Arthropod Defenses

1. Antipredator Defenses Arthropods also use various defenses against predators and parasites. Physical defenses include hardened exoskeleton, spines, claws, and mandibles. Chemical defenses are nearly as varied as plant defenses. Hence, predaceous species also must be capable of evaluating and exploiting defended prey resources. The compounds used by arthropods, including predaceous species, generally belong to the same categories of compounds described previously for plants.

Many insect herbivores sequester plant defenses for their own defense (Blum 1981, 1992; Boyd and Wall 2001). The relatively inert exoskeleton provides an ideal site for storage of toxic compounds. Toxins can be stored in scales on the wings of Lepidoptera (e.g., cardiac glycosides in the wings of monarch butterflies). Some insects make more than such passive use of their sequestered defenses. Sawfly (Diprionidae) larvae store the resinous defenses from host conifer foliage in diverticular pouches in the foregut and regurgitate the fluid to repel predators (Codella and Raffa 1993). Conner et al. (2000) reported that males of an arctiid moth, Cosmosoma myrodora, acquire pyrrolizidine alkaloids systematically from excrescent fluids of certain plants, such as Eupatorium capillifolium (but not from larval food plants) and discharge alkaloid-laden filaments from abdominal pouches on the female cuticle during courtship. This topical application significantly reduced predation of females by spiders, Nephila clavipes, compared to virgin females and females mated with alkaloid-free males. Additional alkaloid is transmitted to the female in seminal fluid and is partially invested in the eggs.

Accumulation of Ni from Thlaspi montanum by an adapted mirid plant bug, Melanotrichus boydi, protected it against some predators (Boyd and Wall 2001) but not against entomopathogens (Boyd 2002). L. Peterson et al. (2003) reported that grasshoppers and spiders, as well as other invertebrates, all had elevated Ni concentrations at sites where the Ni-accumulating plant, Alyssum pintodasilvae, was present but not at sites where this plant was absent, indicating spread of Ni through trophic interactions. Concentrations of Ni in invertebrate tissues approached levels that have toxic effects on birds and mammals, suggesting that using hyperaccumulating plant species for bioremediation may, instead, spread toxic metals through food chains at hazardous concentrations.

Many arthropods synthesize their own defensive compounds (Meinwald and Eisner 1995). A number of Orthoptera, Heteroptera, and Coleoptera exude noxious, irritating, or repellent fluids or froths when disturbed (Fig. 3.7). Blister beetles (Meloidae) synthesize the terpenoid, cantharidin, and ladybird beetles (Coccinellidae), synthesize the alkaloid, coccinelline (Meinwald and Eisner 1995). Both compounds are unique to insects. These compounds occur in the hemolymph and are exuded by reflex bleeding from leg joints. They deter both invertebrate and vertebrate predators. Cantharidin is used medicinally to remove warts. Whiptail scorpions spray acetic acid from their "tail," and the millipede, Harpaphe, sprays cyanide (Meinwald and Eisner 1995). The bombardier beetle, Brachynus, sprays a hot (100°C) cloud of benzoquinone produced by mixing, at the time of discharge, a phenolic substrate (hydroquinone), peroxide, and an enzyme catalase (Harborne 1994).

Several arthropod groups produce venoms, primarily peptides, including phos-pholipases, histamines, proteases, and esterases, for defense as well as predation (Habermann 1972, Meinwald and Eisner 1995, Schmidt 1982). Both neurotoxic and hemolytic venoms are represented among insects. Phospholipases are particularly well-known because of their high toxicity and their strong antigen activity capable of inducing life-threatening allergy. Larvae of several families of Lepidoptera, especially the Saturniidae and Limacodidae (Fig. 3.8), deliver

Lubber Grasshopper Defensive Glands

Defensive froth of an adult lubber grasshopper, Romalea guttata. This secretion includes repellent chemicals sequestered from host plants. From Blum (1997) with permission from the Entomological Society of America.

Defensive froth of an adult lubber grasshopper, Romalea guttata. This secretion includes repellent chemicals sequestered from host plants. From Blum (1997) with permission from the Entomological Society of America.

venoms passively through urticating spines, although defensive flailing behavior by many species increases the likelihood of striking an attacker. A number of Heteroptera, Diptera, Neuroptera, and Coleoptera produce orally derived venoms that facilitate prey capture, as well as defense (Schmidt 1982). Venoms are particularly well-known among the Hymenoptera and consist of a variety of enzymes, biogenic amines (such as histamine and dopamine), epinephrine, nor-epinephrine, and acetylcholine. Melittin, found in bee venom, disrupts erythro-cyte membranes (Habermann 1972). This combination produces severe pain and affects cardiovascular, central nervous, and endocrine systems in vertebrates (Schmidt 1982). Some venoms include nonpeptide components. For example, venom of the red imported fire ant, Solenopsis invicta, contains piperidine alkaloids, with hemolytic, insecticidal, and antibiotic effects.

Lepidoptera Larvae
| Physical and chemical defensives of a limacodid (Lepidoptera) larva, Isa textula. The urticating spines can inflict severe pain on attackers.

2. Antimicrobial Defenses Arthropods also defend themselves against internal parasites and pathogens. Major mechanisms include ingested or synthesized antibiotics (Blum 1992, Tallamy et al. 1998), gut modifications that prevent growth or penetration by pathogens, and cellular immunity against parasites and pathogens in the hemo-coel (Tanada and Kaya 1993). Behavioral mechanisms also may be used for protection against pathogens.

Insects produce a variety of antibiotic and anticancer proteins capable of targeting foreign microorganisms (Boman et al. 1991, Boman and Hultmark 1987, Dunn et al. 1994, Hultmark et al. 1982, A. Moore et al. 1996, Morishima et al. 1995). The proteins are induced within as little as 30-60 minutes of injury or infection and can persist up to several days (Brey et al. 1993, Gross et al. 1996, Jarosz 1995). These proteins generally bind to bacterial or fungal membranes, increasing their permeability, and are effective against a wide variety of infectious organisms (Gross et al. 1996, Jarosz 1995, A. Moore et al. 1996). Drosophila spp. are known to produce more than 10 antimicrobial proteins (Cociancich et al. 1994).

Cecropin, originally isolated from the cecropia moth, Hyalophora cecropia, is produced in particularly large amounts immediately before, and during, pupation. Similarly, hemolin (from several moths) is produced in peak amounts during embryonic diapause in the gypsy moth, Lymantria dispar (K.Y. Lee et al. 2002). Peak concentration during pupation may function to protect the insect from exposure of internal organs to entomopathogens in the gut during diapause or metamorphosis (Dunn et al. 1994). In mosquitoes, cecropins may protect against some bloodborne pathogenic microfiliae (Chalk et al. 1995). The ento-mopathogenic nematode, Heterorhabditis bacteriophora—produces anticecropin to permit its pathogenic bacteria to kill the host, the greater wax moth, Galleria mellonella (Jarosz 1995).

Lepidoptera susceptible to the entomopathogenic bacterium, Bacillus thuringiensis, usually have high gut pH and large quantities of reducing substances and proteolytic enzymes, conditions that limit protein chelation by phenolics but that facilitate dissolution of the bacterial crystal protein and subsequent production of the delta-endotoxin. By contrast, resistant species have a lower gut pH and lower quantities of reducing substances and proteolytic enzymes (Tanada and Kaya 1993).

Cellular immunity is based on cell recognition of "self" and "nonself" and includes endocytosis and cellular encapsulation. Endocytosis is the process of infolding of the plasma membrane and enclosure of foreign substances within a phagocyte, without penetration of the plasma membrane. This process removes viruses, bacteria, fungi, protozoans, and other foreign particles from the hemolymph, although some of these pathogens then can infect the phagocytes. Cellular encapsulation occurs when the foreign particle is too large to be engulfed by phagocytes. Aggregation and adhesion by hemocytes form a dense covering around the particle. Surface recognition may be involved because para-sitoid larvae normally protected (by viral associates) from encapsulation are encapsulated when wounded or when their surfaces are altered (Tanada and Kaya 1993). Hemocytes normally encapsulate hyphae of the fungus Entomophthora egressa but do not adhere to hyphal bodies that have surface proteins protecting them from attachment of hemocytes (Tanada and Kaya

Behavioral mechanisms include grooming and isolation of infected individuals. Grooming may remove ectoparasites or pathogens. Myles (2002) reported that eastern subterranean termites, Reticulitermes flavipes, rapidly aggregate around, immobilize, and entomb individuals infected by the pathogenic fungus Metarhizium anisopliae. Such behavior protects the colony from spread of the pathogen.

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Responses

  • micheal
    What chemicals make romalea guttata toxic?
    6 years ago

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