Justin O. Schmidt
Southwestern Biological Institute, Tucson
Defensive behaviors are the responses of organisms to perceived threats by potential predators. The responses can be active and obvious to an outside observer, including the predator; they can be subtle and difficult to observe; or they can be completely inapparent. Obvious responses might include escape flight or changing to a menacing posture, a subtle response might be the "freezing" of a slowly moving insect, and an inapparent behavior might be the warming of flight muscles by a large moth or beetle in anticipation of flight from a detected predator. The goal of this article is to describe the major types of defensive behaviors of insects and to illustrate how, when, and why these defenses are of survival benefit.
All animals must eat. Food choices for animals are limited to materials derived from other life forms, with flesh from animals being among the richest food sources in energy and nutrients. This sets the evolutionary stage for fierce competition among organisms to eat others, yet not to be eaten themselves. Good defenses and defensive behaviors tip the balance from mere survival of a population (or its extinction) to success and domination of a niche.
Survival and reproduction are the key elements of life. For both elements, defense is a paramount feature; without defense, survival, and therefore reproduction, is unlikely. Insects must defend against microorganisms, parasites, and predators and use different strategies against each. The defenses against these attackers differ. The ultimate defense against microorganisms, including bacteria, viruses, protozoans, and fungi, is the immune system. Parasites pose a different challenge. These multicellular organisms live in or on the insect body, sapping vital nutrients and reserves, sometimes damaging essential tissues or organs and causing death. Defenses against parasites are primarily behavioral and life history strategies, with backup from the immune system after parasite attack. Parasitoids are a curious group of attackers that share properties of both parasites and predators. They, like parasites, live in or on the body of the host insect and feed on its blood and tissues. Like parasites, they also do not immediately kill the insect. Parasitoids resemble predators, in fact some consider them predators, because their mode of delivery usually involves direct physical attack on the prey. Parasitoids differ from parasites because instead of directly killing the host, the parasitoid lays one or more eggs or larvae on or in the host and then the parasitoid larva(e) consumes and kills the host. Insect defenses against parasitoids are a combination of defenses used against predators and against parasites. Attacks by adult parasitoids are met with behavioral and morphological defenses similar to those used against predators. Deposited eggs and larvae are resisted by encapsulation by the immune system and other physiological defenses. Predators, in contrast to microorganisms and parasites, directly attack and kill or paralyze their insect prey. They also possess more complex nervous systems than parasites and use this added ability to combine enhanced sensory awareness, decision-making, and learning to challenge the limits of the insect prey to detect, respond to, and defend against the predator. Consequently insects have evolved a myriad of defenses of dazzling form and complexity against their predatory enemies.
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