Category Archives: Evolution

Bank Holiday Special – Why insects are so colourful: The complex business of survival

Mastering Entomology

In the desperate struggle to evade predators, many insects have evolved toxic or bad-tasting skin, a camouflaged body (‘crypsis’), or a startle response to scare away predators. In this “evolutionary arms race”, adaptations on one side call forth counter adaptations on the other side. One such defensive adaptation is to appear toxic using brightly coloured (‘conspicuous’) body coloration- this is known as ‘aposematism’ (“Ay-PO-Sematism”). This idea that signals are sent by prey to predators to indicate toxicity was first suggested by Wallace to Darwin in 1861- they theorised that this evolved to stop predators attacking toxic prey to benefit both sides.

Aposematic warning coloration is a widely utilised form of defence used in all the animal kingdom (not just insects) and has evolved separately from many different evolutionary lines (convergent evolution). It can warn predators of defences such as a painful sting, repellent spray (such as a Bombardier beetle’s noxious…

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How do Social Insects make Decisions?


Using information passed on by others can greatly improve individual fitness, and has been the fundamental mechanism underlying the evolution of social insects such as bees, wasps, ants and termites. However in some situations it is better to ignore social information and for an individual to use its own prior knowledge and experience. So how do these colony-forming insects tailor their reliance on social information for the benefit of the ‘superorganism’? Scientists have recently reviewed the literature and made theories as to the nature of decision making in insects.

Social information is relatively ‘cheap’ to obtain for hymenopteran foragers, because they can bypass the costs associated with exploration and food sources obtained socially are likely to be better quality. In the truly eusocial western honeybee, Apis mellifera, generations overlap so information passed on by the ‘waggle dance’ (movements conveying location and quality of food sources) increases the fitness of that colony. Foraging choice are further refined by chemical cues (pheromone trails) and simply presence of other foragers.

Relying on social information may also incur costs and may not lend an evolutionary advantage. In the case of the ant forager, if she ignores social information she may find a novel food source that will benefit the colony as a whole, whilst a well-used food source is depleted (I.e. exploration produces more up-to-date information). Honeybees that rely on dance information may take time to find a dancer and may need multiple viewing and excursions to find the communicated food source.

A trade-off between these advantages and disadvantages will adjust how often (and what proportion of) social insects rely on social information. All animals tend to display the most profitable information they know, so relying on social information may be more profitable than exploration. For example honeybees only communicate their dance after finding high quality food sources. ‘Social learning strategies’ in animals are genetically determined in response to environmental and social cues. One such approach is the ‘copy if dissatisfied’ strategy, where animals will use social information if their current information is below a fitness ‘threshold’. These optimum social learning strategies can also be acquired (ironically) through social learning.

Grüter, C., & Leadbeater, E. (2014). Insights from insects about adaptive social information use. Trends in ecology & evolution, 29(3), 177-184.

Acoustic Defence in Bush Crickets: Sex Differences

bush crickets

Many insects produce sounds for mating, territory defence and to communicate with other conspecifics. However only insects in the order orthoptera stridulate their body parts (usually a leg and a wing) to produce sounds for defence. Insects usually protect themselves from predators using distastefulness, odours, colouration (cryptic and aposematic) or by startle and escape behaviour. Sound production has evolved in orthoptera in multiple species  (convergent evolution), and in many tettigoniid species the female has also evolved a ‘response song’.

In most tettigoniid only males have the sound-producing apparatus on their wings, which are rubbed together with a modified toothed vein on the left wing (the file) which is moved against the strong edge of the right wing (the plectrum). However both male and female bush crickets are capable of this defensive behaviour, although they evolved the stridulatory structures independently from one another. Scientists therefore wanted to discover whether there is a difference between the sexes, using the bush cricket (or katydid), Poecilimon ornatus, as a model.

It was found that females had a more varied syllable duration in their defence sound. The male sound last for significantly longer and contains more impulses, which is balanced by their increased tendency to regurgitate gut contents to repulse small predators such as ants and spiders.

It is thought that bush crickets rely on this method to defend themselves because their shorter wings (used for sound production) leave them unable to fly (escape) from predators. Another theory is that both male and female Poecilimon ornatus produce sounds to “evenly distribute” the increased predation risk among the sexes. The different exposure risk to the sexes may explain the differences in acoustic defence between the two sexes.

Kowalski, K. N., Lakes-Harlan, R., Lehmann, G. U., & Strauß, J. (2014). Acoustic defence in an insect: Characteristics of defensive stridulation and differences between the sexes in the tettigoniid< i> Poecilimon ornatus</i>(Schmidt1850). Zoology.