BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (animals)

2.8.6. Phylum Arthropoda (arthropods)

The Classification and Characteristic Features of Arthropods are summarized in Table 2.17. In terms of species richness, the phylum Arthropoda is the largest of all animal groups. More than three-quarters of all known species are arthropods. Insects alone account for more than half of all known species. Arthropods have successfully colonized virtually every habitat on land and in Water.

Class="center">Table 2.17. Classification of the phylum Arthropoda

The basic arthropod body plan has proved to be exceptionally successful. Through a process known as adaptive radiation, a single successfully evolved ancestral form gave rise to a diverse array of species that occupy a multitude of distinct ecological niches (Chapter 26). In insects, for example, adaptive radiation has produced species adapted for flight, burrowing, aquatic life, parasitism, and many other modes of existence.

The insect body plan can be viewed as an evolutionary refinement of the segmented annelid body plan, clearly illustrating how metameric segmentation can be utilized. In primitive arthropods, simple appendages ran along the entire length of the body, likely serving multiple functions such as gas exchange, feeding, locomotion, and sensory reception. In modern arthropods, a trend toward finer specialization compared to Annelids has led to more complex and specialized appendages with a sharper division of labor. Although external segmentation remains visible, the number of segments is reduced relative to annelids.

Below we examine other KEY FEATURES OF arthropods that, combined with the aforementioned evolution of segmentation, help explain their immense evolutionary success.

EXOSKELETON, OR CUTICLE. The cuticle is secreted by the epidermal Cells. It contains Chitin, a nitrogenous polysaccharide very similar in Structure to Cellulose, which forms The plant Cell wall. Chitin has a high tensile strength (it resists being torn apart by pulling forces at both ends). The association of chitin with other chemical compounds can modify The properties of the exoskeleton. The addition of mineral salts (especially calcium carbonate) makes the exoskeleton harder, as seen in crustaceans. Proteins have a similar hardening effect. This allows for A wide variety of exoskeletons differing in hardness, elasticity, and rigidity. The flexibility of the cuticle plays a crucial role at the joints. The presence of an exoskeleton confers several major advantages:

1) it provides structural support, particularly on land;

2) it serves as an attachment site for Muscles, especially those involved in locomotion, including flight;

3) it protects against physical damage;

4) a waxy layer covering the cuticle, secreted by specialized epidermal glands, prevents desiccation in terrestrial environments;

5) the flight capabilities of insects, as well as the jumping abilities of fleas and locusts, rely on highly elastic proteins within the exoskeleton;

6) the exoskeleton has a low density, which is critical for flying animals;

7) it facilitates The formation of flexible joints between body segments;

8) it can be modified into robust mouthparts adapted for biting, crushing, sucking (Fig. 2.61, B), or grinding food;

9) PARTS OF THE exoskeleton can be transparent, allowing light to reach the eyes and enabling camouflage in aquatic environments.

However, the possession of an exoskeleton also entails two major disadvantages.

1. Ultimate body size is limited because, as discussed in the previous section, the surface area-to-volume ratio decreases as an animal grows. The dimensions of the exoskeleton depend on surface area, whereas the animal's mass depends on volume. An arthropod the size of an elephant would either be unable to support its own weight or would require an exoskeleton so massive that it could not move. (Another key constraint on insect size is related to their respiratory mechanism, which relies primarily on the diffusion of oxygen through specialized tubes called tracheae. The largest living insects are stick insects, which can reach lengths of about 30 cm, while some of the largest beetles, such as the Hercules beetle, can weigh up to 100 g — roughly the mass of a mouse.)

2. The exoskeleton restricts continuous growth. This problem is overcome by periodic shedding, or ecdysis (molting). However, until the new cuticle hardens, the animal is extremely vulnerable to predators; consequently, insects typically seek shelter until the molting process is complete.

JOINTED APPENDAGES. The term "arthropod" literally means "jointed FOOT". Jointed appendages are one of the most obvious diagnostic features of arthropods. These appendages are adapted for a wide variety of functions, such as feeding, locomotion, and sensory perception (Fig. 2.59).

Fig. 2.59. Shore crab (Carcinus maenas), dorsal view. Widely found on rocky shores and in the intertidal zone. Gonochoric (having separate sexes). The head is fused with the thorax to form a cephalothorax. The first three pairs of thoracic appendages, known as maxillipeds, are involved in food capture. They are not shown in the illustration as they are not visible from the dorsal side. Note that the legs are jointed.

HAEMOCOEL. During The Development of arthropods and Molluscs, the coelom is almost entirely replaced by another cavity known as the haemocoel (Fig. 2.60). The haemocoel develops from the spaces of the Circulatory system and is therefore filled with Blood. In arthropods, blood generally circulates through the haemocoel and a few associated vessels. Most Organs are directly bathed in blood. The coelom is still present, but it is greatly reduced and restricted to cavities housing the excretory organs and reproductive tracts. The high ratio of blood volume to body volume in arthropods allows them to maintain a High Metabolic Rate, which in turn underlies their exceptionally high activity levels. However, the risk of blood loss in the event of injury is very high.

Fig. 2.60. General body plan of animals with a haemocoel (compare with Fig. 2.55).

SPECIALIZATION OF BODY REGIONS. Division of labour, which is much more pronounced in arthropods than in annelids, has promoted the development of distinct body regions, namely the head and, in many cases, the thorax and abdomen. The head bears sensory receptors, such as eyes and antennae, as well as appendages that assist in obtaining food. In bilaterally symmetrical animals, the anterior end (the head) is the first part of the body to encounter a new environment. It is precisely for this reason that the anterior end has undergone specialization. The Brain in arthropods is considerably larger than in annelids, and cephalization is much more strongly expressed.

FLIGHT. In the course of evolution, insects acquired The ability to fly, which dramatically enhanced their capacity to find food and escape from predators (Fig. 2.61).

Fig. 2.61. A. An insect representative — the large white butterfly (Pieris brassicae), dorsal view; wings are shown on one side, legs on the other. The wings originate from the second and third thoracic segments, the legs from all three thoracic segments. Pairs of spiracles, the openings leading into the tracheae (respiratory tubes), are located on the first thoracic segment and the first eight abdominal segments. B. Details of the head STRUCTURE OF THE large white butterfly.

Insect Life Cycles

Insect life cycles are highly diverse and often remarkably complex. Many insects exhibit development involving metamorphosis (from Greek metamorphosis meaning transformation). Metamorphosis refers to profound Changes in the form or structure of an animal occurring throughout its life cycle. In the most primitive insect groups, the larval stages often resemble the adults (imagos). As a rule, each successive stage (nymph or instar) comes to look more and more like the adult insect. This type of development is termed gradual or incomplete metamorphosis. An example of an insect with this type of life cycle is the locust.

In evolutionarily more advanced groups, the larval stages are radically different from the adults. Their final moult leads to the formation of a pupa, within which metamorphosis takes place, giving rise to adult Tissues formed from the breakdown products of larval tissues. This type of transformation is called holometabolous or complete metamorphosis. An example of this type of life cycle is shown in Fig. 2.62.

Fig. 2.62. Life Cycle of the large white butterfly, a representative of the insect order Lepidoptera (butterflies and moths). A. Schematic diagram of The life cycle (an example of complete metamorphosis). B. Larvae (caterpillars) on a cabbage leaf. C. Pupa. D. Imago (adult female feeding on a butterfly bush).

Thanks to metamorphosis, juvenile and adult forms are able to inhabit different ecological niches and exploit different food resources, thereby reducing competition between them. Dragonfly nymphs, for instance, live in aquatic environments, feed on aquatic insects, and perform gas exchange via gills, whereas adult dragonflies are aerial, feed on terrestrial insects, and breathe through tracheae. Similarly, the caterpillars of Lepidoptera (butterflies and moths) feed mainly on leaves and possess chewing mouthparts, whereas adult butterflies feed on nectar and have sucking mouthparts.

Once an insect's wings are fully formed, moulting becomes impossible and it ceases to grow.