Interspecific Interactions

Beetles exhibit defensive behavior that is mostly rooted in the attributes of their cuticle. Many beetles living an exposed portion of their life cycle on vegetation will use the "dropoff" reflex if disturbed (i.e., simply close the legs and tumble off the leaf or branch and fall to the ground, where their often cryptic coloration helps protect them from visually oriented predators). The drop-off reflex can be combined with thanatosis, in which the beetle lies still with legs appressed to the body. Alternatively, the legs may be held at irregular positions by muscular tetanus (catalepsy), or the individual may roll up into a ball with the antennae, mouthparts, and legs hidden from view. More brightly colored species do not use the drop-off reflex. Chrysomelid flea beetles have enlarged hind femora containing strong tibial extensor muscles; a cuticular femoral spring releases the stored energy, catapulting them into the air.

Defensive chemical secretions that protect beetle adults from predators have evolved numerous times. Toluquinone is a defensive constituent common to several major terrestrial families (Carabidae, Staphylinidae, and Tenebrionidae), suggesting that this was one of the earliest defensive secretion types to have evolved. Since quinones are used in the tanning process of new cuticle, they would have been evolutionarily available in large quantities in well-sclerotized ancestral lineages of these families. Their tanning nature is not restricted to insect cuticle, as attested by the darkly stained fingertips of anyone who picks up an oozing Eleodes tenebrionid beetle.

Perhaps the most famous defensive chemical reaction in beetles is observed in the crepitating bombardier beetles of the carabid tribe Brachinini. These beetles, like other carabids, possess pygidial defensive glands that empty from the lateral edges of the intersegmental membranes between the seventh and eighth abdominal segments, Brachinine bombardier beetles plus carabid beetles of several other tribes (Metriini and Paussini) eject a combination of hydroquinones plus hydrogen peroxide held in one chamber of the gland, and catalases plus hydrogen peroxidase held in a second chamber. These chemicals combined result in an explosive ejection of hot (100°C) secretion, with liberation of the oxygen of H2O2, thus reducing hydroquinone to quinone, with the released O2 propelling the spray (Fig. 50).

In addition to quinone compounds, beetles have evolved to use a variety of other defensive chemicals. The more recently evolved carabid beetle groups spray formic acid, a chemical also utilized as a defensive agent by their omnipresent antagonists, the ants, or Formicidae. Brightly colored or starkly patterned beetles are candidates for chemical protection via defensive gland secretions. The buprestid jewel beetles are often colored in black and yellow stripes to appear like the Hymenoptera with which they cohabit in various flowers. Jewel beetles are highly protected by bitter chemicals named buprestins. Not only are these chemicals distasteful to mammals (viz., organic chemists!), but ants reject sugar solutions laced with buprestins. Jewel beetles form mimetic complexes with lycid beetles, themselves protected by defensive secretions composed of various substituted parazines, reportedly among the most powerful odorous substances known (Fig. 51).

Various other beetle families regularly contribute members to lycid-based mimicry rings, including Cerambycidae, Meloidae, and Oedemeridae. Given that the meloids and oedemerids can synthesize cantharidin, it is likely that most beetles in such rings are distasteful, making Mullerian

Carabid Beetle Defensive Glands

FIGURE 50 Cross section of pygidial defense gland of Brachinus bombardier beetle adult (Carabidae): L, secretory lobes; B, collecting vesicle; M, sphincter muscle; E explosion chamber; G, ectodermal glands that secrete catalase; O, outlet. Vesicle B contains mixture of hydroquinone and hydrogen peroxide, exploded by catalase, when it passes into E. [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 502, Fig. 265. Academic Press, London.]

FIGURE 50 Cross section of pygidial defense gland of Brachinus bombardier beetle adult (Carabidae): L, secretory lobes; B, collecting vesicle; M, sphincter muscle; E explosion chamber; G, ectodermal glands that secrete catalase; O, outlet. Vesicle B contains mixture of hydroquinone and hydrogen peroxide, exploded by catalase, when it passes into E. [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 502, Fig. 265. Academic Press, London.]

mimicry the dominant basis for such common color patterns (Fig. 51). Other mimicry rings center on the dangerously toxic Paederus staphylinid beetles (Fig. 52), the cuticle of which exudes pederin. When such a beetle is scraped or crushed, contact with the pederin released results in human whiplash dermatitis (Fig. 53).

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