Now that we have established the names and locations of many of the important structures, I will discuss their various functions.
The Body Wall: The body wall serves a number of functions. It provides the service of an exoskeleton, and protects the insect from outside shock. It also allows reception of external stimuli. As a result of a high body surface to volume ratio, insects lose much more water to evaporation. The epicuticular layer resists water loss, and some insects can further prevent this by closing their spiracles. It’s also worth noting that the cuticle is not easily wetted, and sprays often need a wetting agent to stick to the shell. Gasses enter the body via the tracheae, although diffusion can occur through the body. Lipid-solubles generally penetrate the cuticle the best.
The exoskeleton is quite rigid, although less so in insects with chemical defenses or color markings. It is significantly stronger than the endoskeleton, and possesses more attachment sites for muscles. This strength is very important for flying, as it affords insects protection against collision. Smell organs can be located just about anywhere on the body, which is advantageous. There are downsides to this system. Discontinuous growth is necessary, and each molt the exoskeleton must be shed. During this time, an insects soft tissues are what supports it. This is the stipulation that limits the size of insects; an insect the size of a mammal would be crushed during this period.
Digestion and Nutrition: Solid food is broken up by the mouthparts as well as the teeth in the proventricus, and then is subjected to digestive enzymes. Most insects eat with their mouths, and saliva serves to prevent coagulation of blood, as that would clog the food channel. Some insects inject digestive enzymes into the food before they even begin ingesting it. Aphids, for example, inject amylase into plants, so they can digest the starch inside. The enzymes vary greatly depending on diet, and may be general or specialized. Omnivorous insects typically produce lipases (for fats), carbohydrates (starches & sugar), and proteolytic (protein) enzymes. Most species cannot digest cellulose, however microorganisms that can may be present. Typically these are bacteria or flagellated protozoans, often housed in special organs near the gut.
Insects require the same ten amino acids that we do: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tyrptophane, and valine. They also require various B vitamines, sterols (such as cholesterol or stigmasterol), several minerals, and some nucleic acid derivatives. They are unable to synthesize the amino acids nor the sterol. The quantity and quality of nutrition has a great impact on development.
Respiration: The respiration is carried out entirely by the tracheal system, which was detailed in the previous section.
Blood & Circulation: An insect’s blood transports salts, hormones, digested material, and metabolic producs throughout the body. There is homeostatic regulation of the water salt ratio in the body. Blood is also very important for inflating the wings, as well as for molting.
An insects blood consists of plasma and hemocytes. There is little oxygen present, and over twenty times as much amino acids compared to mammalian blood. The plasma has a high concentration of uric acid too, comparatively. There are about 50,000 hemocytes per square mm. The functions of these hemocytes varies greatly. Some examples include those that encapsulate foreign bodies inside the blood, migrate to wounds, and conduct phagocytosis (ingestion of bacteria).
Circulation is created by pulsations of the heart, with the help of accessory pulsative organs. The rate varies from 14 to 160 BPM. There is typically an increase in rate with increased activity. The insect circulatory system is very low pressure, and is sometimes even less than atmospheric pressure. When molting or inflating wings, insects may increase the pressure by holding air in the alimentary canal.
Blood is also responsible for homeostasis, or the maintenance of water content, pH (usually 6.0-7.5), salt content, as well as other factors.
Chemoreception: Involved in many insect behaviors. Feeding, mating, and habitat selection ago hand in hand with behavioral responses to chemicals. These reactions are often very specific, and chemically similar compounds can yield entirely different reactions.
Hearing: Insects are usually sensitive to 200-3000 hertz, although certain moths and Orthoptera can hear into the ultrasonic range, at over 100,000 hertz. Airborne sound is detected by small seta, or hairs. Some insects are not adapted to hear airborne sounds, but may hear sounds through substrate. A honey bee is an excellent example of this.
Vision: There are four types of photoreceptors present in insects: dermal receptors, dorsal ocelli, lateral ocelli, and compound eyes. Many types of larvae that lack ocelli or compound eyes will react to light. Dorsal ocelli react to the intensity of the light, but do not form an image. Lateral ocelli form images on retinal cells, and are likely capable of basic color and form recognition. Compound eyes posses light sensitive elements known as rhabdoms. Each cell has a small visual angle, coming together to form a mosaic view.
An advantage of insect vision is its high flicker fusion frequency, or the point when light appears continuous. Think of a flickering fluorescent light versus a working one; both are flickering, one appears continuous to us. Insects can see up to 250 pulses per second, compared to humans 45-53. This allows them to be very sensitive to motion, and perceive forms while rapidly flying. Some insects also posses depth perception.
The visible waves of light to insects are 2,540-7,000λ, compared to human’s 4,000-8,000. This allows them to see ultraviolet, and honeybees even analyze polarized light to find the location of the sun. Many insects appear to be colorblind, however others can observe colors including UV.
Reproduction: Eggs are developed in the ovarioles inside the ovaries. This is controlled by at least one hormone from the Corpora allata. This has been proven; removing the Corpora allata prevents egg function, and reinserting it makes reproduction begin again. Neurosecretory cells in the brain may produce the hormones that affect the Corpora allata.
Impregnation has a lot to do with external stimuli, such as sounds, scents, and body forms. This supersedes the compatibility of the gonads. Interspecies hybridization is quite rare because of the species specific stipulations involved in mating. Male insects have one X chromosome (XY), while females have two (XX). Eggs posses an X chromosome, while sperm may or may not posses an X (1/2 chance), therefore sex is determined when the eggs are fertilized. Some insects, such as Hymenoptera, have haploid (unfertilized) males, and diploid females, so unfertilized eggs will hatch into males.
Muscle Action: Insect muscles are unique in that they are capable of rapid contraction, with wings often stroking at 100/s, or even as high as over 1000/s. These contractions occur much faster than the nerve impulses reaching them, so these results are properties of the muscles themselves, not the brain. This also means that they must be extremely efficient muscles, with a high energy output.
Energy Sources: Obviously, food is where insects derive their energy. Most of the energy used by muscle contractions comes from carbohydrates. This is apparent, as glycogen stores are depleted after extensive muscle activity, with the products of said activity being water, CO2, and energy.
Body Temperature: Insects are primarily cold blooded, or poikilothermous. It is possible for an insect to raise its body temperature through activity of the thoratic muscles. These are the muscles associated with flying, however they may be contracted without moving the wings. Most flying insects are 5-10°C warmer than their environments, however moths and bumblebees may raise their temperature 20-30°C warmer!
Coordination of Bodily Systems: These systems work together via the Nervous and Endocrine systems.
Nervous System: The central nervous system is responsible for rapid adjustment to environmental changes. The three types of nerve cells are motor, internuncial, and sensory. Internuncial cells connect the CNS together, while sensory and motor cells are self explanatory. The Ganglia of the CNS are coordination centers for the body; these are the brain, subesophageal ganlion, and segmental ganglia of the ventral nerve cord. Most of these are autonomous, to some extent. Some activities require use of the brain, however many can occur with the brain absent.
Endocrine System: The endocrine system is made up of hormone producing glands. Hormones are chemical substances that produce effects on physiological processes. They control reproduction, molting, and metamorphosis, as well as other functions. Molting is caused by prothoratic glands secreting ecdysone (C27H44O6), which initiates growth and development. Interestingly, some plants produce substances similar to ecdysone and other juvenile hormones, making them toxic to insects! Evolution at work.
Pheromones: Pheromones are chemical signals used between the same species. They have slow transmission, but last a long time and are often effective at long ranges, even several miles. Pheromones serve many functions. Some of these include alarm substances, sec attractants, individual group recognition, aggregation formation, trails, and caste determination. Many pheromone compositions are known, and farms can even use them for pest control against specific species. Some of these pheromones are specific chemicals, while others are specific quantities of many chemicals. Oftentimes, similar species use the same chemicals in different concentrations.
This concludes Physiology and Function, and the next large post will likely cover insect behavior, which is likely to be less dense!