Insect Physiology Courses

Appendix to Insect Physiology and Biochemistry

James L. Nation (2002) "Insect Physiology and Biochemistry," CRC Press, Boca Raton, FL.

The purpose of this short section is to provide some background information about insects and their near relatives for those who may not be trained as invertebrate biologists or entomologists. I offer an apology in advance to those who may be a specialist on one or more groups of arthropods.

[It is not my intent in this section to make insects (or arthropods) seem simple morphologically, physiologically, or from an evolutionary point of view. My intent in this book, in fact, is just the opposite, to try to demonstrate the extraordinary complexity in physiology and biochemistry of this very successful group of animals. Accounts of their structure, physiology, behavior, and evolution fill volumes.]

The phylum Arthropoda is divided by some authorities into five subphyla, while other authorities merely consider the different groups as classes of the Arthropoda. The subphyla and the classes they contain are the following:

1. Subphylum Trilobita – This subphylum (or Class) is an extinct group of marine arthropods that flourished in the Cambrian period some 500 million years ago. They are very common in the fossil record, were probably quite diverse, and if a modern systematists could have been present to study them when they lived, they would probably be divided into a number of classes.

2. Subphylum Chelicerata. The Chelicerata are commonly divided into four classes. The class Eurypterida, an extinct group called giant water scorpions, is known from fossils of the Paleozoic era. Some were as large as 2.5 meters, and were probably predators, perhaps on evolving fish. The class Pycnogonida contains the relatively rare and exotic sea spiders, marine arthropods found in the oceans and especially in shallow water near the North and South poles. The class Merostomata, nearly all of whom are extinct, includes the living group called horseshoe crabs. They are not true crabs but are the living relics of an ancient line of Chelicerata having changed little over 350 million years. Horseshoe crabs are marine bottom feeders living in shallow water along coasts of North and South America, China, Japan, and the East Indies. Limulus polyphemus, common in North American, has been important in physiological studies, particularly in studies of the compound eyes. The class Arachnida includes about 60,000 species, most of whom are carnivores. The class includes spiders, ticks, mites, scorpions, whipscorpions, daddy longlegs, and a few less common relatives.

Members of the Chelicerata typically have the body divided into two regions, a cephalothorax and an abdomen. They do not have antennae. The name for the group comes from the structure of the first pair of mouthparts, called the chelicerae, which may be pincer-like or fang-like, but not mandibulate. The second pair of mouth appendages are the pedipalps, which serve in a variety of functions in different groups of the Chelicerata, including food manipulation, locomotion, defense, and copulation.

The arachnids are the most diverse of the Chelicerata today, and members exhibit many morphological and physiological adaptations for life in varying terrestrial habitats. The body tends to be divided into a cephalothorax and an abdomen, although the latter is not evident in ticks and daddy longlegs. Arachnids do not have compound eyes nor antennae, but some have simple eyes on the cephalothorax. They typically have six pairs of jointed appendages. The first pair of appendages in spiders is the chelicerae, fang-like structures used to inject poison into the prey. The second pair, the pedipalps, is used by some spiders to manupulate and chew food. In some, the pedipalps are gustatory sensory organs, and in others they serve in courtship display and in sperm transfer. The remaining four pairs of appendages are used for walking. Some arachnids use tracheal tubes in gas exchange, others use book lungs, and some have both book lungs and tracheal tubes. Book lungs consist of a series of thin plates (like pages in a book, hence the name) paired ventrally at as many as four sites in some arachnids. The plates contain blood vessels and gas exchange occurs as blood flows through the plates. Air reaches the book lungs through slits in the outer body wall.

3. Subphylum Crustacea – Crabs, barnacles, shrimps, brine shrimps, crayfish, lobsters, fairy shrimps, water fleas, sand hoppers, and sow (pill) bugs are crustaceans. Crustaceans are a diverse group, and body morphology is highly variable, but most share the characteristics of having two pairs of antennae on the head and mandibulate mouthparts. The mandibles may be adapted for biting and chewing, or piercing and sucking. Most crustaceans also have a second pair of mouthparts called the maxillae that are used for holding and manipulating food. Additional appendages on the body are specialized for walking, swimming, sperm transfer, carrying eggs and young, or serve as sensory structures. Nearly all crustaceans live in aquatic habitats. Some live in marine environments and others in fresh water. The aquatic ones generally have gills for gas exchange and two large antennal glands that open at the base of each antenna and serve as excretory organs. Typically, adult crustaceans have compound eyes. Sow bugs are terrestrial, and have a tracheal system for respiration.

4. Subphylum Uniramia – This largest group of all living animals contains five classes, the Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (0.5-2 mm arthropods living in leaf litter and soil and resembling centipedes, although not necessarily closely related to them), Symphyla (a small group of small arthropods with mouthparts that resemble those of insects in the view of some, but not all, authorities), and the largest group of all other animals put together, the Insecta. Centipedes, millipedes, pauropodans, and symphylans share many characteristics with each other, including a long trunk with many legs, a five- or six-segmented head, and living in leaf litter, loose soil, rotting wood, and similar moist habitats. Some authorities recommend placing them together in the class Myriapoda, or even raising the group to a subphylum level.

In spite of their differences, arthropods share a number of characteristic features, including a chitinous exoskeleton that must be molted periodically as the animal grows, jointed legs, a well developed ventral nervous system, and an open circulatory system with a dorsal vessel or heart. The body of arthropods contains a hemocoel through which the blood flows freely once it leaves the dorsal vessel. The hemocoel is a cavity derived from the embryonic blastocoel and is not a true coelom, which is defined as a cavity lined by mesoderm. Various arthropods have gills (some aquatic ones), a tracheal system, book lungs, or book lungs and a tracheal system for gas exchange. Some arthropods have a blood pigment that aids in transport of oxygen to the tissues.

There are more than 750,000 described species of insects, with new ones being described on a continuing basis. Some authorities, such as E. O. Wilson of Harvard University, suggest that there are millions yet undescribed. Most authorities agree that insects are the most numerous animals, and most diverse in number of species and individuals on earth. The class Insecta is divided by authorities into a variable number of orders. Arnett (2000) lists 31 orders. The largest order, the Coleoptera (beetles and weevils), contains more than 300,000 described species. Those interested in more systematic and taxonomic details may consult one of the several general entomology textbooks listed at the end of this chapter.

Adult insects, and some immature ones, typically have six segmented legs, with one pair attached to each of the three thoracic segments. Some larval insects, such as Hymenoptera larvae and dipterous larvae, are legless. Other larval insects, such as caterpillars (Lepidoptera), have fleshy prolegs that are not jointed and that arise from various segments of the larval body. All insects molt from time to time as they grow to fill the old cuticular exoskeleton; they secrete a new exoskeleton beneath the old one before the old is shed, and then ecdysis of the old cuticle occurs.

Many orders of insects (described as Holometabola by some authorities and in this book) undergo complete metamorphosis, with egg, larva, pupa, and adult forms. Others have a gradual metamorphosis (the Hemimetabola) in which the immature, sometimes called a nymph but increasingly described as a larva by many authorities, looks much like the adult without wings (or with the beginning growth of wings in the later instars). There is no pupal stage in those with gradual metamorphosis, and the last instar molts into the adult. The Apterygota (the orders Protura, Collembola, Diplura, and Thysanura) are wingless both as larvae and as adults, and are considered to lack a metamorphosis, gradually changing into the mature adult with little or no obvious external change except size.

The term "instar" is sanctioned by the Entomological Society of America as a term to describe the immature individual (larva or nymph) between molts (1st instar, 2^nd instar, etc.), and it also is used to describe the duration of time between molts. One should not say or write "2^nd larval instar," because it is redundant.

The body of insects is divided into segments, and there typically is a clearly defined head, thorax, and abdomen. The head in arthropods has evolved from the fusion of a number of segments, from three to seven depending upon different authorities, but it superficially appears to be all of one piece in most insects. Each of the primitive segments probably bore appendages, and these have evolved into the antennae and mouthparts. A pair of antennae occurs on the head of adult insects and some immature ones. The Protura lack antennae. The antennae have evolved into a wide variety of shapes in different groups. They bear a variety of sensory structures, many of which are olfactory. Authorities generally agree that the primitive mouthparts were of the mandibulate type. Other types of mouthparts, such as piercing and sucking, are derived from mandibulate components. The mandibulate type consists of paired ventrolateral mandibles and maxillae, the (ventral) labium, the (dorsal) labrum, and the hypopharynx. The labium and labrum extend beyond the true mouth and form a preoral cavity. The hypopharynx is part of the ventral surface of the head capsule and it extends into the preoral cavity between the labium and labrum. The salivary gland duct empties into the preoral space between the hypopharynx and labium. The maxillae each bear nonsegmented lobes called the galea and lacinia, and a segmented appendage, the maxillary palpus. The labium is derived from the fusion of two primitive segments, and it also bears several nonsegmented lobes and a pair of segmented labial palps. The palps and various lobes of the mouthparts bear tactile and gustatory receptors.

Part of the success of insects has been in the plasticity of their mouthparts and their evolution to support diverse food habits. Chewing mouthparts, often modified for specific trophic functions, are present in many orders of insects. Piercing and sucking mouthparts have evolved in the Hemiptera, adult Siphonaptera, and some Diptera, and sucking mouthparts occur in some Diptera, Hymenoptera, and Lepidoptera. The mouthparts may be reduced or vestigial in nonfeeding adults, such as some Lepidoptera, and in some endoparasitic insects.

The thorax is divided longitudinally into three segments, the prothorax, the mesothorax, and the metathorax. In a schematic form, each of the thoracic segments is like a box with a slightly rounded top, the tergum, that laps over considerably on the sides, two side plates (the paired pleura, singular pleuron), and a ventral plate, the sternum. Each thoracic segment bears a pair of segmented legs attached between the sternal and pleural plates. The mesothoracic and metathoracic segments of many adult insects bear paired wings attached by small wing sclerites (small pieces of cuticle that act like hinges) at the interface between the tergal and pleural plates. Some adult insects are wingless, and this is considered to be a secondarily evolved condition from winged ancestors. Wings probably evolved only once in some early ancestor of winged insects.

Several factors (complex behavior, external and internal morphology, physiology and biochemistry, size, food habits, flight, exoskeleton, and metamorphosis) have contributed to the success of insects in becoming the most diverse and largest group of animals. In fact, just about anything one describes about insects must have contributed to their success. Since this is a book about the physiology and biochemistry of insects, those features are the ones that are stressed, but many morphological, behavioral, and genetic factors also could be cited.

They have a well-developed nervous system enabling complex individual behavior, and social behavior in ants, termites, some Hymenoptera, and to a limited extent in some other groups. Virtually all insects have an extraordinary array of sensory receptors that enable them to gather information about the internal and external environment. An exoskeleton provides protection from the external environment, controls water loss from a body that has a very high surface to volume ratio, and provides for skeletal muscle attachments.

The tracheal system, a system of air-filled tubes, arborizes like the human capillary system to virtually every cell in the body, and allows air to move through an air path to within a few micrometers of mitochondria. Consequently, insects nearly always respire aerobically, even during periods of prolonged flight. Thus, they get the maximum energy release from the breakdown of carbohydrates, and some groups can metabolize fatty acids for even greater amounts of energy during flight.

Flight has certainly been a major factor in the success of insects, allowing rapid and wide dispersal, escape from enemies, and searching for mates, food, and habitats. The use of semiochemicals in communication, and location of mates, food plants, and prey is well developed and seems to have reached an apex in the moths, which fly at night and depend upon olfaction to find food and mates. Sex pheromones play an important role in sexual isolation today, and probably has done so over millions of years.

A great radiation of insects occurred during the evolution of flowering plants about 140 million years ago, and insects feed upon nearly every type of plant. Their food habits and alimentary canal structure evolved together, so that diversity in food is reflected in great diversity in gut structure. Small size has in itself been a major factor in success. They live in many diverse microhabitats, and a small body requires relatively less food to grow and to sustain life.

Insects have a very complex endocrine system involving steroid hormones, neurosecretions, neuromodulators, biogenic amines, second messengers, and possibly a unique hormone, the juvenile hormone. Hormones and neurosecretions regulate growth, metabolism, behavior, molting, metamorphosis, excretion, circulation, reproduction, and probably many processes yet to be discovered. Genetic diversity has enabled them to adapt to changing environmental and food conditions over several hundred millions of years.

Table 1.

A listing of insect orders and type of metamorphosis according to Arnett.

Ametabola Hemimetabola Holometabola

Collembola

Protura

Entotrophi

Microcoryphia

Thysanura

Mallophaga

Anoplura

Ephemeroptera

Plecoptera

Thysanoptera

Grylloblattodea Dictyoptera

Zoraptera

Orthroptera

Homoptera

Hemiptera

Phasmitodea

Psocoptera

Embioptera

Trichoptera

Hymenoptera

Neuroptera

Coleoptera

Diptera

Lepidoptera

Mecoptera

Figure One from Appendix of Nation (2002) textbook.

More Topics on the Wing

About Us