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22 • INTRODUCTION

COMIMT I K MKTAMORNIOSIS In clic insccts that develop by complete metamorphosis, the immature stage, called the larva, looks completely different from the adult. The larvae of some flies are known as maggots; the larvae of many beetles are known as grubs; and the larvae of butterflies and moths are called caterpillars. The larvae feed continuously and go through a number of molts until the final larval stage is reached.

They then stop feeding and search for a suitable place to pupate. In the pupal stage, the reorganization and transformation of the larval tissues into adult structures takes place.

The tissues of the immature insect are broken down and small groups of cells called imaginal disks, which have been present since the egg first hatched, grow and develop into adult organ systems. To protect the pupa, the final larval stage often spins a cocoon or makes a cell out of soil particles or chewed wood fibers. The pupae of some species have moveable jaws and are able to defend themselves to a certain extent. The adult frees itself from the pupal skin and/or cocoon by using its jaws, legs, or by swelling parts of the body.

mate clings to back of female

, luster s of eggs laid on foliage larva pulls itself free of egg case

1. Mating

Courtship may involve the production of sexual odors, sounds, and even light displays, hi the ladybird species shown here, ( Hoccinella septempunctata, the male clings to the bach of his mte. Sperm may be transferred in a matter of minutes, but by maintaining hold for a longer time, the male makes sure that other males do not mate with the same female.

mate clings to back of female

2. Kmkroknt Larval

The eggs are laid in relatively small batches on the leaves of plants, and after about one week the minute first in star larvae emerge, t he cuticle is soft at first but soon hardens and darkens. The larvae must find suitable soft-bodied prey, which in this species are aphids of various kinds.

, luster s of eggs laid on foliage larva pulls itself free of egg case

3. Final Larval stacjk

The dark-colored\ elongate larvae have well developed spines and projections on their bodies, and strong legs, '/'hey can be found on stems and the undersides of leaves wherever there are aphids to eat. A single ladybird larva can eat many hundreds of aphids during its larval development. Its pale spots signal that it is distasteful to potential predators.

spines and projections on body spines and projections on body

orange spots deter predators

pale coloration black thorax with pale yellow spots in this species red color ation darkens over several days

6. Adult Ladybird

The distinctive, hemispherical adult is protected from predators, such as birds, by its bright warning colors and the ability to exude distasteful liquids from its leg joints. I .ike the larvae, the adults feed on aphids and other soft-bodied insects attd are useful in biological control of pests. Many ladybird species overwinter in groups in sheltered places outside or in buildings, and emerge to lay their eggs in the springtime.

5. Nkyy Adult

A fter a week or so, pupation is complete. The pupal cuticle splits down the back, and the pale, soft adult emerges. During the hour or two after emergence, the elytra must be raised and the htndwings expanded from underneath and hardened before tiny can be folded away again ready for flight. The bright colors and contrasting spots of the adult beetle will take a couple of days to appear.

elytra are soft and vulnerable •

cuticle hardens and darkens

Ametabolous Insects

In insect species in which there is no change in shape and molting continues even after sexual maturity is reached, development is said to be ametabolous. Only two orders, bristletails and silverfish,

Silverfish

The streamlined shape gives the silverfish its name.

comprising less than 0.1 percent of all insect species, develop in this way. In addition to thoracic legs, these primitive, scavenging species have short appendages on some of their abdominal segments.

cuticle hardens and darkens

4. Pupation

After about four weeks, depending on conditions such as temperature and the supply of food, the fully grown larva pupates. It attaches itself to the underside of a leaf and sheds its last larval skin. The larva remains immobile and the pupal cuticle underneath hardens and becomes dark.

Tiik Sensory System

ALTHOUGH MOST ARTHROPODS arc .very small creatures, they possess surprisingly sophisticated sensory systems that allow them to respond appropriately to a wide range of internal and external stimuli. Arthropods are able to receive visual, chemical, and mechanical cues, many have temperature and humidity sensors, and some can detect magnetic fields and infrared radiation.

I low Insects Sit:

In most insects, some parasitic groups, and the worker castes of some ants and termites, the main visual organs are compound eyes (see below). Some cave-dwelling species are actually blind. In day-flying species, the image received is made up of numerous tiny spots of differing light intensity. Night- and dusk-flying insects have eyes that arc adapted to dim li^ht conditions, although the images they form are not as sharp as in day-flying species. Many adult insects and some immature ones have simple eyes, called ocelli, either instead of or as well as the compound eyes. Simple eyes respond to light or dark only and are important in determining certain behavioral rhythms, such as when to forage for food or hibernate. Color vision occurs in all orders of insects. Generally, insects see better at the blue end of the spectrum rather than the red. Some insects arc sensitive to ultraviolet li^lu.

dark a ira guides bee to nectar flower m visible light

A A Bee's View

I1 lowers may have very distinctive patterns, called nectar guides, which reflect ultraviolet light and are visible only to bees and some other species. This photograph was taken with I'Y-sensitive film.

dark a ira guides bee to nectar flower m visible light

A A Bee's View

I1 lowers may have very distinctive patterns, called nectar guides, which reflect ultraviolet light and are visible only to bees and some other species. This photograph was taken with I'Y-sensitive film.

I1.yes are important for finding food and mates. Aerial predators often have larger eyes and sharper vision than other insects, sometimes with the eyes covering the entire surface of the head. Insects that mate in swarms also tend to have big eyes. The eyes of the horse fly, shown here, are very large and iridescent.

In m ded pat terns caused by refraction and reflection of light at eye's surface

A Compound Kye

An insect's compound eyes consist of up to 20,000 light receptive units called ommatidia. i.ach ornmatidium comprises several clear lenses that direct the light on to a rod shaped structure called a rhabdom, which is made up of the inner portions of light-sensitive retinula cells.

retinula cell conical tens corneal lens ne ix es leading to brain rhabdom

antennae used to communicate with other ants

Toi CII, SMKLL, AND TASTK

(Chemical sense organs, or chemorcceptors, are present on the mouthparts, antennae, tarsi, and other parts of the body in insects. These enable the insect to detect food, find good egg-laying sites, or to follow marked trails on the ground. Insects pick up airborne odors by means of olfactory sensilla, which are located mainly on the antennae; if present in very large numbers, they can detect extremely low odor concentrations. Insects emit volatile chemicals called phcromones. These can be used for a variety of purposes, but are usually involved with sexual behavior. Attraction phcromones act at a distance to bring the two sexes together; often it is the female who emits the odors and waits for males to find her. Once together, other odors called courtship phcromones are produced.

Communicating Ants

Messages are passed between inserts in several ways. Some use sound or light displays, but touch and taste are more common. Here, two worker ants may be exchanging information about which colony they belong to and perhaps about new food sources.

antennae used to communicate with other ants

"ear" located -on leg

1)1/ri:c ting Sound

Many insects have hairs on their cuticle surface that are responsive to vibrations, air currents, touch, and sound waves. Special hearing structures, called tympanal organs, may be present on various parts of the body (legs, wings, abdomen, or antennae). Depending on the species, these organs are responsive to sound frequencies ranging from well under 1001 1/ (cycles per second) to more than 200k11/.. Male cicadas produce very loud sounds, which can be heard up to 0.6 miles (lkm) away; the tympanal organs in both sexes are located in the abdomen.

Insects may use sound for a number of reasons: for attracting and finding a mate, detecting prey, and avoiding enemies. Many moths, praying mantids, laccwings, and several other species have ultrasonic-sensitive hearing organs, which allow them to receive the sounds of hunting bats.

"ear" located -on leg

Posi tion ot tin-: "Kars"

Hearing organs are found on various parts of inserts' bodies. In katydids, they are located on the tibiae on the front legs. Since the body of a katydid is fairly small, this gives better directional capability. I he tympanal organ on each leg lies below two slits and is connected to special acoustic tracheae, which run back to the thorax.

Food and Feeding

Arthropods eat a variety of foods. l Sometimes the food eaten by the immature stages and the adults is the same, but often the larval stages have very different feeding habits. In some

Predacious Spkciks

Predacious species kill and cat other animals to survive. Most predators rely on more than one type of prey, although some do specialize. Predators do not have to cat as much as herbivores, sincc their food is more nutritious and provides all the protein that they need. Sometimes adult arthropods catch prey and store it for their larvae to cat. As a result of attempting to avoid predators, many arthropods have evolved defense mechanisms, including spines and hairs, cryptic coloring, and toxic secretions.

Praying Man tid >

Binocular vision allows the praying man tid to calculate the exact distance to its prey. 'The strike itself t(ikes less than 100 milliseconds. The tibiae extend then the femora, while the tibiae flex around the prey.

cases, adult insects may not feed at all and simply depend on reserves built up at the larval stage. The main feeding types, together with the symbols that appear in this book, are outlined below.

man tut estimates distance to prey body pushed < forward at tibiae fold start of strike around prey man tut estimates distance to prey

body pushed < forward at tibiae fold start of strike around prey middle and hind < legs maintain firm grip on substrate middle and hind < legs maintain firm grip on substrate ii^ Sapropiiagous Spkciks

Scavenging species that feed on J^ dead or decaying organic matter arc also called dctritivorcs. Some of these scavengers cat primarily plant debris, while others devour mainly animal remains. In practice, it is difficult to distinguish precisely who cats what, and few species rely entirely on one type of food; for this reason, all scavenging species have been classified as saprophagous in this book.

Fungivorous Spkciks

Kungivorous species are those that arc adapted to feeding on fungi (the fruiting body and the hidden hyphae). Typical examples are springtails and the larvae of many beetles and flics, which can be found inside the tissue of fungal fruiting bodies. Leaf-cutter ants and some species of termite cultivate fungal cultures for food.

_ caterpillar uses mandibles to nibble foliage

11krbivoroi s Spkciks

Plant-eating insects may feed on flowers, seeds, or leaves, or may cat inside the plants' tissues. A special case is that of gall-formers, which chemically induce an abnormal growth (gall) to form on a plant, inside which the insect is protected and feeds. Many insects have sucking mouthparts and feed only on plant sap or empty the contents of plant cells.

Caterpillar

One of the most well-known foliage-eaters, caterpillars use their thoracic legs and abdominal prolegs to grip leaves.

_ caterpillar uses mandibles to nibble foliage i<(x)i > and i i:i:dinc; • zi

L HEMATOPI lACiOl IS SPECIES AA l icks, fleas, many flies, and certain ^^^ bugs need the blood of vertebrates to survive or bring their eggs to maturity. Some insects take only mammalian blood, whereas others feed on different hosts, such as birds or reptiles. Irritation from insect bites leads to scratching and sometimes to serious infections, even death. I lowever, the main danger from bites lies in the transmission of various human and animal diseases caused by microorganisms and protozoa. Malaria, yellow fever, and river blindness affect many millions of people in tropical regions; in temperate regions, ticks are significant disease vectors.

Parasitoids and Parasites

Parasitoids are specialized predators that live in or on the body of a host animal. In its lifetime, a parasitoid colonizes only one host, which it eventually kills. Some parasitoids feed internally (endoparasitoid), while others feed externally on the host's body (ectoparasitoid). Examples of this feeding strategy are found in parasitic wasps and some flies. For the purposes of this book, the same symbol is also given to parasites, such as fleas and lice, which feed on another animal's blood, skin, or hair but do not kill their host.

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