Insects and Wildlife

Arthropods and their Relationships with Wild Vertebrate Animals

John Capinera

• Publisher: Wiley-Blackwell
• Year: 2011
• ISBN:
  9781444357844
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Entomología

Insects and Wildlife: Arthropods and their Relationships with Wild Vertebrate Animals provides a comprehensive overview of the interrelationships of insects and wildlife. It serves as an introduction to insects and other arthropods for wildlife management and other vertebrate biology students, and emphasizes the importance of insects to wild vertebrate animals. The book emphasizes how insects exert important influences on wildlife habitat suitability and wildlife population sustainability, including their direct and indirect effects on wildlife health. Among the important topics covered are:

• the importance of insects as food items for vertebrate animals;
• the role of arthropods as determinants of ecosystem health and productivity;
• the ability of arthropods to transmit disease-causing agents;
• an overview of representative disease-causing agents transmitted by arthropods;
• arthropods as pests and parasites of vertebrates;
• the hazards to wildlife associated with using using pesticides to protect against insect damage;
• insect management using techniques other than pesticides;
• the importance of insect conservation and how insects influence wildlife conservation.

Section 1

INTRODUCTION TO THE ARTHROPODS

Chapter 1

INSECTS AND THEIR RELATIVES

Insects are invertebrate animals within the class Insecta and the phylum Arthropoda of the kingdom Animalia. Phylum Arthropoda consist of more than insects, of course, but to most people all arthropods are ‘bugs,’ those small but annoying and damaging organisms that lack fur and feathers, and therefore seem rather alien. Arthropods have jointed legs and generally are hard to the touch because they wear their skeleton externally. Think about shrimp, crabs, and lobsters if you are having trouble envisioning this, as they are arthropods, too. However, most people don’t think of shrimp, crabs, and lobsters as bugs. To most people bugs are insects, ticks, spiders, centipedes, millipedes and similar terrestrial organisms. Here we will learn about the true insects and their near relatives, and you will learn what distinguishes bugs from similar organisms.

NAMING OF TAXA

But first we need to discuss the naming of organisms. The purpose of naming always has been largely practical. We need to have a way of identifying and describing organisms accurately but concisely. Secondly, we want to describe the relationship of organisms to other organisms, usually by giving their biological position from an evolutionary perspective. Initially, organisms typically were grouped according to their appearance, but as we learned more about them and their interrelationships, their positions relative to others often changed. For example, organisms that dwell within ant nests often look quite like ants. Apparently, if you look and smell like an ant, you can gain easy access to ant nests, and the food riches contained there. However, if they are only ant mimics but actually beetles, we want to call them beetles, not ants. So in many cases physical appearance is not adequate, and we need to know about relatedness.

Do you find scientific terminology and the classification of organisms somewhat overwhelming? If you do, you may find it comforting to know that this is normal. All these hard-to-pronounce names are difficult for everyone. Nevertheless, we need to have a way to organize the diversity of life into manageable groups of related organisms or taxa (taxon is the singular form). And insects are truly diverse! Figure 1.1 shows the number of species of common groups of animals. There are far more insects species than other animals. I have used a conservative estimate for the number of species of insects, about one million. In all likelihood, the actual number is probably 3–4 million, as most tropical insect species have yet to be named. As you can see, the number of many other animal species such as reptiles, birds, fish, and mammals is almost insignificant compared to insects.

The insects and their relatives are arranged into taxa that organize similar organisms into categories. The principal taxa, in descending (most inclusive to least) order, are:

Not all of these categories are always used; the most commonly used taxa are class, order, family, genus and species.

Fig. 1.1. The number of species in some important animal taxa. Note that the size of each image is proportional to the number of species in each group.

Every organism has a scientific name, consisting of a genus and species designation. This two-part name follows a system called binomial nomenclature. The rules of binomial nomenclature are standardized to reduce potential confusion, and to confer information about the relatedness of the organism to other organisms. Additional information may be included along with the scientific name. So, you might see something like ‘Schistocerca americana (Drury) (Orthoptera: Acrididae).’ The italics signify the genus (which is capitalized) and species (not capitalized). The name following the genus and species is the person who originally described the species; this person is sometimes called the ‘author’ of the name. If the scientific name (genus and species) is unchanged since the original description, the describer’s name lacks parentheses. If the describer’s name has been modified (usually this indicates transfer to a different genus) then this change is reflected by the parentheses surrounding the author’s name. Therefore, in our example, ‘(Drury)’ signifies that the scientific name has been changed since the original description. In fact, it was originally described as Libellula americana by D. Drury in 1773. After studying the genus, however, in 1899 S.H. Scudder placed this species in the genus Schistocerca, where it now remains. The names within the next set of parentheses are the order and family designations: order Orthoptera, and family Acrididae. This latter information is not always given, but it is beneficial to do so because it tells you about the relationship of this insect relative to other insects. In this case, S. americana belongs to a family that contains many ‘short-horned grasshoppers’ in an order that contains grasshoppers, crickets and katydids. Sometimes a third italicized name is included: the subspecies.

The species is the most fundamental taxon, but for something so fundamental it is surprisingly difficult to define. Most biologists subscribe to the ‘biological species concept,’ which states that a species is an interbreeding group of organisms that is reproductively isolated from other such groups. Species most often develop through geographic isolation (allopatric speciation) from other parts of their interbreeding population. After sufficient isolation and incremental change they can no longer interbreed so they become a new species. Often accompanying this isolation are changes in appearance or behavior, which helps us to identify that they are different species. Sometimes the changes are so subtle that we have trouble identifying groups as being different. Conversely, sometimes we are fooled by the different appearance of organisms into thinking that they are different species when they are only environmentally induced differential expressions of the same species (polyphenisms, see Chapter 2). Geography is not the only means of isolation leading to formation of new species, of course. Another important means occurs when species adopt different host plants, leading to isolation based on feeding behavior, as this can result in host specific associative mating. Speciation that occurs without geographic isolation occurring is called sympatric speciation. Sympatric speciation also commonly results when insects become separated in time (developing at different times of the year) or from the use of different chemical attractants (pheromones).

Species that have populations differing in appearance are sometimes divided into subspecies. A subspecies is usually little more than a color variant, and can interbreed successfully with other subspecies in the same species. Subspecies may be indicative of speciation in progress, however. Butterflies commonly display regional color variations, so subspecies designations are especially frequent in this taxon.

Other categories also exist, and are used when it is necessary or convenient. For example, the class Insecta, along with the class Entognatha (the collembolans, proturans, diplurans) are often placed together into the superclass Hexapoda. In fact, the entognathans are sometimes considered to be insects. The subclass Pterygota is sometimes divided into two divisions, consisting of the hemimetabolous orders and the holometabolous orders. Also, related families are often grouped into superfamilies. Possibly the only level that can be assessed objectively is the species, and even that can be argued. Species are grouped into genera, genera into families, and so forth, but taxonomists differ in the importance of characters used to cluster the taxa, so different arrangements are possible. The names of most orders end in -ptera; of families in -idae; of subfamilies in -inae, and of tribes in -ini.

Remember, all this naming protocol is simply to create order and to show relatedness. Although it may seem confusing, it is really intended to inform you. Admittedly, it takes a while to catch on.

ARTHOPODA

The Arthopoda are a large (about two to three times all other animal species combined) and diverse phylum. All arthropods have some things in common. Among the characteristics that arthropods share, but which help to separate them from other invertebrates, are:

• bilateral symmetry (the left and right halves are mirror images);
• the integument (external covering) contains a great deal of chitin, a structural polysaccharide, and functions as an external skeleton;
• segmented bodies;
• jointed appendages that assist with walking and feeding; the name ‘Arthropoda’ is derived from Greek words meaning ‘jointed feet’;
• a dorsal brain but a ventral nerve cord.

They also have some differences that allow us to separate them into groups (taxa). The principal arthropod taxa of relevance to insects are:

Phylum Arthropoda

Subphylum Trilobita – trilobites (these are extinct)

Subphylum Chelicerata

Class Merostomata – horseshoe crabs

Class Arachnida – arachnids (scorpions, spiders, ticks, mites, etc.)

Class Pycnogonida – sea spiders

Subphylum Crustacea – crustaceans (amphipods, isopods, shrimp, crabs, etc.)

Subphylum Atelocerata

Class Diplopoda – millipedes

Class Chilopoda – centipedes

Class Pauropoda – pauropods

Class Symphyla – symphylans

Class Entognatha – collembolans, proturans, diplurans

Class Insecta – insects

Trilobites (subphylum Trilobita) have been extinct for 250 million years, but they once were very common and over 17,000 species are known. Probably because they had a tough integument (body covering), they preserved quite well, and are second only to the dinosaurs as well-known fossils.

The chelicerate arthropods (subphylum Chelicerata) have two principal body segments, the cephalothorax (fused head and thorax) and the abdomen. They have feeding structures called chelicerae, which are often fang-like and good for grasping and piercing, but not for chewing. They also possess pedipalps, additional segmented appendages that assist in feeding and which may possess claws. The chelicerate arthropods usually have four pairs of legs, but lack antennae (elongate sensory appendages located on the head). The pedipalps, however, often perform sensory functions.

The crustaceans also have a cephalothorax but possess feeding structures called mandibles. Mandibles are jaw-like structures that are good for grasping, holding, and masticating food. The number of legs present is variable, but they have two pairs of antennae.

The classes in the subphylum Atelocerata also have mandibles, and variable numbers of legs, but possess only one pair of antennae. The more important arthropods are briefly discussed below prior to a more detailed discussion of insects.

ARACHNIDA

This class is the most important and familiar of the chelicerate classes, containing many familiar forms (Fig. 1.2). They tend to be carnivorous, and often the digestion occurs largely outside the body (predigestion) because enzymes are secreted into the prey, with the resulting fluid taken up. Four pairs of legs are normal, although the occurrence of enlarged pedipalps in some may give the appearance of five pairs of legs. The most important arachnids are:

Order Scorpiones, the scorpions

Order Pseudoscorpiones, the false scorpions

Order Solifugae, the sun spiders or wind scorpions

Order Uropygi, the whip scorpions

Order Aranaea, the spiders

Order Opiliones, the harvestmen or daddy longlegs

Order Acarina, the mites and ticks

The scorpions (order Scorpiones) seem to be the oldest terrestrial arthropods, and may have been the first to conquer land. They are nocturnal (night-active) and secretive, hiding by day in burrows and beneath stones and logs. They are found widely in both arid and moist environment. Scorpions are well known for their long abdomen that terminates in a sting apparatus. The venom of scorpions is sufficient to kill many invertebrates, but not usually dangerous to humans, producing pain equivalent to a yellow jacket. A few species, however, are quite dangerous, and can kill a human. In the southwestern USA, species of Centruroides fall into this ‘quite dangerous’ category. The front appendages, which appear to be legs, are really enlarged mouth structures (pedipalps) bearing claws for capturing prey. They feed mostly on arthropods, but occasionally on small vertebrates. Scorpions can be up to 18 cm in length. There are about 2000 species of scorpions.

Fig. 1.2. Representative arachnids (phylum Arthropoda, class Arachnida): (a) a spider (order Aranaea); (b) a tick (order Acarina); (c) a pseudoscorpion (order Pseudoscorpiones); (d) a sun spider (order Solifugae); (e) a scorpion (order Scorpiones); (f) a mite (order Acarina).

The pseudoscorpions (order Pseudoscorpiones) are smaller than scorpions, barely exceeding 8 mm in length. About 3400 species are known. Superficially they resemble scorpions because they bear enlarged pincer-like pedipalps, but their abdomen is not elongate and they lack a sting apparatus. They are commonly found in leaf debris, but due to their small size often are overlooked. They feed on small arthropods.

The solifugids (order Solifugae) live in warm, arid environments, and shelter belowground. They are large, up to 7 cm long. They are best known for their large, conspicuous chelicerae. These are in the form of two pairs of pincers that articulate vertically. Like scorpions, they will feed on both arthropods and vertebrates. Unlike scorpions, they are principally diurnal (day-active). About 1000 species have been described.

The whip scorpions (order Uropygi) resemble scorpions except that the abdomen terminates in a long flagellum (the ‘whip’). Some are known as ‘vinegaroons’ because they spray acetic acid as a defensive measure. They vary from small to large in size. Only about 100 species are known.

The spiders (order Araneae) are among the most widely known arthopods. This order is quite species-rich, with over 40,000 named species. They range in size from 0.5 mm to perhaps 9 cm. They feed mostly on insects, but sometimes capture small vertebrates, including birds. Many, but not all, depend on production of silk to ensnare prey. Many hunting spiders such as wolf, crab and jumping spiders stalk their prey, and while they may rest within a web or even tie their captured prey with silk, they do not rely on a web for prey capture. The hunting spiders tend to be heavy bodied. In contrast, the web-producing spiders usually have long, slender legs. Spider webs vary in complexity, but most contain both dry and adhesive strands. The chelicerae of all spiders possess poison glands. The venom of most spiders is not toxic to humans, but a few are quite poisonous. The eyes of spiders are more developed than most arachnids, though they are unable to form an image due to insufficient number of receptors. They usually occur in two rows of four eyes. Courtship of spiders is often a complex process. Eggs are often deposited within an egg sac, and when the young spiderlings (young spiders) hatch they disperse. A common method of dispersal involves ballooning. When ballooning, the spiderlin...