue, or produce silk in caterpillars.
  • Midgut-  Biochemical breakdown (digestion via secretion of enzymes) and nutrient absorption-, Remaining material  enters the hindgut.
    • Food passes through ventriculus that lines the food bolus with a fine chitin/carbohydrate/protein  mesh called the peritrophic membrane. This allows digestion of food bolus, while protecting the epithelial cells from injury by sharp food.  (See Figure 3.16. Gullon and Cranston)
    • Bacillus thuringiensis, a common biological insecticide, kills insects by ripping holes in the peritrophic membrane of the midgut and damaging this fine epithelial wall.
    • Adaptations:
      • Gastric ceacae increase the surface area available for absorption
      • Straight short gut for rapid throughput feeders on solid food like caterpillars (GM).
      • Long, convoluted gut (filter chamber) for liquid feeders like aphids, to maximize absorption.  Hemiptera do not produce peritrophic membranes.

 

  • Hindgut- Absorption of water, salts and other important minerals, elimination of feces through anus. Like many animals, water regulation is critical to its conservation… Most insects need to conserve.  (See Fig 3.17 Gullon and Cranston for Schematic).
    • Malpighian tubules filter nongaseous wastes from haemolymph. Urine is excreted from tubules into the anterior portion of the hindgut.
    • Adaptations:  Cryptonephridia system.   Malpighian tubules are enclosed in a perirectal space and in a common trunk to conserve water and resorb it from relative humidity.  This is useful for insects adapted to dry conditions. (Oseto p. 53).
  1. Circulatory system

 

·        Structure and Function.  Insects have an open circulatory system that moves body fluids around insects bodies. (Figure 3.9 Gullon and Cranston). Circulation of provides nutrients and oxygen to body tissues, and extracts metabolic waste through the tendrils of the Malpighian tubules.  Insecticides that attack insect nervous systems disrupt the muscles that cause fluid circulation.

                                                               i.      Dorsal vessel – Open ended at the head, and closed at the tail end, this functional insect heart is supported by alary muscles. Waves of contraction pumps fluid toward head.  Ostioles are pairs of holes in each segment that via the aid of one-way valves allows hemolymph (=insect blood) to be sucked into the vessel with each contraction. 

                                                             ii.      Ventral diaphragm- Located above the ventral nerve chord acts in concert with thoracic pulsatile organs to pump blood from the head to the tail of the insect.

 

  1. Respiratory System
  • You’ll never see an insect panting like a dog.  They don’t breath through their mouths.  Rather, they breathe through a pair of holes called spiracles at the side wall (pleuron) of each segment.   Smothering agents, like insecticidal soap or oil will clog spiracles. Neurotoxins disable coordinated contractions that facilitate ventilation.

 

  • Air moves through the spiracles though a network vacuum hose like pipes called a tracheal system (Oseto p. 55-56).   Air moves along a main ventral trunk to smaller pipes called tracheoles.  These tracheoles are embedded into muscle tissue to faciliate the inward diffusion of oxygen and outward diffusion of CO2.
    • Ventilation of the tracheoles is enhanced by active contraction of thoracic muscles.   \
  • Adaptations: Most insects have an open respiratory system that allows direct exchange of oxygen from atmosphere.  Aquatic insects and internal parasites that live inside the body of other insects have a closed system that require a gill like apparatus to extract oxygen. Other adaptions for very small soil dwelling insects like Collembolla, lack tracheoles.

 

  1. Nervous System
  • Like other animals, insects transmit signals from sensory organs to central processing centers or ganglia.  These ganglia send signals to muscles and cause them to contract.  They have reflex arcs, like humans that bypass the insect brain. Ganglia are connected along a ventral nerve chord below the digestive tract.  Insect brains are fused ganglia in the head.  (See Oseto page 60).  Compounds used to transmit nerve impulses and innervate mammalian muscle contraction perform the same functions in insects.  Insecticides that target nervous and muscular signal transfer can also be toxic to humans.

 

  1. Reproductive Systems

 

  • Internal Reproductive organs. (Figure 3.20 Gullan and Cranston, Unit 10)

Located inside the abdomen

Female parts

  • Ovarioles, produce oocytes or eggs.
  • Eggs are dropped down from the calyx end of the ovaioles into the oviducts where they may be fertilized as they pass through the genital chamber.
  • Many females have an organ (spermatheca) to store and maintain sperm after mating

Male parts

  • Males produce sperm that pass through the ejaculatory duct through the intromittent organ. 
  • Sperm is deposited with other nutrients into the female genital chamber. 
  • Nutrients deposited by males are often resorbed by females and used to help mature oocytes

 

Kinds of Reproduction

  • Insects reproduce sexually or asexually via a process known as parthenogenisis. Some insects like aphids, will vary in how they reproduce, reproducing asexually at some times of year and sexually at others. Some species, most notably many wasps, have a haplodiploid chromosome system that allows them to control the sex of their offspring.  Males wasps are produced by unfertilized eggs. Female wasps are produced by fertilized eggs. In other species, like black vine weevils a pest of nursery containers, males are unknown.  Some insects like certain aphids give birth to live young (viviparous), others lay eggs (oviparous).  Some eggs produce individual offspring, other eggs divide multiple times to produce 50 or more clones in a process called polyembryony.

 

Mate Finding

  • Some insects need use chemical scents to find each other over long distances. Called sex pheromones, these scents are secreted by females to attract potential mates who reside downwind.  Traps, baited with these materials are often used monitor insect populations. In these cases males have special feathery antenna to allow them to detect the minute quantities of scent they use to follow the plume of pheromone to its source.

 

 

  1. Hormonal control of Insect Systems (Oseto p 78)

 

All insects grow by molting from immature to more mature forms.  The molting process and the rate of maturation during each molt is controlled by two broad classes of hormones that are produced by glands in the brain and the prothorax.

 

  • Ecdysones stimulate the molting process to commence and stimulate maturation. 
  • Juvenile hormones act to counter the activity of ecdysone, delaying moulting and maturation. 
  • The ratio of these 2 compounds determines the level of maturity of the insect during the next molt.
  •  Neuropeptides, secreted by glands in the brain, are protein messengers that regulate many of the systems critical to insect development, homeostatis, metabolism and reproduction.

 

In the 1980’s a new group of insecticides called insect growth regulators have been developed.  These materials alter the delicate balance of these hormones and neuropeptides that kill insects during the moulting process or sterilize them. Advances in molecular biology in the late 90’s have greatly honed the ability to identify specific peptides and map their genes will be producing a wide range of safer insecticides in the near future. 

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References:

Gullan, P.J. and P. S. Cranston 2000.  The insects. An outline of entomology. Blackwell Science, Incorporated. Malden, MA.

 

 

Review questions:

1.      Describe the function of the foregut.  Besure to include the role of the proventriculus and the salivary glands.

2.      What is a peritrophic membrane and how does it protect the midgut?  How is this membrane affected by the biological insecticide Bacillus thuringiensis.?

3.      What is the relationship between the morphology of the gastric cecae and absorbtion of nutrients?

4.      How to liquid feeders (sap feeders) modify the gut to allow them to absorb nutrients?

5.      What is similar between the function of human kidneys and Malpighian tubules?

6.      Unlike humans, insects have an open circulatory system. How does the insect pump its haemolymph and where does it flow?

7.      Insects do not have lungs. What kind of respiratory system do they have?

8.      Most insects do not have respiratory pigments like haemoglobin.  How do they move oxygen to the muscle tissue?

9.      What is a spermathecae and what is its relation to fertilization of eggs.

10.  Wasps are parthenogenetic, with fertilized eggs becoming females and unfertilized eggs becoming males. Given what you just learned about spermathecae how can female wasps determine the sex of their offspring?

11.  What is a sex pheromone?

12.  What are the names of the two kinds of insect hormones that control moulting? How does this relate to the development of new insecticides.

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