We will keep adding to this lecture throughout the term. Here is a very abbreviated introduction to insects and other Hexapods or arthropods having six legs.

There are three small groups of Hexapods that are believed to have branched off an ancestor before the evolution of insects. in these groups, the mouthparts are located inside a pouch and are not visible externally.

All are small, with the most common encountered being springtails or Collembola. The ventral tube can secrete adhesive, but is mainly used to transfer water to various parts of the body. The furcula is used for jumping.

Insecta

The Clade (Class)Insecta is the largest group of eukaryotes and there are at least three times as many species of insects as there are all other kinds of animals combined. No one knows exactly how many kinds there are, as many previously unknown species are being discovered each year.

Body is divided into head, thorax and abdomen.

There is a pair of antennae and a pair of compound eyes.

The circulatory system is open, hearts are usually tubular shaped.

The thorax consists of three segments with three pairs of legs and usually two pairs of wings.

Respiration commonly takes place by trachea.

Malpighian tubules are the common excretory organs and their product is usually uric acid.

Many insects have sucking mouth parts. An extreme example is the butterfly which uses its long tongue to withdraw flower nectar. Chewing with mandibles is another common feeding style as illustrated by the locust. Movies below were made by Dr. Heatwole.

Modification of mouthparts to suit lifestyle is in itself a semester's worth of material. Examine the grasshopper mouth parts below and then follow the transformation of the labrum and labium in the following animation.

It is common knowledge that many insects feed on plants and can cause considerable damage to crops. The larval stage of insects can do as much damage to plants as the adults.

There are also many carnivorous insects that prey on arthropods. Some of these, like the famous Lady Beetle, aid humans by destroying harmful insects.

 

Insects are sometimes classified on their life cycle. There are two fundamental kinds of life cycles in insect.

In hemimetabolous metamorphosis (sometimes called incomplete or gradual metamorphosis), the young insect that hatches from the egg resembles the adult in general characteristics except for its smaller size and the absence of functional wings. It is called a nymph.

Holometabolous or complete metamorphosis is characterized by a drastic change in body form as growth occurs.

 Larvae of holometabolous insects are being used to help heal wounds.

The eight most common or conspicuous groups (orders), are:

 o Coleoptera: beetles. There are more beetles than any other kind of insect.

          o Lepidoptera: butterflies and moths

          o Hymenoptera: bees, wasps and ants

          o Hemiptera: the true bugs

          o Diptera: the flies

          o Isoptera: the termites

          o Orthoptera: grasshoppers, crickets, katydids

          o Odonata: dragonflies and damselflies

Courtship in insects frequently makes use of pheromones: chemicals that are released by one animal to influence the behavior of another. A female ready to mate will release a sex attractant odor into the air to announce her location to males. The use of pheromones in this way is especially widespread in moths and butterflies. Sound is used in the courtship ritual of mosquitoes, cicadas, grasshoppers and crickets.

Vision is utilized at close range by many insects for courtship displays, and fireflies use visual light signals at a distance.

Comment on fireflies

Most of the time, males will emit a single type of flash pattern that is species-specific and females emit a stereotyped response. The response latency, the delay between call and response, is a critical factor for species recognition. This has frequently been used as a handy taxonomic reference.
However, Photuris females have learned to "mimic" females of other species to attract heterospecific males and eat them. Predation in adult Photuris species is apparently unique in fireflies, other species will either feed on plant fluids or use larval fat storage. The term "aggressive mimicry" has been coined to describe this type of predatory behavior.

 Insects copulate at the end of the courting ritual, and fertilization is almost always internal. Often a complex spermathophore, sperm sac, is transferred that contains nutients for the female as well as sperm. Few insects provide care for their offspring after eggs are laid, but eggs are deposited in places where the newly hatched larvae can find food. Some insects, notably the wasps, provide paralyzed prey for their offspring or deposit eggs in the larvae of other insects. There are some insects that guard eggs, such as water bugs.

Variation in mating behavior and associated morphology is great:

One example, Dragonflies:

Dragonflies: Male dragonflies and damselflies are peculiar among insects in having a secondary copulatory organ located at the anterior end of the abdomen (second segment). Before the male can mate it must transfer sperm from his primary copulatory organ at the tip of the abdomen to the structure on the second segment. Once this is done males then chase flying females and grab them by the thorax with their legs. After a male has caught a female he then curves his abdomen towards her and latches onto her head with a set of special posterior abdominal appendages, called anal claspers. These claspers maintain a firm hold on the female and they can even at times dent her eyes. The male then releases his legs so that the pair remains attached in a head to tail position, often referred to as the tandem position. The female then bends her abdomen towards the male until her primary genitalia come into contact with the male's accessory genitalia. This position is often referred to as the wheel. Dragonflies often assume this wheel position while still in flight which is quite an acrobatic accomplishment. While in this position sperm is transferred to the females' reproductive tract via the male's penis.

In some species the males have a specialized penis that is designed to scrape out any sperm that already exist in the female's reproductive tract from previous matings before sperm from the current mating are transferred (Thompson & Dunbar 1988). This adaptation helps to ensure that the last male to mate with a female is the one who's sperm fertilizes most of her eggs. It is, therefore, important for males to make sure that no other male mates with the female before she lays her eggs. As a result many male dragonflies guard their mates until they have laid their eggs (McMillan 1991 and Thompson & Dunbar 1988).

Development of insect embryos is quite different from that of other animals and has been intensively studied in fruit flies.

Insects also vary in the complex relationships they show with other species. They are often used as the classic example of co-evolution, pollinators and flowers.

Example: Ants and Acacia

Janzen worked with ants (Pseudomyrmex) and Acacia.

http://waynesword.palomar.edu/acacia.htm

Other coevolutionary relationships

http://waynesword.palomar.edu/pljune99.htm

http://waynesword.palomar.edu/ww0902a.htm

Many unique species

The Bombardier beetle can serve as an example of a unique species; it can shoot a hot, burning liquid which is, at the very least, annoying and can cause injury to the eyes.

Bees, wasps and hornets cause stings that are painful, but not dangerous unless an unlucky human is allergic to the venom or attacked by a swarm. The so-called "Killer Bee", introduced from Africa, is unusually aggressive in defense of its hive. Their "nesting" behaviors make them less desirable to use as pollinators and the fear is that africanization of N. Amer. bee colonies will devastate this industry.

 Houseflies and cockroaches do not bite or sting, but can contaminate food, causing food poisoning and a variety of serious diseases.

  Biting flies and mosquitoes can carry serious diseases which are transmitted when the insect sucks blood. Mosquitoes are difficult to eradicate, since they can breed in any body of standing water

Human botflies employ mosquitoes to transport their eggs.

 Fleas can be a problem for both pets and humans, especially when they infest a home. The larval stage of fleas can serve as the intermediate host for dog and cat tapeworms. Fleas do not usually spread disease, but are potential carriers of bubonic plague, the Black Death of medieval history.

Several types of lice can parasitize humans. Head lice and pubic lice are irritants, but not very harmful. The body louse has been known to carry disease organisms and was responsible for spreading Trench Fever during the first World War.

Using lice to tell us things about human evolution.

Bedbugs are more prevalent overseas than in the USA, but recent outbreaks have been reported in a few states. This parasite has been with humans since they lived in caves and is undesirable, but not considered a health threat.

One of the most unique features of insect biology is their social systems. Examples: Termites, bees and ants

Many cultures have used insects as food.

http://www.hollowtop.com/finl_html/finl.html