Orb

Spirals of sticky silk are constructed across open spaces. Some webs can be strong enough to catch birds.

Trap Door

This is a silk-lined tunnel with a hinged lid\ to provide shelter and protection as the spider waits for prey.

Cast Web

Some spiders make small webs that they hold in their legs and throw over passing prey.

Cob Wkb

Tangles of silk seen in buildings may be made by daddy-long-legs. Other species make cob webs in vegetation.

Wi I at is a Crustacean?

Marink Crustaceans

M ost crustaceans are abundant in the sea and belong to the group Malacostraca. This includes the more familiar crab, shrimp, and lobster species. The front of the carapace often extends to form a projection, the eyes are stalked and compound, and the abdomen ends in a taillike telson. In crabs, the abdomen is short and tightly curled to fit under the broad carapace.

swimming legs thoracic limbs covered P with fine hair swimming legs thoracic limbs covered P with fine hair

A Common Lobstkr

In many larger species, such as the common lobster (above), the first pair of thoracic legs is en la/get/, with strong claws. These are used for defense, handling food, and courtship.

mobile, protective body platt

Divkrsk in appkaranck, crustaceans range from waterfleas, barnacles, and sand-hoppers to shrimps, crabs, and lobsters. The group ranges in size from microscopic plankton to giant lobsters that reach lengths of more than 3()in (75cm). They are primarily aquatic, and typically have a distinctive hardened carapace. They occur in freshwater and marine habitats throughout the world.

Some crustacean species have adapted to life on land; woodlice, for example, are exclusively terrestrial and arc common and widespread. Most crustaceans are scavengers, but there are predatory and herbivorous species, and some, such as barnacles, filter minute particles of food from the water using modified, strainerlike legs.

'I'll k Paris op a Cki stack an

The carapace of crustaceans is similar to the exoskeleton of other arthropods, but is often strengthened with deposits of calcium carbonate. The head and thorax are often covered by a single carapace. Crustaceans have a second pair of antennae, and their appendages are specialized for a number of functions, ranging from collecting sensory information to movement, respiration, and egg brooding. Their appendages are double-branched, a ba> portion bearing an inner part, which is used for walking, and an outer part, which is used for swimming.

head cannot be tucked in

<PlLL woodloisk

Descended from an aquatic species, pill woodlice still prefer damp places.

mobile, protective body platt cuticle protects against • dehydration

LArmamujmum a I. hum

Woodlice roll into a ball when threatened but cannot tuck their heads in, unlike pill millipedes.

What is a Myriapod?

Thk Pari s op a Myriapod

These typically elongate, terrestrial animals are distinguished from all other arthropods by their numerous pairs of legs, and by having a trunk that is not divided into a separate thorax and abdomen. They have one pair each of antennae and mandibles. The cuticle of a myriapod is not as waterproof as an insect's, and myriapods cannot close off the spiracular openings to the tracheal system; as a result, they are mostly confined to humid microhabitats, such as soil and leaf litter, and are usually nocturnal. Different gaits, determined by leg length and number, are seen in fast-running and burrowing species.

segment behind head has pair of poison daws

ACkntipkdk

Typically fast-moving and predacious, centipedes have trunk segments that carry one pair of legs each.

<Milupkdk

These are typically slow-moving, burrowing species. Most of their trunk segments (ire fused in pairs, called diplosegments, each bearing two pairs of legs.

<] sympiivi van

Closely related to the centipede, this soft-bodied creature lives in soil and leaf Utter. It has fairly long antennae and 12 pairs of legs.

>pauropod

A close relation of the millipede, this myriapod inhabits leaf litter and soil. It has a soft body, short, branched antennae, no eyes, and nine pairs of legs.

Self-defense

Centipedes use their poison claws for self-defense (the bite can cause vomiting and fever in humans). Pill millipedes can roll into a ball with their head tucked in under the last tergite (abdominal plate).

9 head tucked in

Mil in

Myriapods ark similar to insects in many ways, and the two groups are considered to be close relatives. Both have mandibles and lack the branched legs and second pair of antennae found in crustaceans. They also have some similar internal organs such as the tracheal system and Malpighian tubes.

I lowever, some evidence suggests that insects may be closer to crustaceans, and that the legs and antennae of insects have evolved differently in response to life on land. V N

20 • INTRODUCTION

Lite Cycle

All arthropods must shed their Lexoskeleton at intervals in order to grow, but the development from egg to adulthood varies between the different groups. Myriapods and arachnids molt throughout their lives, and immature stages typically look like smaller versions of the adults. Insects, however, with the exception of bristletails and silverfish

(see p.23), change their appearance from the immature to the adult stage. In more primitive insects, change is gradual and the metamorphosis is described as "incomplete" (see below); in advanced insects the change is often extremely dramatic and the metamorphosis is known as "complete" (see pp.22-23).

tip of female's abdomen joins mate's genitalia

elongate nT

1 eggs laid in small batches

gilts project from end of abdomen male holds female using clasping organs at the end of its abdomen

1. Mating

In the damselfly spedes shown here, (locnagrion puella, the male transfers sperm from its primary genital organs on the ninth abdominal segment to secondary genitalia on the third abdominal segment. The male clasps the female behind the head\ while the female bends her abdomen around to join with the secondary genitalia of the male. Sperm is transferred via the penis to the female's sperm storage organ. Eggs are laid inside aquatic plants.

gilts project from end of abdomen i emergent nymph is relatively pale

INCOMIMTIL MLTAMORPIIOSIS In insects that undergo incomplete metamorphosis, the immature stages are called nymphs. The nymphs look very similar to adults hut lack wings and reproductive structures. Wings develop gradually on the outside of the body, inside wing buds or pads. After a series of molts - the precise number varying between species - the final molt to adulthood occurs with the expansion of the wings In aquatic orders, such as dragonflies and damselflies, the nymphs are less like the adults.

2. Kmlrgknt Nymimi

After the pale nymph emerges from the egg, it develops through a series of stages called instars - the number of which varies between species, and according to temperature and food supply. 'The first few instars do not have visible wing pads. Pads become more noticeable in older nymphs. \lthough predacious, the nymphs are themselves prey to many creatures, such as water • beetles and fish.

yf elongate nT

egg tip of female's abdomen joins mate's genitalia

1 eggs laid in small batches

abdomen is final body part to emerge empty nymphal skin remains attached to stem

bright colors swollen thorax caused by pressure of hemolymph nymph •— uses dims to crawl up plant stem relatively dark coloration bright colors

5. Adult Damsklily

Once free of the nymph a I shin, the a finit can expand its abdomen and wings to full size by pumping hemolymph into them. The cuticle will harden in a few hours, but it will be several days before the bright adult coloration develops. The female is not ready to natte immediately; it feeds Jo ta week or two while its ovaries mature.

nymph •— uses dims to crawl up plant stem swollen thorax caused by pressure of hemolymph relatively dark coloration

3. final ins tar When a nymph is fully grown, it begins to emerge from the water for increasingly long periods of time and eventually leaves for good. An increase in hemolymph pressure in the thorax causes a split along the back of the nymph a I cuticle between the wing pads. The adult's head and thorax separate from the old skin first and emerge.

4. Kmkrginc; Adult

Anchored by the nymphal skin, the emerging adult pushes upward and fakes hold of a stem using its legs. Pulling itself forward, it gently eases the wings out of the wing buds and then removes its abdomen. At this stage the thorax has not acquired its f inal shape, the body is soft, and the wings are crumpled and unexpanded.

abdomen is final body part to emerge empty nymphal skin remains attached to stem

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