Defensive Display

Deimatic behaviour or startle display means any pattern of bluffing behaviour in an animal that lacks strong defences, such as suddenly displaying conspicuous eyespots, to scare off or momentarily distract a predator, thus giving the prey animal an opportunity to escape. The term deimatic or dymantic originates from the Greek δειματόω (deimatóo), meaning "to frighten".

Deimatic display occurs in widely separated groups of animals, including moths, butterflies, mantises and phasmids among the insects. In the cephalopods, different species of octopuses, squids, cuttlefish and the paper nautilus are deimatic.

Displays are classified as deimatic or aposematic by the responses of the animals that see them. Where predators are initially startled but learn to eat the displaying prey, the display is classed as deimatic, and the prey is bluffing; where they continue to avoid the prey after tasting it, the display is taken as aposematic, meaning the prey is genuinely distasteful. However, these categories are not necessarily mutually exclusive. It is possible for a behaviour to be both deimatic and aposematic, if it both startles a predator and indicates the presence of anti-predator adaptations.

Vertebrates including several species of frog put on warning displays; some of these species have poison glands. Among the mammals, such displays are often found in species with strong defences, such as in foul-smelling skunks and spiny porcupines. Thus these displays in both frogs and mammals are at least in part aposematic.

In insects

Deimatic displays are made by insects including the praying mantises ( Mantodea) and stick insects (Phasmatodea). While undisturbed, these insects are usually well camouflaged. When disturbed by a potential predator, they suddenly reveal their hind wings, which are brightly coloured. In mantises, the wing display is sometimes reinforced by showing brightly coloured front legs, and accompanied by a loud hissing sound created by stridulation. For example, the grasshopper ''Phymateus'' displays red and yellow areas on its hind wings; it is also aposematic, producing a distasteful secretion from its thorax. Similarly the threat display of the walking stick phasmid (''Peruphasma schultei'') is not a bluff: the insect sprays defensive dolichodial-like monoterpene chemical compounds at attackers.

Among moths with deimatic behaviour, the eyed hawkmoth (''Smerinthus ocellatus'') displays its large eyespots, moving them slowly as if it were a vertebrate predator such as an owl. Among butterflies, the peacock butterfly ''Aglais io'' is a cryptic leaf mimic with wings closed, but displays 4 conspicuous eyespots when disturbed, in a display effective against insectivorous birds (flycatchers).

An experiment by the Australian zoologist A. D. Blest demonstrated that the more an eyespot resembled a real vertebrate eye in both colour and pattern, the more effective it was in scaring off insectivorous birds. In another experiment using peacock butterflies, Blest showed that when the conspicuous eyespots had been rubbed off, insectivorous birds (yellow buntings) were much less effectively frightened off, and therefore both the sudden appearance of colour, and the actual eyespot pattern, contribute to the effectiveness of the deimatic display.

Some noctuid moths, such as the large red underwing (''Catocala nupta''), are cryptic at rest, but display a flash of startlingly bright colours when disturbed. Others, such as many species of genus ''Speiredonia'' and ''Spirama'', look threatening while at rest. Also saturniid moths of the genera ''Attacus'' and ''Rothschildia'' display snake heads, but not from the frontal position.

Many arctiid moths make clicks when hunted by echolocating bats; they also often contain unpalatable chemicals. Some such as dogbane tiger moths (''Cycnia tenera'') have ears and conspicuous coloration, and start to make clicks when echolocating bats approach. An experiment by the Canadian zoologists John M. Ratcliffe and James H. Fullard, using dogbane tiger moths and northern long-eared bats (''Myotis septentrionalis''), suggests that the signals in fact both disrupt echolocation and warn of chemical defence. The behaviour of these insects is thus both deimatic and aposematic.

File:Flügel Peruphasma schultei.jpg, Deimatic display of the phasmid ''Peruphasma schultei''
File:Haaniella dehaanii-subadult threaten female.JPG, Threat pose of the phasmid ''Haaniella dehaanii''
File:Gottesanbeterin Abwehr.JPG, An adult female Mediterranean mantis, ''Iris oratoria'', in threat pose
File:Smerinthus ocellatus MHNT Female dos.jpg, Female eyed hawkmoth, ''Smerinthus ocellatus'', mounted to show the large eyespots
File:Inachis io bottom side.jpg, Peacock butterfly, ''Aglais io'', is a cryptic leaf mimic when its wings are closed
File:Watching you watching me - geograph.org.uk - 235513.jpg, Peacock butterfly displays startling eyespots.
File:Speiredonia spectans.jpg, ''Speiredonia spectans'' resting mimicking a brooding head
File:Pseudocreobotra wahlbergii defence.jpg, ''Pseudocreobotra wahlbergii'' flashing its wings in deimatic pose
File:Gray_plate8.jpeg, A fine large "''Phasma''" illustrated by George Robert Gray in 1833, showing cryptic resting pose and dramatic wing flash

In arachnids

Both spiders and scorpions are venomous, so their threat displays can be considered generally aposematic. However, some predators such as hedgehogs and spider-hunting wasps ( Pompilidae) actively hunt arachnids, overcoming their defences, so when a hedgehog is startled by, for instance, the sounds made by a scorpion, there is reason to describe the display as deimatic.Edwards, 1974. pp. 158–159

Spiders make use of a variety of different threat displays. Some such as '' Argiope'' and '' Pholcus'' make themselves and their webs vibrate rapidly when they are disturbed; this blurs their outline and perhaps makes them look larger, as well as more difficult to locate precisely for an attack. Mygalomorphae spiders such as tarantulas exhibit deimatic behaviour; when threatened, the spider rears back with its front legs and pedipalps spread and fangs bared. Some species, such as the dangerous Indian ornamental tree spider (''Poecilotheria regalis'') have bright colouring on the front legs and mouthparts which are shown off in its threat display when it "rears up on its hind legs, and brandishes the fore limbs and palpi in the air".

Scorpions perform non-bluffing threat displays, as they have powerful defences, but various predators still eat them. When provoked, they spread their pincers and in some cases raise their abdomens, their tails standing near-erect with the sting ready for immediate use. Some scorpions in addition produce deimatic noises by stridulating with the pedipalps and first legs.

File:Tarantula, Attacking Position, Photo by Sascha Grabow.jpg, Aposematic threat display of Brazilian tarantula
File:Female Poecilotheria regalis, ventral shot.jpg, Belly of the spider ''Poecilotheria regalis''. The bright yellow forelegs are used in deimatic displays.
File:Skorsh.jpg, Scorpion's threat display with pincers spread wide, abdomen raised to present sting.

In cephalopods

Deimatic behaviour is found in cephalopods including the common cuttlefish ''Sepia officinalis {{redire ...