Insect Defences

For insects, danger lurks everywhere. Insects are eaten by many animals - mammals, birds, amphibians, reptiles, spiders and other insects - yet they thrive. We can, of course, point to several reasons why insects are among nature�s most successful creatures. For one thing, many species can breed very quickly and in prolific numbers. They can afford to lose a large percentage of their offspring to predators with little or no long-term impact on population size. For another, most insects have highly sophisticated escape reactions and strategies of self-defence against the onslaught of predators. It is these escape and defensive ploys that are described here.

Escape Reactions

For many insects, a quick escape by running or flying is the primary mode of defence. A cockroach (order Dictyoptera), for example, has mechano-receptive hairs (setae) on its tail appendages (cerci) that are sensitive enough to detect the change in air pressure caused by a fast approaching object, like a predator (or your foot!). Nerve impulses from these receptors travel through giant neurons to thoracic ganglia at speeds up to 3 meters per second, triggering an evasive response by the legs which enables the insect to dart away in less than 50 milliseconds. House flies (order Diptera, family Muscidae) have a similar reaction time when you try to swat them. It takes only 30-50 milliseconds after sensing a threat for them to leap into the air and start beating their wings in flight.

Compare your response time to the speed of a fly's escape reflex. Click "start" to begin and then click "swat" as soon as a fly appears in the white window. Can you beat 50 milliseconds?

Java script courtesy of Dynamic Drive

Jumping Insects >>>

Bat Detecting Insects >>>

Other Escape Reactions >>>

Hard Bodied Insects >>>

Spiny & Hairy Insects >>>

Limb-Shedding Insects >>>

Case-Bearing Insects >>>

Protective Chemicals

Many insects are equipped to wage chemical warfare against their enemies. In some cases, they manufacture their own toxic or distasteful compounds. In other cases, the chemicals are acquired from host plants and sequestered in the haemolymph (blood) or body tissues. When threatened or disturbed, the noxious compounds may be released onto the surface of the body as a glandular ooze, into the air as a repellent vapour, or aimed as a spray directly at the offending target. Defensive chemicals typically work in one of three ways:

• Repellency
   
  A foul smell or a bad taste is often enough to discourage a potential predator. Shield bugs (often called stink bugs; order Hemiptera), for example, have specialized glands located in the thorax or abdomen that produce foul-smelling hydrocarbons. These chemicals accumulate in a small reservoir adjacent to the gland and are released onto the body surface only as needed. The larvae of certain swallowtail butterflies (order Lepidoptera, family Papilionidae) have eversible glands, called osmeteria, located just behind the head. When a caterpillar is disturbed, it rears up, everts the osmeteria to release a repellent vapour, and waves its body back and forth to ward off intruders. Similar eversible osmeteria, but located on each side of the thorax and abdomen, are found in adult beetles (Coleoptera) belonging to the family Malachidae. The larvae of some leaf beetles (order Coleoptera, family Chrysomelidae) cover themselves with their own slimy, black excreta - no doubt distasteful and certainly not an appetizing sight!

• Irritation & Pain
   
  Irritant compounds often induce cleaning behaviour by a predator, giving the prey time to escape. Some blister beetles (order Coleoptera, family Meloidae) produce cantharidin, a strong irritant and blistering agent that circulates in their haemolymph. Droplets of this blood ooze from the beetle's leg joints when it is disturbed or threatened - an adaptation known as reflex bleeding. Many ladybirds (order Coleoptera, family Coccinellidae) and leaf beetles (order Coleoptera, family Chrysomelidae) have similar reflex bleeding mechanisms.
   
  Irritant chemical sprays are produced by some termites (order Isoptera), cockroaches (order Dictyoptera), earwigs (order Dermaptera), stick insects (order Phasmida) and beetles (order Coleoptera). In many cases these sprays also contain pungent, repellent substances and their release is sometimes accompanied by a threatening posture or display. Some darkling beetles (order Coleoptera, family Tenebrionidae), for example, adopt an aggressive head-stand when disturbed and squirt a pungent, irritating mixture of quinones at their attacker from large, glandular reservoirs in the tip of their raised abdomen. Rove beetles (order Coleoptera, family Staphylinidae) curl the abdomen upwards and over their back in a scorpion-like manner whilst releasing an irritant spray or vapour from the abdomen tip. The notorious bombardier beetles (order Coleoptera, family Carabidae) store chemical precursors for an explosive reaction mixture in very specialized, reinforced abdominal glands. When the beetles are threatened, these precursors are mixed together to produce a forceful discharge of boiling hot quinone and water vapour (steam), together with an audible pop.
   
  Perhaps the most effective chemical deterrents are irritant compounds that also induce pain. Caterpillars of the io moth (Automeris io: order Lepidoptera, family Saturniidae), and various other lepidopteran larvae, have hollow body hairs that contain a painful irritant. Simply brushing against these urticating hairs will cause them to break and release their contents onto your skin. The consequence is an intense burning sensation that may last for several hours. No doubt most vertebrate predators would experience similar effects. Many female ants, bees and wasps (the aculeate Hymenoptera) deliver venom to their enemies by means of a formidable sting (the modified ovipositor or egg-laying tube). The venom is a complex mixture of proteins and amino acids that not only induces intense pain but may also trigger an allergic reaction in the victim.

• Adhesion
   
  Some insects secrete sticky compounds that harden like glue to incapacitate an attacker. Several species of cockroach (order Dictyoptera) guard their rear-end with a slimy anal secretion that quickly cripples any worker ants that launch an attack. Similarly, members of the soldier caste in nasute termites (order Isoptera, family Termitidae) have nozzle-like heads equipped with a defensive gland that can shoot a cocktail of sticky, defensive chemicals at intruders. The compounds, which are both irritating and immobilizing, have been shown to be highly effective against ants, spiders, centipedes and other predatory arthropods. Paired abdominal cornicles are a characteristic feature of aphids (order Hemiptera, family Aphididae). These tubes are the openings of specialized wax glands, from which the aphid can exude large droplets of waxy fluid when attacked. The fluid quickly dries and may deter and immobilize some of its smaller predators and parasites.

Chemically Protected Insects >>>

Protective Colouration

Biologists recognize that there is usually an underlying rationale for the great diversity of shapes and colours found in the insect world. We may not know why a particular species has parallel ridges on the pronotum or black spots on the wings, but we can be reasonably certain that this shape or colour has contributed in some way, however small, to the overall fitness of the species. It is obvious that at least some of the colours and patterns serve a defensive function by offering a degree of protection from predators and parasites. These patterns, collectively known as protective colouration, fall into four broad categories:

• Crypsis
  Insects that blend in with their surroundings often manage to escape detection by predators and parasites. This tactic, called cryptic colouration, involves not only matching the colours of the background but also disrupting the outline of the body, eliminating reflective highlights from smooth body surfaces, and avoiding sudden movements that might betray location. Obviously, this tactic loses much of its effectiveness if an insect moves from one type of habitat to another. Well camouflaged insects usually stay close to home or make only short trips and return quickly to the shelter of their protective cover. Many ground-dwelling grasshoppers and crickets (order Orthoptera), for example, have colours of mottled grey and brown that help them "disappear" against a background of dried leaves or gravel. On the other hand, closely related species that live in foliage are usually a shade of green that matches the surrounding leaves. Similar examples of cryptic colouration can be found in most insect orders, especially among the Lepidoptera (butterflies and moths), Coleoptera (beetles) and Hemiptera (true bugs). The larvae of some lacewings (order Neuroptera) improve their camouflage by attaching bits of moss or lichen from their environment onto the dorsal side of their body.

  Cryptic Insects >>>

• Mimesis
  Some insects "hide in plain sight" by resembling other objects in the environment. A thorn could really be a treehopper (order Hemiptera, family Membracidae); a small twig might be a stick insect (order Phasmida), an assassin bug (order Hemiptera, family Reduviidae) or the caterpillar of a geometer moth (order Lepidoptera, family Geometridae); and sometimes a dead leaf turns out to be a bush-cricket (order Orthoptera, family Tettigoniidae), a moth or even a butterfly (order Lepidoptera). This "mimicry" of natural objects is known as mimesis and goes far beyond imitation of plant parts. Some butterfly and moth larvae (order Lepidoptera) resemble bird droppings, others have false eyespots on the thorax that create a convincing imitation of a snake or animal head. Likewise, many adult butterflies and moths have eyespots on the wings that emulate the face of an owl or some other large animal. Some lantern flies (order Hemiptera, family Fulgoridae) have a large frontal extension of the head which resembles the head of an alligator or crocodile.

  Mimetic Insects >>>

• Warning
  Insects that have a chemical means of defence (like a sting, repellent spray or or toxic substances present in their body tissues) frequently display bright colours or contrasting patterns that tend to attract attention. These visually conspicuous insects illustrate warning or aposematic colouration, a term derived from the Greek words apo- (from a distance) and sema (a sign or signal) - meaning "a signal from afar". A predator quickly learns to associate the distinctive colouration with an "unpleasant" outcome, and one such encounter is usually enough to ensure avoidance of that prey in the future. A few individuals will die as sacrifices, but for the species as a whole, it pays to advertise!

  Aposematic Insects >>>

• Mimicry
  If a distinctive visual appearance is sufficient to protect an unpalatable insect from predation, then it stands to reason that other insects might also avoid predation by adopting a similar appearance. This ploy, essentially a form of "false advertising", was first recognized and described by Henry W. Bates (1861). Today, it is commonly known as Batesian mimicry. Many species of bee flies (order Diptera, family Bombyliidae), hover-flies (order Diptera, family Syrphidae), robber flies (order Diptera, family Asilidae), clear-winged moths (order Lepidoptera, family Sesiidae) and longhorn beetles (order Coleoptera, family Cerambycidae) are protected from predation because they mimic the appearance (and often the behaviour) of stinging bees and wasps (order Hymenoptera). Batesian mimicry is usually a successful strategy as long as the model and mimic are found in the same location, the mimic's population size is smaller than that of the model, and predators associate the model's appearance with an unpleasant effect.

  Batesian Mimics >>>

  In 1879, Fritz M�ller recognized that two or more distasteful species often share the same aposematic colour patterns. Many species of wasps (order Hymenoptera), for example, have similar alternating bands of black and yellow on the abdomen. Viceroy and monarch butterflies (order Lepidoptera, family Nymphalidae), both of which are now considered more or less unpalatable to birds, have almost identical wing patterns. This defensive tactic, commonly known as M�llerian mimicry, benefits all members of the group because it spreads the liability for "educating the predator" over more than one species. In fact, as the number of species in a M�llerian complex increases, there is a greater selective advantage for each individual species.
   
  Mimicry has been carried to extremes in some tropical Lepidoptera where both related and unrelated species resemble each other in size, shape, colour, and wing pattern. Collectively, these butterflies (and sometimes moths) form mimicry rings that may include both palatable and unpalatable species. In South America, for example, some heliconid or longwing butterflies (order Lepidoptera, family Nymphalidae) form complex mimicry rings that include at least twelve different species.

  M�llerian Mimics >>>

Sound Producing Insects >>>

An Evolutionary Arms Race

Although natural selection favours individuals in a population with the best chemical defence, camouflage or mimicry, it also favours the predator or parasite with the best prey-finding acumen. As a result of these competing interests, co-evolution between predator and prey populations inevitably leads to an ongoing escalation of offensive and defensive measures - a scenario that Leigh Van Valen of Chicago University describes as an evolutionary "arms race". In order to survive in the arms race, both predator and prey must constantly evolve in response to the other's changes. Failure to "keep up" concedes a competitive advantage to the opponent and may lead to extinction. The idea that perpetual change is necessary just to maintain the status quo has been coined the Red Queen's Hypothesis. This name refers to a scene from the stories of Alice in Wonderland by Lewis Carroll. In "Through the Looking Glass", Alice meets a chess piece, the Red Queen. After running hard to follow the Queen, Alice discovers that she has not moved from where she started. Asked about this paradox, the Red Queen replies, "Here, you see, it takes all the running you can do to keep in the same place."