Apart from the more usual running or flying, a few insects
have evolved greatly enlarged hind-legs for jumping. They can quickly leap away
when danger threatens, leaving a predator unsure quite where the insect has
gone. Grasshoppers, locusts (order Orthoptera, family Acrididae) and bush-crickets
(order Orthoptera, family Tettigoniidae), as well as fleas (order Siphonaptera),
are perhaps the most familiar of these jumping insects, but many beetles - the
so called flea beetles (order Coleoptera, family Chrysomelidae) - also have this
ability. Likewise, many springtails (order Collembola) can jump when disturbed,
but here the leap is not powered by modified legs; instead they have a specialized,
forked springing organ (called the furcula), hinged at the tail-end of the insect
and, at rest, folded forward underneath the body. When released, the furcula
springs backwards and downwards against the ground, forcing the whole insect to
leap forward through the air.
Jumping Insects >>>
Tiger moths (order Lepidoptera, family Arctiidae), yellow
underwing moths (order Lepidoptera, family Noctuidae) and some scarab beetles
(order Coleoptera, family Scarabaeidae) can detect the ultrasonic echo-location
used by bats to find their night-flying insect prey. At low sound intensity,
these insects merely fly away from the bat, but if the bat's call increases to
a certain threshold, indicating close proximity, the insects quickly drop from
the air in an evasive, looping dive.
Bat Detecting Insects >>>
Other escape reactions may be less dramatic, but just as
effective: some cuckoo wasps (order Hymenoptera, family Chrysididae) curl up
into hard, rigid balls; tortoise beetles (order Coleoptera, family Chrysomelidae)
have strong adhesive pads on their leg tarsi and hold themselves tight and flat
against a leaf or stem. Many other insects simply "play dead" when disturbed -
behaviour called death feigning or thanatosis. Generally, they
release their grip on the substrate and fall to the ground where they can be hard
to find as long as they remain motionless. In many cases the antennae and legs are
folded tightly away under the body, sometimes into specially recessed grooves, so
that even if found there are no protruding appendages for a predator to sieze.
Other Escape Reactions >>>
Spines, bristles, hairs and scales may be effective mechanical
deterrents against predators and parasites. A mouthful of sharp spines, hairs or
scales can be an unpleasant experience for a predator. Long, dense hairs may also
prevent parasitic flies or wasps getting close enough to the body of an insect
to lay their eggs. Some moth caterpillars (order Lepidoptera) incorporate body
hairs into the silk of their cocoon as an additional defence against predation.
Many aphids and related plant-lice (order Hemiptera) secrete long strands of wax
over the body surface, which may provide a defensive mechanical barrier to
predators and parasites, in much the same way as the body hairs of other insects.
Spiny & Hairy Insects >>>
Some insects have a "fracture line" in each leg (often
between the trochanter and the femur) that allows a leg to break off easily if
it is caught in the grasp of a predator. This phenomenon, called autotomy,
is most common in crane-flies (order Diptera, family Tipulidae), stick insects
(order Phasmida), grasshoppers (order Orthoptera, family Acrididae), bush-crickets
(order Orthoptera, family Tettigoniidae) and other long-legged insects. In most
cases, sacrificing a limb in this manner creates only a minor disability. In fact,
stick insects (especially young nymphs) may regenerate all or part of a missing
appendage over the course of several molts.
Limb-Shedding Insects >>>
Most caddis fly larvae (order Trichoptera) and the
caterpillars of some moths, notably the bag-worm moths (order Lepidoptera,
family Psychidae), live inside a protective, case-like body covering which they
construct from secreted silk and strengthen with materials gathered from their
surroundings, such as sand grains, small stones, shells, twigs, fragments of
bark, leaves and similar debris. These case-bearing larvae carry their case
around with them and can make a quick retreat inside if disturbed or attacked by
a predator. Covering the case with material from the surroundings has, of course,
the additional benefit of camouflage. The caterpillars of many moths and some
butterflies (order Lepidoptera) constuct more or less fixed refuges or retreats
in which to live. For example, the caterpillars of tortrix moths (order
Lepidoptera, family Tortricidae), commonly known as leaf-rollers, spin silk to
pull and fasten leaves or other parts of their food-plant together as a protective
cover under which they can feed in relative safety, unseen by predators.
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 Müller 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 Müllerian 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 Müllerian 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.
Müllerian Mimics >>>
Protective Sounds
Many insects, such as some shield bugs (order Hemiptera,
family Pentatomidae), dung beetles (order Coleoptera, family Scarabaeidae),
longhorn beetles (order Coleoptera, family Cerambycidae), ants (order Hymenoptera,
family Formicidae) and tiger moths (order Lepidoptera, family Arctiidae),
produce rasping, buzzing or hissing sounds when disturbed or handled, in most
cases by rubbing or vibrating one part of the body against another part
(a mechanism called stridulation). Death's head hawk moths (Acherontia
sp.: order Lepidoptera, family Sphingidae) can make a high pitched squeaking noise
when disturbed, by forcing air out of the proboscis (mouthparts); the hissing
cockroach (Gromphadorhina portentosa: order Dictyoptera, family Blaberidae)
from Madagascar, as its name suggests, can make a hissing sound by expelling air
through a modified pair of its abdominal breathing pores (spiracles). These various,
apparently defensive, insect sounds may serve to startle or confuse a predatory bird or mammal.
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."
