Topic: Thanatosis (feigning death) in spiders and insects
Beetles that "play possum"? A rather interesting example of convergence…
Pretending you are dead, playing possum, can often work; humans have used it on many occasions. Amongst animals, death feigning as a defensive response is very widespread, occurring in groups as diverse as frogs (e.g. Scinax hiemalis), fish (some cichlids), snakes (e.g. brown snakes Storeria dekayi) and iconically in the mammals (e.g. opossums Didelphis marsupialis). However, there are some particularly interesting examples in ants, beetles and spiders, with the latter case also involving nuptial gifts.
Thanatosis in arachnids
Nursery web spiders
In many cases, death feigning occurs in response to a predator, but there are also intraspecific instances, such as in nursery web spiders (Pisauridae).Here, thanatosis is unusual in that it occurs in a sexual context prior to mating. If there is one occasion where the capacity to drop “dead” at a moment’s notice might come in useful, it is with the ticklish business of a male spider approaching his potential mate, whose mood may swerve from amatory to predatory – sexual cannibalism is a constant risk if you are a male spider. Males of the species Pisaura mirabilis approach a female with a nuptial gift – a juicy insect that is wrapped in silk. If she plays along and starts eating it, the male will begin to transfer his sperm. But if things don’t go according to plan, he will collapse into a motionless state, looking for all the world dead. As both are holding the gift, the female may drag the corpse around, and should she resume feeding, then he revives to continue courtship. Initially, it was assumed that this behaviour reduces the risk of sexual cannibalism for the male, thus serving as a protection strategy. Recent studies, however, have provided evidence that thanatosis enhances a male’s success at obtaining copulations and also increases the length of copulation. Therefore, it seems to be an adaptive male mating strategy.
Harvestmen (Opiliones), also known as “daddy longlegs” owing to their extremely long walking legs, are arachnids as well, but do not belong to the spiders (Araneae). Some employ thanatosis as one of several defensive adaptations against potential predators. The large species Hoplobunus mexicanus, for example, appears to rely on its bright warning colours (aposematism) as the first line of defence, but might also employ stridulation or thanatosis, for example at night. The final (and most powerful) defence mechanism is the use of irritating chemicals. Thanatosis, where the legs are either partially extended or retracted over the body, can last for half an hour in Hoplobunus, and similarly long-lasting death feigning has been recorded in harvestmen of the families Gonyleptidae, Trogulidae and Escadabiidae. However, these are all smaller in size and cryptically coloured.
Thanatosis in insects
A remarkable example of intraspecific death feigning comes from the fire ants, insects that “far exceed human beings in organised nastiness” (Hölldobler and Wilson 1995, Journey to the ants: a story of scientific exploration, Harvard University Press, p. 59). In the highly territorial species Solenopsis invicta, neighbouring colonies are at war, and their aggressive workers fight each other to the death. Here, we see an interesting range of defensive behaviours that depend on the worker’s age. The relatively young flee, whilst the older workers slog it out, stinging and biting opponents with their powerful mandibles. And the youngest? They feign death, and when the danger has apparently passed, then look around before fully “reviving”. Given they are the most vulnerable and ineffective in fight due to their relatively soft cuticle, this seems a sensible strategy, and it was shown that death feigning increases their chances of survival fourfold compared to older workers. This is important, because these young workers have the longest life expectancy and are hence most valuable to the colony.
Red flour beetles (Tribolium castaneum) feign death upon encountering a predator such as a jumping spider. Artificial selection experiments have shown that the duration of death feigning is variable and heritable and that it is adaptive to remain motionless for longer. Beetles selected for long-duration thanatosis had a lower frequency of predation when exposed to the jumping spider Hasarius adansoni and, accordingly, a higher rate of survival. The intensity of death feigning can also change with external and internal conditions. In two seed beetles (Callosobruchus maculatus and C. chinensis), frequency and duration of thanatosis were found to increase at lower temperatures (when the beetles are less able to run away from a predator), whereas mating activity reduced thanatosis time in sweetpotato weevils (Cylas formicarius). Curiously, the Colorado potato beetle (Leptinotarsa decemlineata) feigns death in response to vibrations of 30 Hz, although no predator is known to generate such vibrations.
A particularly intriguing case of death feigning is found in a pselaphid beetle (Claviger testaceus), which is a guest in the nests of the ant Lasius flavus. In many situations, animals live as guests of others, very often in burrows. This is known as inquilism and usually of little consequence to the host, whilst providing a safe refuge for the guest. This beetle, however, has something rather more sinister in mind. Entering an ant nest is not an easy proposition, but by feigning death it is treated as a cadaver, and the ants attempt to dismember it as they drag this Trojan horse to the nest. Once safely ensconced, the beetle “unexpectedly” revives and not only manages to trick the ants into feeding and caring for it, but it also preys on its host’s eggs, larvae and pupae – not, perhaps, the ideal guest after all.
Death feigning has been reported from several other groups of insects and in different contexts. It serves as an anti-predator adaptation in the nymphs of the blue-tailed damselfly (Ischnura elegans) and in the pygmy grasshopper (Criotettix japonicus). Emerging queens of the stingless bee Melipona beecheii feign death to avoid being attacked by their workers and female robber flies Efferia varipes to evade harassment by males. Males of the praying mantis (Mantis religiosa) “freeze” immediately after mating, apparently so they don’t end up as a meal for the female.
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Map of Life - "Thanatosis (feigning death) in spiders and insects"
December 8, 2016