CHAPTER 1

The Success of Insects

Many entomologists (unusual and sometimes weird people who study bugs—most are weirder than you or me, of course) think that insects and their relatives are the most successful living organisms on the earth, while many others might quickly point out that humans rule the earth. Both points of view have merit, but consider the following: one measure of success is endurance. Fossil records indicate that insects and their relatives have been on earth for more than 300 million years. This number becomes even more meaningful when one considers that human existence dates back only 3 million years. Such a long-term record has allowed insects and their relatives to occupy almost every available ecological habitat and certainly has given them time to develop a huge variety of behaviors that lead to their success.

It is estimated that there are between 4 and 14 million insect species worldwide—most of which are yet to be discovered. Excluding the other arthropods (spiders, ticks, scorpions and the like), there are only about 1 million species of plants and animals combined. There is only one species of human (Homo sapiens). The current human population is approximately 6 billion; however, at any one time it is estimated there are 4 quadrillion individual insects on earth. Therefore, insects outnumber us by almost one million to one. It may be difficult to realize how big a number 4 quadrillion is. To help in this realization, consider the following: a housefly is typically thought of as an average-sized insect—measuring about 1/4 inch in length.  If we took 4 quadrillion houseflies and lined them up end to end, this line would be long enough to reach around the world 150,000 times.

In terms of sheer biomass (total weight), insects outweigh us 10 to 1. That is, for every pound on human on the earth, there are 10 pounds of insects.  Whether or not one believes that insects are the dominant animals on the planet, it would be difficult to argue that they are not a very successful group. This leads to the next question: why are they so successful? One reason for the success of these animals is their small size—most insects are smaller than a housefly. Small animals need little energy (food) in order to develop and survive. In addition, smaller organisms live in microclimates (very small areas) which, in many cases, allow them to thrive in otherwise intolerable conditions. For example, the humidity on the surface of a plant leaf nears 100% and the temperature is considerably lower than that of the surrounding area. Consequently, leaf-feeding insects survive in the hottest, driest deserts of the world—areas where they would quickly perish if exposed to the everyday temperatures and humidities of a macroclimate. Also, small organisms can hide more easily from potential predators.

Most insects are winged; this gives them the advantage of great mobility to seek new environments when necessary. In addition, the majority of insects exhibit complete metamorphosis; that is, they have an egg, larval (caterpillar, grub or maggot), pupal (cocoon) and adult stage. This type of development has an advantage, in that the 2 active and feeding stages (adult and larva) live in different environments and feed on different materials; consequently, they do not compete with each other.

Insects generally have a tremendous reproductive capacity. Again, take for example the common housefly. The female is capable of laying several hundred eggs and this insect can complete one generation from egg to adult in as little as 10 days. Given this type of reproductive capacity, if a male and female fly mated in April and all their offspring survived and reproduced, and this cycle were repeated every 10 days, by mid-August of the same year there would be enough adult flies to cover the earth by 45 feet. Obviously this does not occur, but this type of reproductive capacity gives insects a tremendous advantage, in that an average of only 2 offspring need survive and reproduce every generation for success and survival of the species.

Finally, one of the main reasons for the success of insects is their tremendous diversity of capabilities. Discussed below are some extreme examples of these capabilities. Fecundity is the egg laying capacity of insects that varies considerably from species to species. An Australian ghost moth holds the highest recorded fecundity among nonsocial insects. One female is capable of laying 44,000 eggs. Although astounding, this is meager when compared to social insects such as the queen African driver ant that can produce 4 million eggs every 23 days or the African termite queen that can produce 10,000 eggs a day for 30 years (Figure 1A). On the other end of the spectrum is the louse fly (Figure 1A) that produces a meager average of 4.5 eggs per female. In this case, the young are well cared for, insuring maximum survival. After the eggs hatch, the maggots remain inside the female where they feed on a specialized milk gland until fully developed.

Figure 1A. Left. An African queen termite can lay 10,000 eggs a day for 30 years. Right. Louse fly-a wingless parasitic fly.

The life cycle of an insect is defined as that sequence of events that begins with the egg and ends with the reproductive adult. Aphids probably have the shortest life cycle among the insects, with some species completing one generation, or life cycle, in as little as 5 to 6 days. On the other end of the spectrum is the periodical cicada that requires 17 years to complete one life cycle (Figure 1B). In some insects and their relatives, the normal life cycle can be greatly extended in extreme environmental conditions. An example is the long horned beetles that normally develop in rotting logs and typically require 1 to 4 years to pass through one cycle (Figure 1C). However, if a tree containing the grubs of these beetles is cut into lumber, the normal environment changes drastically (less available nutrients and moisture) and their life cycle can be extended 50 or more years. There is also a case of extended dormancy, where water was added to some dried moss that had been in a British museum for 106 years and tartigrades (insect relatives) emerged.

Figure 1B. A 17-year locust or periodical cicada lives 17 years below ground as a nymph (right) and then dies in a few days once it emerges as an adult. Right image courtesy Peter Chew, Brisbane Insects.

Figure 1C. A. longhorned beetle-holds the longest known longevity while in the larval stage.

Insects have a tremendous ability to withstand very low temperatures. Surprisingly, the insect most able to survive the lowest temperatures is not found in Polar Regions, but in tropical West Africa. There is a fly maggot that inhabits shallow pools which are subject to drying out repeatedly. These maggots can withstand severe desiccation or drying of their body moisture. In a laboratory, these maggots can be desiccated or dried out and then dipped in liquid helium for 5 minutes (-270 F) and when warmed up and rehydrated (body water replenished), they will recover 100 percent of the time. This is not totally surprising because the main factor leading to insect deaths after exposure to extreme cold is the formation of ice crystals in individual cells which causes the cells to rupture.

On the other hand, insects in general can not withstand extremely high temperatures. The insects recorded as most tolerant to high temperatures during every day life are several species of Sahara Desert ants that forage over sand temperatures exceeding 140 (F) degrees. However, some insects can withstand fairly high temperatures for a short duration. For example, a fly maggot from Nigeria can be dipped in boiling water (212 (F) degrees) for one minute and still survives.

Some insects are capable of flying very fast. It is believed that some of the fastest flying insects are the hawk or sphinx moths, with recorded speeds exceeding 40 miles per hour (Figure 1D). However, recent research indicates that this record is far exceeded by a male horsefly chasing a female at a speed exceeding 100 miles per hour. (She must have been really cute!)  It is not surprising that insects can fly so fast when one considers the efficiency and speed of their wing beats. The record for this is held by a species of midge that is capable of 1046 beats per second.

Figure 1D.  A tobacco hornworm or hawk moth is not only one of the fastest flying insects (40mph) but also has the longest mouthparts or proboscis (uncoiled) of any insect. Image compliments of Bob Spencer.

Actually the caption in Figure 1D is not totally correct. Darwin in an expedition into the jungles of Madagascar discovered a rare species of orchid that had it nectaries (glands that produce nectar-a favorite food of insects) located 12 inches deep in the throat of the flower. At that time he predicted that there must be an insect that has the mouthparts that could reach this source of food. He was greatly criticized for this seemingly ridiculous prediction. Approximately 180 years later a scientist found one of these rare flowers and decided to test Darwin’s suggestion. After sitting in the jungles of Madagascar for several hot, humid nights he filmed a hawk moth (very similar to the above hornworm) appear and uncoil its mouthparts. In this case the beak or proboscis measure approximately 13 inches or over 4 time the body length of the moth.

Finally the speed at which the snap jawed ant closes its mandibles (jaws) is the fastest of any anatomical structure ever recorded in the animal kingdom. The full strike, from the instant at which the fully opened mandibles start to close until they clash together takes as little as a third of a millisecond.-that is one-three-thousandths of a second. Previously the fastest recorded movement had been the jump of a springtail-it seems significant that these ants feed on springtails and would of course be faster. Other recorded (fast movements) include the escape response of a cockroach (42 milliseconds), the foreleg strike of a praying mantis (42 milliseconds) and the leap of a flea (1.2 milliseconds). This ant hunts with its disproportionately long jaws wide open with a sensory hair-like trigger projecting forward between the opposing mandibles.

These ants are also referred to as the popcorn ants due to their unusual behavior of popping up into the air when disturbed. Apparently this occurs when the tips of closing jaws hit a hard surface causing the ant is flipped backwards into the air. This behavior is thought to be advantageous in that these flying ants land on and readily attack (sting) potential predators.

A Republic of Insects and Grass. As we have discussed, insects are an amazing group of animals. Some can be dipped in boiling water for a minute or in liquid helium (-270) and still survive. Some live in the harshest environments on earth where humans can not survive, but what about their tolerance to irradiation? There have been many studies where a variety of plants and animals have been exposed to irradiation to ascertain its sterilizing and lethal dose of each. One of the reasons for these studies was to ascertain the potential effects of a nuclear war on the earth’s inhabitants. The lethal dose for most mammals and birds lies between a few hundred to a thousand rads of gamma irradiation. On the other hand the lethal dose for some insects approaches a hundred thousand rads. In essence, if a large scale nuclear attack were to occur, it appears evident the major survivors would be certain species of insects (ants and cockroaches to name a few) and plants (grass is also quite tolerant to irradiation).

CLASSIFICATION and TAXONOMY

Any study of plants and animals necessitates some scheme of classification into groups, or taxa. Organisms can be grouped by any number of common characteristics, but the one followed by scientists is based on similarities in structure—with those organisms having certain characteristics in common being placed in one group and those having others placed in another.

The animal kingdom is divided into a dozen or so major groups called phyla (phylum, singular). Each phylum is further subdivided into classes and so on. The main categories (taxa) of classification from the largest to the smallest are phylum, class, order, family, genus, and species, with frequent intermediate categories such as superfamily and subfamily. Humans, for convenience and the need to organize for study, create most taxa. Probably the only natural grouping is species. In nature a species if generally defined as a group of organisms capable of interbreeding and producing fertile offspring.

Because the determination of taxa is more or less arbitrary, different researchers often arrive at different schemes of classification and also use different names for the same group. For example, depending on the source (e.g. different textbooks), the order of scorpions has been named Scorpiones and Scorpionida. We have attempted to use the simplest names possible in this book.

The scientific name of a species is binomial; that is, it has two names, the genus and species. In a manuscript these names are either written in Italics or underlined. The genus name is always capitalized, but the species name is not (e.g. Musca domestica). If a species is referred to, but not named, it is written "sp." For example, Musca sp. refers to an unknown species of the genus Musca. Most taxa names are derived from Greek or Latin and, in many cases, translation is significant in describing characteristics of a particular group. For example, in the order of the flies (Diptera), 'di' in Greek means two, while 'ptera' means wings. One of the main characteristics of this order is having two wings.

The phylum with which we will be dealing in this course is Arthropoda (The Arthropods). Worldwide, this phylum is the largest and contains far more species than all other plants and animals combined. This phylum also possesses some characteristics of both higher and lower forms of animal life. Some of these characteristics include a chitinous or hard exoskeleton, a segmented symmetrical body, and segmented appendages. The main classes dealt with in this text are the Chilopoda (centipedes), Diplopoda (millipedes), Arachnida (arachnids) and Insecta or Hexapoda (insects). The crustaceans are briefly mentioned here, but these forms are mainly marine. Some common terrestrial crustaceans are the pillbugs and sowbugs.