At times it may appear that insects and their relatives are capable of thought. However, close examination reveals that they do not think but are merely reacting to their environment. Insects can be compared to a complex computer, which has been programmed through the processes of nature. As a result they are equipped with a large number of behaviors, such as walking, running, tasting, eating, flying, mating, avoiding light and many, many, more. In order for a behavior to occur, the proper button on this computer (insect) has to be pushed. This is accomplished through the sensory system of the insect (e.g. compound eyes, ocelli, tactile hairs, antennae, maxillary palps, etc.). Once an insect perceives a stimulus (e.g. tastes food with its maxillary palps), then the preprogrammed behavior unfolds. In this case the insect eats food.
It can be demonstrated easily that there are no thought processes going on by placing the abdomen of a dragonfly in its own mouth, which it will readily consume. This occurs as a thoughtless behavioral reaction to the stimulus of protein being placed in its mouth. Of course in nature it would indeed be rare for a dragonfly to find its own abdomen in its mouth, so this behavior would be of no disadvantage to the species.
Many insects share some characteristics with higher animals including humans. Two signs of intelligence are the ability to think and communicate those thoughts to others. As we have already discussed, insects cannot think but they do have limited abilities of communication.
Communication in insects is most commonly found in, but not limited to social insects. There are documented cases of nonsocial insects communicating with individuals of the same species, cases of insect communicating with insects of a different species and cases of insects communicating with higher animals.
Although the last situation discussed above is extremely rare, a few species of moths communicate with bats. Most if not all insect-eating bats use echolocation to find their prey. A feeding bat sends out a high frequency beep that bounces off its prey (moths), with the returning beep being detected by the bats large ears. This system works similarly to a destroyer's sonar in detecting a submarine. Some species of moths are capable of detecting the beep of an echolocating bat. If the detected beep is weak, indicating the bat is far away, the moth will fly away as fast as it can in an attempt to avoid the bat. When the beep is strong, indicating that the bat is close, the moth will fly erratically or merely drop to the ground. Although amazing, none of the above is a good example of communication. However, in at least one species of tiger moth, the beep of an echolocating bat will cause the moth to return a squeak of its own which is detected by the bat. This tiger moth is foul tasting and any bat that has previously fed on another squeaking tiger moth of the same species will associate that sound with the foul taste and break off the hunt.
Unfortunately, a few questionable entrepreneurs have exploited the public by selling pest control devices based on this principle of avoidance displayed by insects and other pests upon hearing certain types of sound. One such device is a small electrical box that is supposed to produce a high frequency sound that drives cockroaches and other insect pests from the home or disrupts their mating biology. Another is a probe that, when placed in the ground, produces a beep that drives gophers from the ground. Unfortunately these devices are totally worthless. I once walked into the backyard or a homeowner who had placed 8 of these probes in the ground in attempts to eliminate one gopher. Of course the gopher was still there after 4 weeks of beeping.
Interspecies Insect Communication. Although rare, there have been a few documented cases of one species of insect communicating with another species of insect. There is a species of caterpillar in Costa Rica that possesses glands which secrete a sweet tasting fluid that readily attracts ants. The ants obviously benefit from the relationship by obtaining nutrients from the liquid and the caterpillars benefit by the ants' protection from potential predators. The caterpillar has a ridged area on the front of the prothorax which, when rubbed by the back margin of the head, produces a squeaking sound. If a predator attacks the caterpillar, it produces this sound, which alerts any nearby ants to the danger so they can quickly rush to its aid.
Insect communication within the species is accomplished with the use of all possible media including smell, sight and sound.
Chemical Communication. Insect pheromones are a component of insect behavior that has been studied in great detail. A pheromone is a hormone-like chemical that is released by one individual of a species that elicits a behavioral response in another individual of the same species. There are many different types of these chemicals including sex, aggregation, trail-following, alarm and oviposition pheromones.
Sex pheromones-Sex pheromones serve to stimulate the opposite sex to mate and are found throughout the insect world. They have been studied most thoroughly in the Lepidoptera (butterflies and moths) and the Coleoptera (beetles). In the former order, moths rely considerably more on sex pheromones for mating communication than do butterflies. Butterflies are diurnal and rely much more on sight to recognize the opposite sex. However, with nocturnal insects such as moths, vision is of little value and consequently, they utilize chemical communication.
When a female moth is ready to mate, she exposes a gland on the tip of her abdomen which releases a sex pheromone continuously into the air. Biologically speaking the female of the species is the logical sex to release such a chemical as typically she is more heavy-bodied (full of eggs) and consequently a less agile flyer.
Once released, the sex pheromone floats downwind from the female forming a continuous odor plume. The distance this plume travels while remaining intact may be several hundred yards, depending on the species of moth involved and the speed and turbulence of the prevailing wind. Because the odor plume will remain intact longer and therefore be more effective when winds are calm and slow moving, most moths mate very late at night or in the early morning hours (when these conditions occur). Any sexually receptive male moth of the same species exposed to the female sex odor plume will immediately begin flying in the direction of the releasing female. The main environmental clue as to the direction at the female is not the pheromone itself, but the direction of the wind. Since the pheromone travels downwind, the responding male orients or flies upwind, thus traveling toward the female. This phenomenon of upwind orientation is referred to as positive anemotaxsis.
Once near the female, the male does not recognize her; he relies entirely on a concentration of her sex pheromone that is more or less uniform at close distances in all females of the same species. At that point, he releases a sex pheromone of his own--thus alerting her to his presence.
The author has conducted considerable research with insect sex pheromones. On more than one occasion I have spilled small amounts of these chemicals on myself and then gone out at night only to have large numbers of male moths flock to, and attempt to mate with--me! One warm summer evening our daughter drove a car containing a vial of synthetic sex pheromone (used in research) in the glove box to pick up her brother from his job. As she waited under the lights with the windows down, approximately a hundred moths flew into the car and circled her frantically. Others waiting in their cars were understandably astounded.
The sex pheromone of many of the more important agricultural pests has been chemically identified. This is usually accomplished by first establishing a large laboratory colony of a species. The sex pheromone glands of many thousand females is then clipped off the abdomen in an attempt to chemically extract enough pheromone in a pure form so its chemical structure can be identified. Once identified, the chemical can be synthetically produced in the laboratory.
One of the more effective uses of synthetically produced sex pheromone has been in the control of the pink bollworm, a small moth that is major cotton pest from U.S. to Southern America. This moth is a weak flier and only feeds and mates in cotton fields. The control technique consists of saturating entire cotton fields with the smell of artificially produced sex pheromone over the entire growing season. The purpose of this procedure is to prevent the male moths from finding females and consequently preventing mating and reproduction. If this can be accomplished, the moths will not reach sufficient numbers to become pests.
Two theories as to why this technique works have been called the male confusion technique and the male inhibition technique. The male confusion theory is based on the premise that is if the air of a field is saturated with artificially produced sex pheromone, when females release her pheromone in that field, there will be no discernible odor plume for him to follow and find her.
The male inhibition theory is based on a biological phenomenon that is common to many animals including humans and moths. For example, when someone is continuously exposed to a strong odor (e.g. perfume or cologne), after a short period of time, one can no longer smell it. This is because the olfactory sense cells in the nose adapt to the presence of this odor. The same phenomenon occurs in a male moth sitting in a field saturated with the smell of artificial pheromone. If a female moth released her pheromone in that field, a male would not be able to detect it and therefore could not find her to mate.
Trail Following Pheromones-Trail following pheromones are found in a number of insects but can most often be seen in the ants, honeybees and termites. These insects forage from their nest randomly in search of food. Once successful they carry some of the food back to the nest, utilizing visual cues in their environment and time compensated sun orientation. This term refers to the ability to use the sun as a compass and adjusting orientation to the sun as it appears to move across the sky (see later discussion in this chapter).
As an ant returns to the nest, it drops small amounts of a trail-following pheromone from the tip of its abdomen onto the ground or other substrate on which it is traveling. Upon reaching the nest, it shares the food with other workers, which excites them to leave the nest and search for food. Upon leaving, the new foragers follow the partially established trail and reinforce it with pheromone of their own. Trail-following pheromones are quite volatile and if not constantly reinforced will soon disappear, a characteristic that is advantageous once the food source runs out. This phenomenon can easily be demonstrated by rubbing one's hand through a trail of foraging ants and thus erase the chemical trail. Once other foraging ants reach this point in the trail they appear confused and head off in any of a number of directions.
Because the worker and soldier castes lack wings and thus never fly, and the reproductives use their wings for just a brief amount of time, termites predominantly rely upon their legs to move about. Foraging behavior depends on the type of termite. For example, certain species feed on the wood structures they inhabit, and others harvest food that is near the nest. Most workers are rarely found out in the open, and do not forage unprotected; they rely on sheeting and runways to protect them from predators. Subterranean termites construct tunnels and galleries to look for food, and workers who manage to find food sources recruit additional nestmates by depositing a feeding pheromone that attracts workers. Foraging workers communicate with each other via pheromones. When workers begin to forage outside of their nest, they release trail following pheromones from their special glands. In one species, Nasutitermes costalis, there are three phases in this foraging expedition: first, soldiers scout an area. When they find a food source, they communicate to other soldiers and a small force of workers starts to emerge. In the second phase, workers appear in large numbers at the site. The third phase is marked by a decrease in the number of soldiers present and an increase in the number of workers.
Foraging Worker Termites Accompanied by Soldiers. Image Courtesy Bernard Dupont. CC BYSA 2.0 International
Another insect that utilizes trail following pheromones is the honeybee. When a beehive becomes crowded, a new queen is raised and she subsequent leaves the hive with approximately 1/2 the worker bees (up to 40,000) in the form of a swarm. The newly emerged swarm will not typically fly far from the hive and will quickly settle down on a nearby fence post, branch or other structure. Because such locations are not ideal for setting up a new home, a number of the worker bees in the swarm will search out a more suitable location. Unfortunately, a newly found location often ends up being the hollow wall of an apartment building or house. Once a worker bee finds this new location, she will return to the swarm and lead it to that location by releasing a trail following pheromone as she flies.
Honeybees also utilize trail following pheromones to organize or “call in” foraging bees returning from the field. When large numbers of returning bees begin to gather near a hive entrance, a degree of confusion may occur in this location. This is especially true if there are several hives in a small area. As a result a number of hive bees will line up at the entrance of the hive, raise their abdomen, fan their wings and produce a trail following pheromone from a white gland on the tip of the abdomen. Of course this produces an air born trail of pheromone leading to the entrance (see elbow).
Worker Bee Producing a Trail Following Pheromone at Hive Entrance.
Aggregation pheromones-Aggregation pheromones differ from sex pheromones in that they can be released by either sex of a species and attract both sexes for mating and feeding. They are best developed in the bark beetles. Bark beetles are one of the major insect pests of timber, especially when trees are in a weakened condition. Healthy trees generally possess a natural defensive mechanism against beetle attack in the form of sap flow which will drown most boring pests. If a tree is weakened due to disease, drought, fire, or any of a number of other reasons, sap flow decreases and bark beetles is attracted in large numbers. These insects bore beneath the bark forming extensive tunnels which can eventually girdle and kill the tree.
Aggregation pheromones provide an efficient mechanism for bark beetles to find susceptible trees. If a bark beetle flies to and bores into a healthy tree, sap flow will kill it quickly. However, if it bores beneath the bark of a weakened tree with reduced sap flow and begins to feed, it releases an aggregation pheromone that in turn attracts huge numbers of beetles to the tree. This mechanism greatly increases the chances of beetle survival insuring both mates and a copious source of food.
Aggregation pheromones have been identified for a number of species of bark beetles. Again, humans have used these chemicals in the attempt to control these pests. One technique, which has resulted in marginal successes, is to paint aggregation pheromone on healthy trees, letting the natural sap flow kill those beetles that are attracted. Aggregation pheromones also have been used as 'bait' in large traps that are painted with a sticky material that ensnares beetles. When these traps are placed in forested areas, each literally can trap buckets of beetles in a few nights.
Alarm pheromones-Alarm pheromones are found in many species of insects but are best developed in social insects, such as bees. When a honeybee stings, it releases an alarm pheromone from a gland located at the tip of. In this case the pheromone draws other bees to the site of the attack and stimulates them also to sting. Because honeybees are rarely found together except in the vicinity of a beehive, this is a mechanism for protecting the colony. Many a beginning beekeeper has been stung by a bee inside the veil and made the mistake of attempting to remove it only to find many more angry bees on the attack.
This reaction was recently demonstrated in a beekeeping class at Cal Poly. There was an individual in the class who had gained the reputation as being cool. He was what we call a professional student having been at the university for nearly 15 years and having completed 4 majors and 9 minors. The first day of laboratory he got a bee under his veil which stung him on the ear. Forgetting what we had warned him and not remembering the warning, off comes the veil and on came the hasty retreat. He quickly outdistanced the bee until he ran into a patch of prickly pear cactus. After a few days he healed except for his bruised dignity.
Oviposition Pheromones-Oviposition pheromones are associated with the process of egg deposition. They are best developed in the parasitoids, or those insects that parasitize other insects. A well-known parasitoid is the tarantula hawk, a wasp that deposits an egg inside a tarantula. Once the egg hatches the larval wasp feeds and completes its development inside the tarantula and eventually kills it.
Many parasitoids will place a dot of oviposition pheromone on their host immediately after they have deposited an egg in or on it. This phenomenon is referred to as the spore effect. In these cases the pheromone serves to alert other parasitoids of the same species that this host is already parasitized. O course this prevents over-parasitization and insures an adequate food supply for the developing immature parasitoids.
Visual Communication. As previously mentioned, butterflies utilize sight and moth use odor to attract the opposite sex for mating. Correspondingly, butterflies are typically brightly colored and moths tend to be dully colored. This fact was recently well documented on a recent insect collecting trip to Costa Rica. Several of the students had spent most of the morning attempting to collect the large brilliantly blue colored Morpho butterflies. They had limited success as these butterflies tend to be quite fast flying and elusive. Upon observing their frustration, I pulled out a blue piece of paper and waved it in the air attracting several excited males which were easily captured.
Another insect that uses visual clues to communicate mating readiness to the opposite sex is the firefly. The fireflies are soft-bodied beetles in which the head is not visible when viewed from a dorsal angle. During the spring and summer months these insects are quite conspicuous due to their blinking yellow lights. There are small members of this group in California, but these are not capable of producing light. The light-producing species are rather common in many areas of the world including the southern and eastern United States.
The light emitted by these insects is unique in that 100% of the energy produced is in the form of light. In the common light bulb only about 10% of the energy produced is light while the rest is heat. The light is emitted from a gland located on the underside of the firefly's abdomen and is produced by oxidation of a material called luciferin in the presence of an enzyme called luciferase. The gland is richly supplied with tracheal breathing tubes and the beetle has the ability to supply oxygen to the gland when it is needed to oxidize the luciferin to produce light.
Each species has its own flashing pattern with variations occurring in the flash length and intervals between flashes. The blinking is a form of sexual communication within a species. At twilight the males of most species fly low over the ground and begin to flash. While sitting on the ground or vegetation receptive females of the same species begin to flash back, thus drawing in the males for mating.
In a few species large numbers of males will gather in one bush and flash in unison. This draws both sexes for mating. This phenomenon has also been observed is some of the "eyed" click beetles and is similar to some the behavior of some of the long-horned grasshoppers or cicadas that sing in unison to draw mates. This cooperative behavior intensifies the signal which then is carried over longer distances than can that of individuals.
There is one species of predatory firefly that mimics the blinking pattern of a smaller species. In this case the males of the smaller species that respond are consumed rather than finding a mate. Surprise!
Honeybee Communication and Foraging Behavior. One of the more interesting behaviors of the honeybee is foraging communication. Once foraging honeybees that have found a strong source of food return to a hive they have the ability to communicate to other foragers the direction and location of the source. This is done two types of dances (round dance and waggle dance) that are performed on the vertical honeycomb. The remarkable thing about this is that the foragers do not follow the scouts back (the scouts may remain in the hive for hours). So the scout bees have communicated to the foragers the necessary information for them to find the food on their own.
If the source is within 50 to 75 meters of the hive the round dance is performed. Since it is dark within a hive vision does not play an active role in the other foragers perceiving the dance. But instead they gather around the dancer and interpret this activity (by movement and vibrations) with their antennae. Once perceiving the dance the new foragers leave the hive and search in all directions (within 50 to 75 meters of the hive) for the source.
Round dance (A) and waggle dance (B) of honeybee.
On the other hand if the distance to the source is over 70 meters then the returning bees perform the waggle dance. In this case the bee dances what is best described as a figure eight. As the bee dances the straight portion (middle of figure eight) she vigorously waggles her abdomen. One of the amazing aspects of this dance is that the dancer can actually communicate not only the distance but also the direction of the source. There is a correlation between how vigorous the dancer shakes or waggles its abdomen and the distance to the source. Generally speaking the faster the rate of waggle and overall speed of the dance, the further is the distance to the source. There is a direct correlation between the speed of the dance and distance of the source from the hive and the foraging bees are able to compute the correlation.
The direction of the sources is communicated by the straight portion of the dance which actually correlates to the position of the sun (see below). In example A (the flowers) the straight portion of the dance is at a right angle or 90 degrees to vertical with the dancer heading to the left. This is correlates to a ninety degree angle (to the left) to the position of the sun. In Figure B the straight portion is at a 45 degree angle to the right of vertical. In this case the source is at a 45 degree angle to the right of the position of the sun.
A B C
Location of Nectar Source Based on Waggle Dances. A. Dance is Performs Straight up on Vertical Comb. B. Dance is Performed 80 Degrees of Left of Vertical. C. Dance is Performed Straight Down on Vertical Comb. Image Dr. Kaae.
If that isn’t amazing enough another factor comes into play. The position of the sun in the sky obviously changes with time. This would cause a problem if a bee was foraging to the same location over a long period of time. The amazing thing is that the bee can actually adjust and computed the change in position of the sun and this can even be done if the bee was confined to a hive over this period. This phenomenon is referred to as time compensated sun orientation.
The time sense of honeybees has long been known to people who have sweet snacks in their garden at a set time every day. Within minutes of the regular time, foraging bees arrive for their share of the jam. The speed of the bee's clock seems to be related to its metabolic rate. If normally punctual bees are chilled (to lower their metabolic rate) or exposed to an anesthetizing concentration of carbon dioxide they arrive late to the picnic table.
The question may arise how bees forage on cloudy days. In actuality the scouts (and foragers) don't actually have to see the sun to navigate. As long as they can see a small patch of clear blue sky, they get along fine. This is because sky light is partially polarized, and the plane of polarization in any part of the sky is determined by the location of the sun. Try it by rotating a pair of Polaroid® sun glasses!
The waggle dance is also use with swarming bees. Once a swarm first settles somewhere, e.g., on a tree branch, scouts leave to search for a permanent home. Once a promising site is found, they return to the swarm and dances as though she had found food. Eventually, the swarm departs for the location.
Sound in Insect Communication. Katydids, crickets and cicadas are the most common insects that use sound in mating communication. Male crickets produce their familiar chirping by rapidly raising and lowering their wings. In doing so a hardened area on the forewing (scraper) is rubbed over a ridge area (file) on the hindwing. Each cricket has its own characteristic chirp--thus avoiding mating attempts between different species. Mating communication in cicadas is the loudest sound in the insect world. This sound is produced by a pair of flap like structures (tymbals) located on the underside of the abdomen. In the jungles of Malaysia the mating calls of the largest species of cicada in the world (Pompone imperata) can be nearly deafening.