From so simple a beginning

Evolution and Biodiversity

Category: predation (Page 2 of 2)

Second hand meal

23 November 2017 /

Sea slug consumes the food of its prey

the pilgrim hervia steals the prey of its prey

The diet of the pilgrim hervia Cratena peregrina, a sea slug, not only consists of the hydroids on which it lives; this animal also feeds on prey that was captured by the polyps, Trevor Willis and colleagues report.

The pilgrim hervia Cratena peregrina is a fairylike beautiful creature. Its white back bears tens of red protuberances with a luminescent blue tip, much like little candles. The sea slug occurs in the Mediterranean Sea and the eastern Atlantic Ocean on the branched colonies of hydroids such as Eudendrium ramosum and Eudendrium racemosum. The colonies provide shelter, the polyps are edible and possess defensive weapons that are useful. And according to Trevor Willis and colleagues, there is even more: the polyps capture food which the sea slug then may steal and consume.

Hydroids are cinidarians, just like jellyfish, and their mouth is surrounded by tentacles which grasp their prey, mainly zooplankton. They also possess stinging cells that are able to eject a harpoon (nematocyst) with a toxic content to paralyze prey or to deter predators from attacking.

Cnidosacs

But the pilgrim hervia is not deterred. It devours polyps without being bothered by stinging cells, as has already been known for a long time. In one way or another it is protected against injury by nematocysts that are fired while it is feeding, and many nematocysts that are ingested remain undischarged. Undischarged harpoons are not digested, but remain structurally intact while passing through the digestive system; some of them are discarded in the faeces, but others are sequestered and stored in cellular vesicles (cnidosacs) at the tips of the dorsal appendages.

So, the sea slug incorporates its prey’s weapons, and it can use them to frighten off hungry fish predators. Because of the bright aposematic coloration, predators quickly learn not to touch this beautiful but unpalatable morsel.

Prey of prey

Now, Willis shows that Cratena peregrina not only obtains second hand weapons from the hydroids, but also takes the prey that the polyps have captured. Laboratory experiments revealed that the sea slugs preferentially feed on polyps that are handling prey, for instance brine shrimp. By doing so, the sea slug ingests two food types in one bite: its prey and its prey’s prey. The second hand food, zooplankton, appears to be a substantial part of its diet – and the sea slug doesn’t have to capture this mobile prey by himself.

Willy van Strien

Photo: Cratena peregrina on Eudendrium ramosum. Français (Wikimedia Commons, public domain)

Watch the sea slug on YouTube

Sources:
Willis, T.J., K.T.L. Berglöf, R.A.R. McGill, L. Musco, S. Piraino, C.M. Rumsey, T.V. Fernández & F. Badalamenti, 2017. Kleptopredation: a mechanism to facilitate planktivory in a benthic mollusc. Biology Letters 13: 20170447. Doi: 10.1098/rsbl.2017.0447
Greenwood, P.G., 2009. Acquisition and use of nematocysts by cnidarian predators. Toxicon 54: 1065-1070. Doi:10.1016/j.toxicon.2009.02.029
Aguado, F. & A. Marin, 2007. Warning coloration associated with nematocyst-based defences in aeolidiodean nudibranchs. Journal of Molluscan Studies 73: 23-28. Doi:10.1093/mollus/eyl026
Martin, R., 2003. Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod Cratena peregrina. Marine Biology 143: 533-541. Doi: 10.1007/s00227-003-1078-8

Saving energy

1 November 2017 /

Venus flytrap controls the trapping process in several ways

Venus flytrap has several mechanisms to save energy

Carnivorous plants should control their energy budget, otherwise the benefits of capturing insects will not compensate for the costs. The Venus flytrap has several mechanisms to limit waste of energy, Andrej Pavlovič and colleagues discovered.

To us, it is almost impossible to catch a fly. But the Venus flytrap has no difficulty. The plant (Dionaea muscipula) occurs in North and South Carolina (United States), where it grows in sunny, wet areas on poor soil; it can grow there by ‘eating’ insects. The catch of a fly yields lots of nutrients, but the process also demands lots of energy, and the balance between yield and costs must be positive, otherwise the plant will not grow. So, it has evolved a number of control mechanisms to minimize waste of energy, as Andrej Pavlovič and colleagues point out.

The leaves of the Venus flytrap end in a two-lobed trapThe leaves are arranged in a rosette, and each leaf has a double-lobed trap at the top with a row of ten to twenty teeth at the edge of each lobe. Glands along the edges secrete a sugary substance that attracts insects. Each lobe has a few trigger hairs that respond when touched by an insect, causing the trap to snap shut rapidly. The central zone of the trap contains glands secreting enzymes that digest a trapped prey and proteins that enable the glands to absorb the nutrients that are released upon digestion.

The Venus flytrap has to invest a lot of energy to keep the traps operational and to produce lures, digestive enzymes and absorption proteins. How does the plant control these costs?

1: Two times

First of all, a trap will not snap shut until trigger hairs are touched at least twice within twenty seconds, when there is a fair chance that an insect has landed. So, a trap will not close when a for instance a wind-blown dust grain touches a trigger hair.

2: Panic

But not every animal that landed turns out to be a nice fat fly. Upon closure, small gaps between the marginal teeth allow little insects that are not worth the effort to digest them to escape. If the trap is empty, it will reopen again after a few hours. But if a large insect is encased, it will struggle in panic, and his movements induce the trap to seal hermetically. After the trigger hairs have been touched at least five times, the secretion of digestive enzymes and absorption proteins starts, and the more movements the prey makes, the more enzymes and proteins will be secreted.

3: Limited reaction

Still, a trap may snap shut, close tightly and secrete digestive enzymes and absorption proteins in vein. This happens when it is damaged. The cause of the error is to be found in the evolution of carnivorous plants, for the habit to capture insects probably evolved from defence mechanisms against herbivorous insects. In ordinary plants, herbivory generates an electrical signal, which in turn stimulates the accumulation of plant hormones, jasmonates. These will induce the plants to synthesize toxins that harm the insects, not only locally on the place of damage, but also elsewhere in the plant, as a precaution. In carnivorous plants, such as the Venus flytrap, things have a bit changed. In these plants, the presence of an insect triggers an electrical signal that induces the accumulation of jasmonates; these hormones stimulate the secretion of digestive enzymes and absorption proteins. The electrical signal also induces the trap to close.

Now, Pavlovič conducted an experiment in which he repeatedly wounded a trap of Venus flytraps by piercing it with a needle to mimic herbivory, and noticed that the trap showed the same response as when the trigger hairs wouldhave been touched by an insect: the trap closed and jasmonates accumulated; if he continued damaging every few minutes, the traps secreted digestive enzymes and absorption proteins – to no end. But the misplaced reaction was limited to the local trap that was damaged and did not occur elsewhere in the plant, in contrast to defence reactions agains herbivores.

4: Process stops

The secretion of digestive enzymes and absorption proteins does not run at full speed from the start. Only when certain substances from an enclosed prey are released, the rate of secretion increases to the highest speed. If there is no prey, the process will stop. So, the plant doesn’t waste much energy when it is misled.

After about ten days the fly is digested and the fall will reopen again.

Willy van Strien

Photos
Large: ©Andrej Pavlovič
Small: Olivier License (via Flickr, Creative Commons CC BY-NC-ND 2.0)

Watch the trapping process

Sources:
Pavlovič, A., J. Jakšová & O. Novák, 2017. Triggering a false alarm: wounding mimics prey capture in the carnivorous Venus flytrap (Dionaea muscipula). New Phytologist 216: 927-938. Doi: 10.1111/nph.14747
Böhm, J., S. Scherzer, E. Krol, I. Kreuzer, K. von Meyer, C. Lorey, T.D. Mueller, L. Shabala, I. Monte, R. Solano, K.A.S. Al-Rasheid, H. Rennenberg, S. Shabala, E. Neher & R. Hedrich, 2016. The Venus flytrap Dionaea muscipula counts prey-induced action potentials to induce sodium uptake. Current Biology 26: 286-295. Doi: 10.1016/j.cub.2015.11.057
Libiaková, M., K. Floková, O. Novák, L. Slováková & A. Pavlovič, 2014. Abundance of cysteine endopeptidase dionain in digestive fluid of Venus flytrap (Dionaea muscipula Ellis) is regulated by different stimuli from prey through jasmonates. PLoS ONE 9: e104424. Doi:10.1371/journal.pone.0104424
Pavlovič, A., V. Demko & J. Hudák, 2010. Trap closure and prey retention in Venus flytrap (Dionaea muscipula) temporarily reduces photosynthesis and stimulates respiration. Annals of Botany 105: 37-44. Doi:10.1093/aob/mcp269

Floral dress

7 September 2017 /

Pollinators are deceived by flower mimicking crab spider

Epidacus heterogaster sucessfully mimics a flower

The spider Epicadus heterogaster is coloured strikingly like a flower, and bees are lured by the colour to become prey, as Camila Vieira and colleagues show. Its masquerade is completed by a conspicuous abdomen, mimicking a flower’s shape.

The crab spider Epicadus heterogaster, which lives in Brazil, always seems to be dressed in a carnival costume that makes it look like a flower: it has a white, yellow or purple body colour and conspicuous abdominal protuberances. By mimicking a flower, it attracts insects that use to visit flowers to collect nectar, meanwhile pollinating the flowers. All it has to do next, is extend its legs and grab the victims – and the pollinators become prey.

Now, Camila Vieira and colleagues present proof that the crab spider’s colour attracts pollinators.

Sunscreen

Like many flowers, Epicadus heterogaster has an ultraviolet component in its body colour. We cannot see that colour, but insects do and some insects prefer it. The spider’s colour stands out clearly against the green leaves on which she awaits her visitors.

In order to demonstrate that the spiders’ colour indeed lures insects, the researchers treated anesthetized females with sunscreen that blocks ultraviolet light. When they applied the sunscreen on a female’s back, passing pollinators no longer saw an ultraviolet colour and didn’t approach the spider; in contrast, they avoided it. But when the sunscreen was applied not on the dorsal side, but on the ventral side of a spider, it remained attractive to pollinators. Its flowerlike appearance undoubtedly  contributes to the deceit.

Inconspicuous

Juvenile female spiders are coloured like adults, also mimicking a flower, but they exploit their disguise in another manner. They’re not sitting on a leaf to attract pollinators, as they are too small to be of any interest to them anyway, and by being conspicuous, they would attract predators. Instead, youngsters are observed mostly on flowers, where they are perfectly camouflaged.

Willy van Strien

Photo: Alex Popovkin (Wikimedia Commons, Creative Commons CC BY 2.0)

Epicadus heterogaster on YouTube

Source:
Vieira, C., E.N. Ramires, J. Vasconcellos-Neto, R.J. Poppi & G.Q. Romero, 2017. Crab spider lures prey in flowerless neighborhoods. Scientific Reports 7: 9188. Doi: 10.1038/s41598-017-09456-y

Glue-coated silk

30 June 2017 /

Ground spider immobilises prey with sticky threads

Ground spider captures prey with sticky threads

Many ground spiders can handle large or dangerous prey. They swathe their victims’ legs with threads that are coated with a tough glue, as Jonas Wolff and colleagues show.

Sticky threads

Ground spiders (Gnaphosidae) do not capture their prey by building a web, but hunt for them on the ground. For some of them, this is a risky venture because they aim for large or dangerous prey, such as ants and other spiders. Jonas Wolff and colleagues found out how they managed to capture these prey.

The researchers recorded the behaviour of a number of ground spiders during an attack with a high speed camera and analyzed the footage. When a ground spider perceives a prey, they observed, it first tries to grab it with its front legs. If it fails because the victim is large or dangerous, it quickly switches to another tactic and applies sticky silk threads to the soil and to the opponent’s legs and mouth parts to immobilise it; sometimes, an entangled prey spider still tries to defend itself by biting – hunting is never without risk.

Other spider species don’t produce such sticky silk threads. Spider silk is made in glands, and comparing the silk glands of ground spiders that swathe the legs of their prey with those of other species, the researchers found clear differences.

Spiders possess different types of silk glands, each of which makes another type of silk. The glands produce a liquid mixture of silk proteins and have a tapering duct ending in a nozzle-like opening (the spigot) on a spinneret; spiders have one to four pairs of spinnerets on their abdomen and make threads by pulling the protein mixture from the spigots.

In most spider species, the largest pair of spinnerets bears the spigot of a single large ampullate gland. This gland produces the strong silk of which draglines are made: the structural threads of webs of web-building species and the drop lines that spiders make when they drop. In addition, there are the spigots of numerous tiny piriform glands; they produce short glue-coated fibres of which spiders spin attachment disks to link the threads of their web at the intersections, or to anchor a dragline to, for instance, a tree.

Modified glands

In ground spiders that use sticky threads to capture their prey, the silk glands are strongly modified. The ampullate glands are relatively small, while the piriform glands are enlarged and have wide spigots. Ground spiders entangle their prey with the silk threads pulled from these enlarged piriform glands. The threads are coated with a layer of ductile, tough glue, perfect for swathing and immobilising struggling prey.

So, ground spiders developed a novel use of piriform silk, and the morphology of the silk glands is adapted to this novel use. The consequence is that they are not able to make functional draglines and fully functional attachment disks. Like many other spider species, they build a silk shelter in which they hide (of yet another type of silk), but they can’t anchor that shelter to the substrate as well as other spider species do. That’s the price they have to pay for their exclusive hunting method.

Willy van Strien

Photo:
Mouse spider Scotophaeus blackwalli. Richard Pigott (via Flickr; Creative Commons CC BY-SA 2.0)

Source:
Wolff, J.O., M. Řezáč, T. Krejčí & S. Gorb, 2017. Hunting with sticky tape: functional shift in silk glands of araneophagous ground spiders (Gnaphosidae). Journal of Experimental Biology 220: 2250-2259. Doi: 10.1242/jeb.154682

Fast trapping device

2 June 2017 /

Water flea has no chance against bladderwort bladders

Southern bladderwort possesses fast trapping device

There is no escape for a water flea that hits one of the submerged suction traps of bladderwort. Within a split second, the trapdoor opens and closes and the water flea has disappeared, as Simon Poppinga and colleagues show.

Some carnivorous plants, which feed on small animals, possess motile traps to capture insects or other prey. The fasted motile trapping device belongs to aquatic bladderwort species, according to Simon Poppinga and colleagues.

greater bladderwort, leaves with trapsThe stems with yellow flowers of these floating water plants are visible above water level, while the leaves, bearing bladder-like suction traps, are below surface. Using a high-speed camera, the researchers recorded the action of the suction process in Southern bladderwort bladders when trapping a water flea.

Each trap is filled with water, sometimes also with some air, and because water is continuously pumped out, there is a negative pressure within. The bladder entrance is closed with a flexible door which is fixed along the upper part, resting on a threshold and bulging outwards; it bears trigger hairs that are sensitive to touch.

Immobilized

As soon as a water flea touches a trigger hair, the door will invert its curvature, bulging inwards. In that position it can’t resist the water pressure and swings open. The water flea is sucked into the bladder with a velocity that increases to 4 meters per second. It is unable to interfere with the process in any way. As soon as it is in, the water flow decelerates. The strong acceleration and deceleration immobilize the animal and maybe even kill it. And if still alive, the water flea will die soon due to anoxia.

The door recloses and regains the convex curvature. The whole process took only 0.01 second, and within a couple of hours, the prey will be digested.

Willy van Strien

Photos:
Large: Southern bladderwort. Abalg (via Wikimedia Commons. Creative Commons CC BY 3.0)
Small: leaves with suction traps of greater bladderwort. H. Zell (via Wikimedia Commons. Creative Commons CC BY-SA 3.0

Source:
Poppinga, S., L.E. Daber, A.S. Westermeier, S. Kruppert, M. Horstmann, R. Tollrian & T. Speck, 2017. Biomechanical analysis of prey capture in the carnivorous Southern bladderwort (Utricularia australis). Scientific Reports 7: 1776. Doi:10.1038/s41598-017-01954-3

Glow in the dark

16 February 2017 /

Flashlight fish turns headlights on to catch prey

flashlight fish turns headlights on to find prey

It is difficult to find food in the dark. But the splitfin flashlight fish Anomalops katoptron has no problem: it turns its headlights on when it hunts on zooplankton, as Jens Hellinger and colleagues report.

Only in complete darkness, the splitfin flashlight fish Anomalops katoptron will leave its hiding place. During daytime the fish, which lives in shallow coral reefs in the Pacific, resides in cavities and cracks in the reef where it is invisible to its predators, thanks to its dark colour. But in dark moonless nights it ventures to the open water to forage in a school of conspecifics. The diet consists of swimming zooplankton, prey that is difficult to find in the dark.

But Anomalops katoptron has a light organ under each eye that emits blue light, Jens Hellinger and colleagues point out. The light is produced by symbiotic bacteria that live densely packed within these organs. The bacteria have got a safe place to live in, in exchange for producing light.

Blinking

The bacteria glow continuously, but the fish can turn his lights off by rotating them, exposing their dark backsides instead of the transparent sides. During the day, the lights are almost always off, otherwise the fish would be visible in spite of its dark colour. Occasionally, he blinks.

When the splitfin flashlight fish is active, at night, he blinks more often, Hellinger observed when he studied a number of fish in a tank in the laboratory, and the lights shine about half of the time. And if the fish detects prey, it has its lights on almost continuously.

Many animal species exist that emit light, particularly in the sea, and their luminescence has several functions. Most luminescent species emit light to chase off or embarrass predators. Anglerfish lure prey: their dorsal fin is modified to a ‘fishing rod’ with a luminous bulb that attracts little creatures. And still others lure or recognize partners by flashing patterns; male ostracods, for instance, perform a spectacular light show to attract females, much like fireflies do on land.

Until now, it was not clear where the splitfin flashlight fish Anomalops katoptron uses its light for. It now turns out that it is mainly to detect prey in the dark.

Photo: California Academy of Sciences (via Flickr. Creative Commons CC BY-NC-ND 2.0)

Source:
Hellinger, J., P. Jägers, M. Donner, F. Sutt, M.D. Mark, B. Senen, R. Tollrian & S. Herlitze, 2017. The flashlight fish Anomalops katoptron uses bioluminescent light to detect prey in the dark. PLoS ONE 12: e0170489. Doi:10.1371/journal.pone.0170489

The trick of a snake

8 February 2017 /

Puff adder sticks out its tongue to lure a frog

puff adder extends its tongue to lure a frog

A South African frog that perceives and approaches a tasty worm may be deceived. The worm may turn out to be the tongue of a snake, as Xavier Glaudas and Graham Alexander write, and if it is, the frog is in trouble.

The venomous puff adder (Bitis arietans), which lives in South Africa, hunts its prey by lying in ambush. Mostly nocturnal, camouflaged and hidden in the vegetation, it waits unobtrusively until a victim comes close, and then it strikes. But the striking range is only ten centimetres, so a prey often will stay out of reach.

Luring prey

But the snake uses a trick, Xavier Glaudas and Graham Alexander noticed when they reviewed  a large amount of video recordings they had made of puff adders in ambush in the field. A puff adder often lures a prey by extending and moving the black tongue, the two points spread. The tongue then looks like a squirming worm and apparently, a frog is easily deceived. It hops closer to inspect the snack, and as soon as it comes within striking range, the snake will try to seize it. The frog that thought to find a meal is eaten himself.

Is the puff adder really mimicking a worm by extending the tongue to lure prey? According to the researchers, it does. The snake only extends its tongue if there is a frog or a toad close by, they argue; it doesn’t upon perceiving the presence of other prey, such as a mouse that doesn’t eat worms. Also, snakes use their tongues to sample odours, but chemosensory tongue flicks only take half a second while ‘lingual luring’ bouts take much more time.

The puff adders also wave their tails, and according to Glauda and Alexander that behaviour is also performed to lure prey. But they don’t have any recordings to show this, because their camera had been focussed on the heads of the animals.

Willy van Strien

Photo: Joachim S. Müller (via Flickr, Creative Commons CC BY-NC-SA 2.0

Xavier Glaudas explains his research

Source:
Glaudas, X. & G. J. Alexander, 2017. A lure at both ends: aggressive visual mimicry signals and prey-specific luring behaviour in an ambush-foraging snake. Behavioral Ecology and Sociobiology 71:2. Doi: 10.1007/s00265-016-2244-6

Prepared to have a meal

20 December 2016 /

Even a sedentary antlion has a capacity for learning

Antlion larva builds a pitfall

All that an antlion larva has to do once he has made his pitfall, is sit there and wait for a prey to come. Over the weeks, he learns to anticipate the arrival of a prey, Karen Hollis discovered.

Antlion larvae need a lot of food, consisting of little critters. They don’t go after their prey, but they take what comes along. While some species wait in ambush for their food, others build traps: a larva of such a species digs a funnel-shaped pit with steep walls in loose sandy soil and buries itself at the vertex, only head and jaws remaining visible. Tiny animals that wander along the edge lose their footing and tumble into the pit, from which it is difficult to escape. And when a nearby prey fails to fall in, the antlion larva tosses sand to him, so that the victim is disoriented, stumbles and comes down in a sand avalanche.

Vibrational signal

Once an antlion has dug his pit – which is a big job -, he only needs to wait until a prey is trapped. That’s all, and yet such a buried larva is learning something, as Karen Hollis reports.

adult antlion is a winged insectWorldwide, there are a few thousand species of antlions, many of them with larvae that dig a pit on a sheltered place, for instance under overhanging branches. Ants are a common prey. Adult antlions are graceful, winged insects.

Hollis and colleagues show that larvae learn to perceive when a prey is approaching. They brought a number of larvae into the laboratory and housed each of them in its own sand-filled plastic bowl. Half of the larvae received a prey item each day at a randomly determined time, always a few seconds after the researchers had dropped some sand grains beside their pits. This was an imitation of the natural situation: an animal that approaches a pit, causes a similar vibrational signal. The other half were presented with a daily prey item at the same time as the first group, but received a vibrational cue at a different, randomly selected time. The larvae were treated as described until they pupated.

Prepared

If a victim falls into the pit, an antlion larva will pick it up, drag it under the sand, bite and deliver an immobilizing poison and digestive enzymes. He then sucks the liquefied prey contents and throws the empty exoskeleton out. If it is necessary, he will repair his trap.

Antlions that used to get their prey each day after a vibrational cue, began to prepare themselves after receiving this cue, the experiments revealed. They responded faster than the untrained larvae when a prey arrived and extracted the contents of the prey at a higher rate and more efficiently, probably because they started to produce digestive enzymes earlier. Apparently, they had learned to associate the vibrational cue with the gain of a prey, in contrast to the other group.

Other researchers, Karolina Kuszewska and colleagues, reported that antlion larvae can learn to distinguish between large and small prey, as large prey causes stronger vibrations. Antlions abandoned a small prey when they noticed that a large one was approaching.

Faster

Because antlions learn to anticipate the capture of a prey, they are able to handle it efficiently, the authors conclude. This is advantageous: in the laboratory, the larvae that learned to associate the vibrational cue with prey grew faster, were bigger and pupated sooner than the larvae that had not been given the opportunity to learn. Trained larvae thus shorten the larval stage, which is the most vulnerable stage of their life cycle because larvae are exposed to wind and rain and accessible to predators. Moreover, the more food a female larva consumes, the bigger and stronger the eggs she will produce later as adult.

So, even an animal that captures prey with a sit-and-wait strategy proves to be able to improve this strategy by learning.

Willy van Strien

Photos:
Large: an antlion larva, probably Myrmeleon formicarius. Aiwok (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: adult Myrmeleon formicarius. Gilles San Martin (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Watch a video of an antlion and its pitfal

Sources:
Hollis, K.L., 2016. Ants and antlions: The impact of ecology, coevolution and learning on an insect predator-prey relationship. Behavioural processes, online December 6. Doi: 10.1016/j.beproc.2016.12.002
Kuszewska, K., K. Miler, M. Filipiak & M. Woyciechowski, 2016. Sedentary antlion larvae (Neuroptera: Myrmeleontidae) use vibrational cues to modify their foraging strategies. Animal Cogntion 19: 1037-1041. Doi: 10.1007/s10071-016-1000-7
Hollis, K.L., F.A. Harrsch & E. Nowbahari, 2015. Ants vs. antlions: An insect model for studying the role of learned and hard-wired behavior in coevolution. Learning and Motivation .50: 68-82. Doi: 10.1016/j.lmot.2014.11.003

Who is the prey?

1 November 2016 /

Assassin bug sneaks in the web to eat the deadly spider

giraffe-assassin-bug-f-soley

Thanks to a stealthy hunting tactic, the giraffe assassin bug is able to prey on web building spiders. Fernando Soley unravelled how the bug manages to approach a spider unnoticed.

With their sticky webs and their venom, web building spiders are formidable predators of insects and other small animals. Webs and venom also make a powerful defence and it is nearly impossible for small animals to prey on spiders. Still, some critters feed on these predators, risking their lives.

One of the most remarkable spider eating species is the giraffe assassin bug, Stenolemus giraffa, an inhabitant of the northern, tropical part of Australia. It has a weird appearance, as a long ‘giraffe’s neck’ (pronotum) extends between the head with part of the thorax and the rest of the thorax with the thin abdomen. Antennae and front legs are placed at one end of the neck, middle legs and hind legs at the other end. The bug lives on rock escarpments and is often found associated with spider webs because it is araneophagic: it preys on web building spiders. After grasping one, it inserts the tip of its rostrum (piercing mouthparts) into the spider’s abdomen to feed on the contents. Feeding takes an hour or longer.

Cautious

How is this delicate looking bug able to approach and attack a dangerous spider in her web? To find out, Fernando Soley observed the bugs under natural conditions and conducted experiments in the lab.

The giraffe bug spends nearly half his time on stalking resting spiders, as Soley noticed. If he perceives one, he first tries to access it without touching the web, for the spider will perceive any vibration of the threads. Hanging from the rock on middle and hind legs the enemy cautiously progresses. If he is close enough, he bends to his prey and stretches his forelegs to grab it. The long legs and giraffe neck facilitate this way of hunting.

But often, he cannot reach the spider without invading the web. Soley made artificial webs of spider’s silk and placed them in front of a laser vibrometer in order to measure the vibrations produced when a bug steps on a thread. It turned out that those vibrations are very soft and hardly detectable. Because of the long body and the long legs, the bug’s weight is distributed over a large surface area. Besides, it moves slowly, pausing after each step.

Wind

Great difficulties arise when the spider rests at the opposite side of the web. Then the assassin bug has to break some threads next to the spider with its forelegs to be able to capture it. He does so very carefully. To prevent the ends from snapping back after the break and alerting the spider, the bug holds on to the loose ends for a while, causes them to sag and then releases them carefully. After releasing the first loose end, he waits several seconds or even minutes before he releases the second one to space out the vibrations in time. In this way, Soley reports, the bug strongly attenuates the vibrations produced, making them often indistinguishable from background noise.

If a wind blows, things are easier, as vibrations are less conspicuous and the bug can move faster. So, it prefers to breaks threads in the presence of wind.

Long rest

In spite of this stealthy behaviour, there is a risk that the spider notices the steps of the giraffe assassin bug or the breaking of threads and either escapes or attacks. Occasionally, the hunting bug becomes prey himself. The success rate of his attacks is only 20 per cent, but if he succeeds, rewards for his patience and careful behaviour are high. After having overpowered and eaten a spider, the bug can rest for days, digesting this meal.

Willy van Strien
This is an update of an earlier version in Dutch

Photo: Giraffe assassin bug Stenolemus giraffa. © Fernando Soley

Sources:
Soley, F.G., 2016. Fine-scale analysis of an assassin bug’s behaviour: predatory strategies to bypass the sensory systems of prey. Royal Society Open Science 3: 160573. Doi: 10.1098/rsos.160573
Soley, F.G. & P.W. Taylor, 2012. Araneophagic assassin bugs choose routes that minimize risk of detection by web-building spiders. Animal Behaviour 84: 315-321. Doi:10.1016/j.anbehav.2012.04.016
Soley, F.G., R.R. Jackson & P.W. Taylor, 2011. Biology of Stenolemus giraffa (Hemiptera: Reduviidae), a web invading, araneophagic assassin bug from Australia. New Zealand Journal of Zoology 38: 297-316. Doi: 10.1080/03014223.2011.604092

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