Chapter 31

1. A soft body covering maximizes flexibility and body conformation, allows movement into confined spaces, and permits direct exchange of gases and fluids with the environment. Movement of soft-bodied organisms is largely through the actions of muscles acting on a hydrostatic skeleton. A hard body covering, in contrast, provides greater protection from predation and dehydration and provides a structure to anchor muscles that attach to appendages, which can assist in movement and feeding. Respiration in a hard-bodied organism must be accomplished through external openings and movement of gases inside the body.

2. A-34

   Their jointed appendages and rigid exoskeletons provide support for walking on land. The exoskeletons also provide protection against dehydration, which is important in a terrestrial environment.

1. The dorsoventrally flattened body of flatworms ensures that each cell is near a body surface to allow gas exchange directly with the surface. In addition, the highly branched gut increases the surface area for transfer of nutrients to the nearby cells.

2. Most annelids have a thin, permeable body wall that serves as a surface for gas exchange. Annelids rapidly lose water across this body wall if they are removed from moist environments.

3. The basic body organization of a mollusk includes a muscular foot, a visceral mass, and a mantle that typically secretes a hard, calcareous shell. The primary modifications of the major groups of mollusks have involved the foot and the shell. Over the course of evolution, the foot has been modified as a crawling structure (as in gastropods), a burrowing structure (as in bivalves), a clinging structure (as in chitons), or a sensory and feeding structure (as in the arms and tentacles of cephalopods). The shell has been modified to form a series of flexible but protective plates (as in chitons), a hinged pair of valves (as in the burrowing bivalves), a greatly reduced structure for internal support (as in many cephalopods), or a mobile, spiraling chamber to protect the visceral mass (as in gastropods).

1. In many wormlike ecdysozoans, the cuticle is relatively thin and flexible. This provides only modest body protection and support but allows the exchange of water, gases, and minerals across the body surface. Species in which the cuticle is thin are typically restricted to moist environments. In contrast, most arthropods have a thicker, more rigid cuticle that protects against dehydration and predation and provides support for muscle attachment, which allows for colonization of drier environments.

2. Many nematodes act as scavengers in the soil and are important in decomposition and soil formation (which is critical to agriculture). The nematode C. elegans is an important model organism that is widely used by geneticists and developmental biologists. Other species of nematodes parasitize humans, causing diseases such as trichinosis and elephantiasis.

1. The segmented bodies of arthropods, with rigid exoskeletons and jointed appendages, provide support for walking, swimming, and flying, so arthropods are well suited to life in many different environments. Their exoskeletons also provide protection against dehydration and predation.

2. Incomplete metamorphosis involves a series of gradual changes among instars. Complete metamorphosis involves a dramatic morphological change between two developmental stages, as between caterpillars and butterflies.

3. One factor contributing to the success of insects is that flight gives insects greater access to plants. Many insect species are specialists on one or a few plant species, and plant diversity is far greater on land and in freshwater environments than in the oceans. Although some insects live in fresh water for part or all of their life cycles, these freshwater environments are closely associated with surrounding terrestrial environments. Crustaceans have been much more successful in the oceans than have insects, and crustaceans may simply outcompete insects in marine environments.

1. Estimates:

   1. Number of host-specific beetles species in the forest canopy = (Number of beetles specific to L. seemannii) ×(Number of species of canopy trees) = (163) ×(70) = 11,410.

   2. Number of non-host-specific beetles species in the forest canopy = 1,200 – 163 = 1,037.

   3. Number of beetle species on the forest floor = one-third of the number of species in the canopy (75% of beetles are in the canopy, and 25% are on the forest floor). Based on (a) and (b), there are 11,410 + 1,037 = 12,447 species in a hectare of canopy. Therefore we can estimate 12,447/3 = 4,149 species of beetles on a hectare of forest floor.

   4. Number of species of all insects other than beetles = 1.5 times the number of beetles (40% of insects are beetles, so 60% of insects are non-beetles). From (a), (b), and (c), we can estimate 11,410 + 1,037 + 4,149 = 16,596 beetles in a hectare of Panamanian forest.

   Therefore the number of insects other than beetles in a hectare of forest = (1.5) ×(16,596) = 24,894.

   This gives a total estimate for the number of insect species in an average hectare of Panamanian forest of 16,596 + 24,894 = 41,490.

2. 1. The estimate for the number of host-specific tropical canopy insects = (50,000) ×(163) = 8,150,000.

   2. Add 1 million for generalist and temperate canopy beetles: total beetles = 8,150,000 + 1,000,000 = 9,500,000.

   3. As in Question 1, beetle species on the forest floor = one-third of the number of species in the canopy. Therefore the number of species of ground beetles = 9,500,000/3 = 3,166,667 species.

      As in Question 1, species of all insects other than beetles = 1.5 times the number of beetles. Our worldwide estimate for number of beetle species is 9,500,000 + 3,166,667 = 12,666,667. Therefore we estimate non-beetle species diversity at (12,666,667) × (1.5) = 19,000,000.

   4. Summing the number of species of beetles and non-beetles, we get an estimate of worldwide insect species diversity of 12,666,667 + 19,000,000 = 31,666,667 insect species.

1. The graph shows tracheal density of the insects’ legs (red line) and total body (black line) plotted against body length. Researchers wanted to understand the upper limits of tracheal density as a possible limitation for insect size. The tracheal density of the leg is more limiting because it reaches the upper limit threshold at a smaller body size.

2. Insects take in oxygen through pores called spiracles, which open through the exoskeleton to the outside of the body. Oxygen exchange occurs in the tracheal network, an extensive system of tubes that branch into every part of the body. All living cells in the animal are within micrometers of the tracheal network. In contrast, humans and other vertebrates transport oxygen and carbon dioxide in blood and through a network of arteries, veins, and capillaries.

3. Based on the graph, one would predict that the largest living beetle could reach a body length of about 15 cm (with 95% confidence limits of about 12–22 cm). This is the approximate length at which the upper limit of tracheal leg density is reached. The largest living beetle (T. giganteus) is slightly bigger than the regression line would predict, but it is within the confidence limits of the prediction.