Defensive Behavior

Most insects are capable of defending themselves against predators. Mandibulate species frequently bite, and haustelate species may stab with their stylets. Kicking, wing fanning, and buzzing also are effective against some predators (Robinson 1969,T.Wood 1976). Many species eject or inject toxic or urticating chemicals, as described in Chapter 3 (see Figs. 3.7 and 3.8). Insects armed with urticating spines or setae often increase the effectiveness of this defense by thrashing body movements that increase contact of the spines or setae with an attacker. Many caterpillars and sawfly larvae rear up and strike like a snake when attacked (Fig. 4.8).

Insects produce a variety of defensive compounds that can deter or injure predators, as described in Chapter 3. Many of these compounds are energetically expensive to produce and may be toxic to the producer as well as to predators, requiring special mechanisms for storage or delivery. Nevertheless, their production sufficiently improves the probability of survival and reproduction to represent a net benefit to the producer (Conner et al. 2000, Sillen-Tullberg 1985). Such species usually are conspicuously colored (aposematic) to facilitate avoidance learning by predators (Fig. 4.9).

Defense conferred by camouflage reduces the energy costs of active defense but may require greater efficiency in foraging or other activities that could attract attention of predators (Schultz 1983). Insects that rely on resemblance to their background (crypsis) must minimize movement to avoid detection (Fig. 4.10). For example, many Homoptera that are cryptically colored or that resemble thorns or debris are largely sedentary while siphoning plant fluids. Many aquatic insects resemble benthic debris and remain motionless as they filter suspended matter. Cryptic species usually restrict necessary movement to nighttime or acquire their food with minimal movement, especially in the presence of predators (Johansson 1993). Such insects may escape predators by waiting until a predator is very close before flushing with a startle display, giving the predator insufficient warning to react. However, some birds use tail fanning or other scare

Caterpillar Snake Startle Display

Defensive posture of black swallowtail, Papilio polyxenes, caterpillar. This snake-like posture, together with emission of noxious volatiles from the orange protuberances, deters many would-be predators.

Apposematic Coloration Behavior

| Aposematic coloration. Seed bugs (Lygaeidae) often sequester toxins from their host plants and advertise their distasteful or toxic condition (Puerto Rico).

Plants That Sequester Toxins

| Examples of cryptic coloration. Creosote bush grasshopper, Bootettix argentatus, in creosote bush, Larrea tridentata (New Mexico, United States) (top); moth with leaf-mimicking coloration and form (Taiwan) (bottom).

tactics to flush prey from a greater distance and thereby capture prey more efficiently (Galatowitsch and Mumme 2004, Jablonski 1999, Mumme 2002).

Disruptive and deceptive coloration involve color patterns that break up the body form, distract predators from vital body parts, or resemble other predators. For example, many insects have distinct bars of color or other patterns that disrupt the outline of the body and inhibit their identification as prey by passing predators. Startle displays enhance the effect of color patterns (Robinson 1969). The underwing moths (Noctuidae) are noted for their brightly colored hind wings that are hidden at rest by the cryptically colored front wings. When threatened, the moth suddenly exposes the hind wings and has an opportunity to escape its startled attacker. The giant silkworm moths (Saturniidae) and eyed elater, Alaus oculatus (Coleoptera: Elateridae), have conspicuous eyespots that make these insects look like birds (especially owls) or reptiles. The eyespots of moths usually are hidden on the hind wings during rest and can be exposed suddenly to startle would-be predators. The margin of the front wings in some sat-urniids are shaped and colored to resemble the heads of snakes (Fig. 4.11) (Grant

Defensive Behavior Lizards

Image of a snake's head on the wing margins of Attacus atlas. From Grant and Miller (1995) with permission from the Entomological Society of America.

Image of a snake's head on the wing margins of Attacus atlas. From Grant and Miller (1995) with permission from the Entomological Society of America.

and Miller 1995). Sudden wing movement during escape may enhance the appearance of a striking snake.

Mimicry is resemblance to another, usually venomous or unpalatable, species and usually involves conspicuous, or aposematic, coloration. Mimicry can take two forms, Batesian and Mullerian. Batesian mimicry is resemblance of a palatable or innocuous species to a threatening species, whereas Mullerian mimicry is resemblance among threatening species. Both are exemplified by insects. A variety of insects (representing several orders) and other arthropods (especially spiders) benefit from resemblance to stinging Hymenoptera. For example, clear-wing moths (Sessidae) and some sphingid moths, several cerambycid beetles, and many asilid and syrphid flies resemble bees or wasps (Fig. 4.12). A variety of insect and other species gain protection through adaptations that permit them to mimic ants (Blum 1980, 1981). Mullerian mimicry is exemplified by sympatric species of Hymenoptera and heliconiid butterflies that sting, or are unpalatable, and resemble each other (e.g., A. Brower 1996, Sheppard et al. 1985).

Mimicry systems can be complex, including a number of palatable and unpalatable species and variation in palatability among populations, depending on food source. For example, the resemblance of the viceroy, Limenitis archippus (Nymphalidae), butterfly to the monarch, Danaus plexippus (Daneidae), butter-

Black Stinging Insects

| Batsian mimicry by two insects. The predaceous asilid fly on the left and its prey, a cerambycid beetle, both display the black and yellow coloration typical of stinging Hymenoptera.

| Batsian mimicry by two insects. The predaceous asilid fly on the left and its prey, a cerambycid beetle, both display the black and yellow coloration typical of stinging Hymenoptera.

fly generally is considered to be an example of Batesian mimicry. However, monarch butterflies show a spectrum of palatability over their geographic range, depending on the quality of their milkweed, and other, hosts (L. Brower et al. 1968). Furthermore, populations of the viceroy and monarch in Florida are equally distasteful (Ritland and Brower 1991). Therefore, this mimicry system may be Batesian in some locations and Mullerian in others. Conspicuous color patterns and widespread movement of the co-models/mimics maximizes exposure to predators and reinforces predator avoidance, providing overall protection against predation.

Sillen-Tullberg (1985) compared predation by great tits, Parus major, between normal aposematic (red) and mutant cryptic (grey) nymphs of the seed bug, Lygaeus equestris. Both prey forms were equally distasteful. All prey were presented against a grey background. Survival of aposematic nymphs was 6.4-fold higher than for cryptic nymphs because the birds showed a greater initial reluctance to attack, learned avoidance more rapidly, and killed prey less frequently during an attack. The greater individual survival of aposematic nymphs indicated sufficient benefit to explain the evolution of aposematic coloration.

Some insects alert other members of the population to the presence of predators. Alarm pheromones are widespread among insects. These compounds usually are relatively simple hydrocarbons, but more complex terpenoids occur among ants. The venom glands of stinging Hymenoptera frequently include alarm pheromones. Alarm pheromones function either to scatter members of a group when threatened by a predator, or to concentrate attack on the predator, especially among the social insects. A diverse group of ground-dwelling arthropods produce compounds that mimic ant alarm pheromones. These function to scatter attacking ants, allowing the producer to escape (Blum 1980). Alarm pheromones released with the venom are used by stinging Hymenoptera to mark a predator. This marker serves to attract, and concentrate attack by, other members of the colony.

3. Mutualistic Behavior Insects participate in a variety of mutualistic interactions, including the well-known pollinator-plant, ant-plant, and wood borer-microorganism associations (see Chapter 8). Usually, mutualism involves diversion of resources by one partner to production of rewards or inducements that maintain mutualistic interactions. Various pollinators and predators exploit resources allocated by plants to production of nectar, domatia, root exudates, etc., and thereby contribute substantially to plant fitness. At the same time, the plant limits the nectar reward in each flower to force pollinators to transport pollen among flowers. During dispersal, bark beetles secrete lipids into mycangia to nourish mutualistic microorganisms that subsequently colonize wood and improve the nutritional suitability of woody substrates for the beetles. Obviously, the benefit gained from this association must outweigh these energetic and nutritional costs (see Chapter 8). Resources directed to support of mutualists could be allocated to growth and reproduction. These resources may be redirected if the partner is not present

(e.g., Rickson 1977), although some species maintain such allocation for long periods in the absence of partners (Janzen and Martin 1982).

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