Frightening days

Crayfish avoid light when renewing their armour

red swamp crayfish is anxious when moulting

Normally, the red swamp crayfish is rather fearless. But if it has to replace its carapace with a new one, its bravery disappears, as Julien Bacqué-Cazenave and colleagues report.

Crustaceans do not have a skeleton inside their body, like we do. Instead, they have a carapace, an external skeleton. This sturdy box in which they are packed protects them from physical harm. But there is a drawback: the carapace limits body growth. That is why the animals must, from time to time, replace their carapace with a larger one. The old one is shed, a new one is formed.

That is no trifle, as Julien Bacqué-Cazenave and colleagues show.

Process takes a month

The researchers wanted to know how the red swamp crayfish, Procambarus clarkii, is doing during a moult. The species originally occurs in Mexico and the south of the United States and has been introduced in many other places; it has settled as an exotic species in Europe.

Its moult is a lengthy and complex process. The chitin, of which the carapace consists, is secreted by the epidermis and the carapace is attached to it. So, it is must be separated from the epidermis, which has to form a new one. The attachments of the muscles that are anchored to the armour have to be transferred.

As soon as the old carapace is shed, the new one is exposed. This leaves the crayfish unprotected and vulnerable, as the newly formed carapace is thin and fragile in the beginning. It has to thicken and harden before it can protect the animal. The entire process of moulting takes about a month: two weeks before the old armour is shed and two more weeks until the new armour has hardened.


The red swamp crayfish normally is courageous, but during the month of moulting, especially during the third week, it is not at ease, as experiments conducted by Bacqué-Cazenave show. He tested the animals every two or three days in a plus-maze with two illuminated and two dark arms. Crayfish that did not experience any stress spent 40 percent of their time in the illuminated part of the plus-maze. But when they were about to shed their carapace, they began to avoid the light a few days in advance, and the first week after moulting they stayed in the dark areas almost continuously. From earlier work, the researchers knew that the animals behave like this when they are anxious.

The aversion to light was indeed associated with moulting, according to tests in which the animals were given a hormone that initiates the moulting process, a so-called ecdysteroid. But when the animals were also given a tranquilizer, they did not avoid the illuminated areas. From this, the researchers conclude that the light aversion is an anxiety reaction.

Obviously, the period of moult is hard. But when it is over, the crayfish is safe in its armour for the next two to six months.

Willy van Strien

Photo: Andrew C (Wikimedia Commons, Creative Commons CC BY 2.0)

Bacqué-Cazenave, J., M. Berthomieu, D. Cattaert, P. Fossat & J.P. Delbecque, 2019. Do arthropods feel anxious during molts? Journal of Experimental Biology 222: jeb186999. Doi: 10.1242/jeb.186999

Romantic sea

Fairytale light shows of Cypridinid ostracods

ostracod produces light to escape from predator

With an amazing show of light pulses, male cypridinid ostracods try to attract a mate. Each species has its own specific show program, with either very short lasting flashes or bulbs that glow for several seconds. Nicholai Hensley and colleagues examined the chemistry behind.

It looks like a fairytale scene: dozens of blue lights dancing in the dark waters of the Caribbean Sea. The spectacle is visible to those who dive or snorkel at the beginning of the night. The light artists are ostracods of the Cypridinidae family, tiny crustaceans (less than two millimeters long) with a carapax consisting of two valves, like a clam shell.

They are also known as sea fireflies. Nicholai Hensley and colleagues study their behaviour and the chemistry behind their light.


Ostracods produce light by expelling mucus containing a reactant, vargulin, and the enzyme c-luciferase, which react with oxygen in seawater emitting blue light. The ostracods use their light mainly to avoid predation. If a fish picks up an ostracod, the prey will produce a cloud of blue mucus that is pumped into the water via the gills of the fish. It makes the fish visible to its own predators. Startled, it will spit out the bite.

In ostracods of the family Cypridinidae that live in the Caribbean Sea, males use the same light reaction in a much more subtle way with a completely different purpose: they place luminescent slimeballs in the water in order to seduce a female into a mating. This courtship behaviour produces the fairytale scenes.

Train of lights

The light artist best known is Photeros annecohenae, one of the most abundant species off the coast of Belize. In the first dark hour of the night, when the sun is down and the moon is not shining, groups of males display above seagrass beds. They have to perform well, because competition is high. While there are as many females as males, most are unavailable. This is because they incubate fertilized eggs in a brood pouch, and during this period, they will not mate.

American biologists examined male courtship behaviour in the lab, using infrared light. A displaying male will first swim in a looping pattern just above the tips of the seagrass blades and place about three bright flashes of light, probably to draw attention. Then, while spirally swimming upward, it places weaker light pulses at regular intervals. It swims at high speed, slowing down when it releases a luminescent slime ball.

By doing so, it creates a train of about twelve consecutively flashing lights that can be 60 centimetres long. When finished, it descends to start a new series. Often other males join and start displaying in synchrony.


To choose a mate, females assess the light pulses that the males produce. If a female is attracted to a particular male, she will swim to him without producing any light herself. Thanks to his regular flashing pattern, she manages to meet him just above his last light pulse. Mission accomplished.

Sometimes males try to obtain a mate without producing light themselves. Instead, they intercept a female that is on her way to a performing male.

Starting a show, following another male’s show or sneaking to get a female are different tactics to acquire a mate and a male can easily switch among them.

Species-specific shows

In the Caribbean Sea, many other species of Cypridinidae also occur, and about ten species commonly live at the same place. Because they all have their own characteristic light show, a female has no difficulty finding a conspecific partner. The shows vary in the trajectory a courting male swims, the number of light pulses, the brightness of the light, the interpulse distance and time interval and the time that a pulse remains visible.


Hensley investigated the cause of the variation in light pulse length. For although all species perform the same chemical reaction to make light pulses, the duration of the pulses varies greatly: some species, such as Photeros annecohenae, show flashes that last only a fraction of a second, others make light bulbs that continue to glow for 15 seconds.

The structure of the enzyme c-luciferase appears to vary between species, resulting in the light reaction to proceed faster in one species than in another. This determines how soon the light extinguishes. In addition, the reaction rate depends on the amount of vargulin compared to the amount of enzyme: the more vargulin, the longer it takes before it is all converted and the light disappears.

Courting males produce far less light than an animal that avoids predation. Romantic lights don’t have to be that big and bright.

Willy van Strien

Photo: Luminous cloud around a fish that intended to consume an ostracod. It will spit it out. © Trevor Rivers & Nicholai Hensley

Fifteen-scaled worm emits light to defend itself in another way

Hensley, N.M., E.A. Ellis, G.A. Gerrish, E. Torres, J.P. Frawley, T.H. Oakley & T.J. Rivers, 2019. Phenotypic evolution shaped by current enzyme function in the bioluminescent courtship signals of sea fireflies. Proceedings of the Royal Society B 286: 20182621. Doi: 10.1098/rspb.2018.2621
Rivers, T.J. & J.G. Morin, 2013. Female ostracods respond to and intercept artificial conspecific male luminescent courtship displays. Behavioral Ecology 24: 877–887. Doi: 10.1093/beheco/art022
Rivers, T.J. & J.G. Morin, 2012. The relative cost of using luminescence for sex and defense: light budgets in cypridinid ostracods. The Journal of Experimental Biology 215, 2860-2868. Doi: 10.1242/jeb.072017
Morin, J.G. & A.C. Cohen, 2010. It’s all about sex: bioluminescent courtship displays, morphological variation and sexual selection in two new genera of Caribbean ostracodes. Journal of Crustacean Biology 30: 56-67. Doi: 10.1651/09-3170.1
Rivers, T.J. & J.G. Morin, 2009. Plasticity of male mating behaviour in a marine bioluminescent ostracod in both time and space. Animal Behaviour 78: 723-734. Doi: 10.1016/j.anbehav.2009.06.020
Rivers, T.J. & J.G. Morin, 2008. Complex sexual courtship displays by luminescent male marine ostracods. The Journal of Experimental Biology 211: 2252-2262. Doi: 10.1242/jeb.011130

Fiery character

A brave great tit probably is either big or hungry

The corage of a great tit depends on its body size and condition

How much risk is a great tit prepared to take? The answer differs greatly among individuals. Maria Moiron and colleagues show that how much the courage the birds exhibit, depends on their size and condition.

Just like humans, animals have a personality, a stable set of coherent behavioural traits. For example, animals differ from each other in how brave they are, the extremes being an aggressive, brutal, curious and enterprising character on the one hand and a shy, cautious and withdrawn nature on the other. As biologists pointed out, these personality types also occur in the great tit (Parus major). Maria Moiron and colleagues wondered if the personality of a great tit is linked to its physical characteristics.

To find out, they weighed a number of males and measured the length of leg, beak and wing. Also, they tested the animals for their willingness to take risks by assessing how aggressive the animals behaved to another male, which intruded into their territory. They also tested how quick they were to explore an unfamiliar test cage.

Fear or courage?

Great tits differ greatly from each other in all triats measured, as it turned out. After a statistical analysis of the data, the researchers conclude that large individuals on average are less anxious than smaller conspecifics. That may be, they speculate, because a large animal has a greater chance of winning when it comes to fighting and will be hurt less severely. An alternative explanation is that it will take more risk in acquiring food because it needs more energy.

Another finding is that the condition of the birds, in the sense of their energetic reserves, also determines their behaviour. An animal in need of some food generally takes more risks than an animal that is well-fed. A hungry bird cannot afford to be careful, it has to take action, the authors explain. It is also possible that a well-fed bird is more cautious because it has more difficulty taking off in case of danger.

Conclusion: the personality of a great tit is indeed related to its physical characteristics. That is not unexpected – but it had not been demonstrated before.

Willy van Strien

Photo: Tbird ulm (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Moiron. M., Y.G. Araya-Ajoy, K.J. Mathot, A. Mouchet & N.J. Dingemanse, 2019. Functional relations between body mass and risk-taking behavior in wild great tits. Behavioral Ecology, online January 18. Doi: 10.1093/beheco/ary199

Giving everything he’s got

Hummingbird male shines for a split second

broad-tailed hummingbird male performs spectacular dive

In order to seduce as many females as possible, a broad-tailed hummingbird male performs tight diving courtship flights. He combines movement, colour and sound into a spectacular whole, Ben Hogan and Cassie Stoddard show.

With a striking display, a broad-tailed hummingbird male (Selasphorus platycercus) tries to gain a female’s interest. He performs a number of U-shaped dives, getting down from great height (up to 30 meters!) while his wings are trilling. The lowest point of the dive is close to the targeted female, which is perched. At that point, he will give everything he’s got: he rushes past her with a top speed of more than 20 meters per second while his tail feathers produce buzzing sounds. The female perceives his iridescent gorget rapidly shifting from bright red to dark green. Then he climbs up to enable a new dive.

The show is so fast that we can’t see what exactly happens. But Ben Hogan and Cassie Stoddard made video and audio recordings of a large number of shows and analyzed them.

Blink of an eye

An entire dive takes about 6.5 seconds. At the lowest point, the small bird appears to tightly synchronize the components of the show, as the analysis revealed. As a result, top speed, buzzing sounds and colour change almost coincide, all occurring within 300 milliseconds, a human blink of the eye. When he rapidly rises again from the lowest point, the pitch of wing- and tail-generated sounds drops sharply, as when a car with a siren is passing by (the Doppler effect).

The whole is meant to make an overwhelming impression on her. But she is used to see shows like his, because all males perform them. The hummingbird males do not contribute to nest construction or care for the young, leaving all of the work to the females. They try to sire young with as many females as possible. With their tightly synchronized dive, they advertise their genetic quality, promising healthy and attractive offspring.

But is he able to seduce a female? The researchers have not yet figured out what exactly makes a show appealing and how it is performed perfectly in her eyes.

Willy van Strien

Photo: Greg Schechter (Flickr/Wikimedia Commons, Creative Commons CC BY 2.0)

Hogan, B.G. & M.C. Stoddard, 2018. Synchronization of speed, sound and iridescent color in a hummingbird aerial courtship dive. Nature Communications 9: 5260. Doi: 10.1038/s41467-018-07562-7

Escape scene

Young aphids try to hitch a ride on adults

young pea aphid hitches a ride on an adult

Young pea aphids demand a piggyback ride on the back of an adult when they have to walk on the soil, Moshe Gish and Moshe Inbar report. But they don’t always get what they want, because the adults try to remove them.

When their host plant starts shaking and a warm, moist air is blowing over it, sap feeding pea aphids (Acyrthosphon pisum) are in danger: a mammalian herbivore is approaching. Just before the plant is eaten by the grazer, they massively drop off the plant to the ground to escape. But they are not safe there either; they can be trampled, dessicate, starve or fall prey to predators that hunt on the ground, like spiders. And so they start walking in search of a new plant.

That is not easy for young aphids (the nymphs). The ground is bumpy with clumps of soil, cracks, stones, fallen twigs and leaves. The young critters are moving much slower than adults. But there is a solution, Moshe Gish and Moshe Inbar discovered: piggybacking on a large aphid.

Tiny drama

In a series of lab experiments, the researchers simulated escape scenes. They placed about ten female aphids on a fava bean plant. Next morning, young aphids had been born; pea aphids can reproduce parthenogenetically, females giving birth to daughters without having been mated. By tapping the plant and exhaling over it, the researchers induced the aphids to drop off. At some distance from the fava bean plant, a circle of lentil plants was placed to offer a destination, the bottom was covered with soil.

A tiny drama takes place in such a case, as the researchers observed. Immediately after landing, young aphids try to climb on an adult. They also climb on green plastic beads and on dead aphids that they happen to stumble upon, but from these, they quickly disembark. If a living aphid stays immobile after they climbed on, they get off after a while too. But when their carrier starts walking, they ride to a new plant, where they arrive faster than when they would have walked the distance on their own.

But adult aphids are not really willing to help young ones.


On the contrary: passengers seem to be a nuisance to them. When a nymph tries to climb on an adult aphid, this aphid will often raise its body to make it difficult, or it runs away. If there are already nymphs on its body, it often stays motionless, waiting until the passengers leave. Or it repeatedly lowers its head or posterior end to the surface to get rid of them. The back is the best place for a nymph to sit on. Eventually, an adult aphid will start walking with one passenger at most.

Once it has left, after a delay, it will reach a new plant as soon as an aphid without a hitchhiker, so the walking speed is not affected by the burden. But it is energetically costly to bear it.

Willy van Strien

Photo: © Stav Talal

Also these tadpoles try to catch a ride

Gish, M. & M. Inbar, 2018. Standing on the shoulders of giants: young aphids piggyback on adults when searching for a host plant. Frontiers in Zoology 15: 49. Doi: 10.1186/s12983-018-0292-7
Gish, M., A. Dafni & M. Inbar, 2010. Mammalian herbivore breath alerts aphids to flee host plant. Current Biology 20: R628-R629. Doi: 10.1016/j.cub.2010.06.065

Unopened flower

Moth larva enforces self-pollination in Canada Frostweed

Canada Frostweed may be enforced to self-pollination by a moth larva

The larva of the moth Mompha capella inhabits a flower bud of Canada Frostweed and prevents it from opening, as Neil Kirk Hillier and fellow researchers show. Pollinators cannot visit the flower, which has to pollinate itself instead.

Canada Frostweed (Crocanthemum canadense), a perennial plant of Northern America, is attractive to the moth Mompha capella, which lays its eggs on it. Then something unusual happens: the plant loses control over its reproduction.

The plant produces yellow flowers that normally open just after sunrise, revealing the female pistil and male stamens. Bees and flies visit the flowers, transferring the pollen from one to the next, so that the flowers are cross-pollinated. Multiple stamens lay against the five yellow petals, retracted from around the pistil to prevent self-pollination. Within a few hours, a flower’s own pollen has disappeared and the pistil is covered with pollen from other flowers. The petals fall off, the green sepals close over the pistil and protect the developing fruit with seeds within.

But when a moth has left its eggs on the plant, the larvae that hatch from these eggs crawl into a flower bud, one larva per bud. And then things are very different, Neil Kirk Hillier and colleagues discovered.


The larvae start to eat. And they don’t do it randomly, but first feed on the bases of the still folded petals. The severed petals no longer grow and don’t unfold when the flower should open, but remain folded like a cap over stamens, pistil and developing fruit, keeping the flower closed. Pollinators cannot enter. Because the stamens are compacted around the top of the pistil, the pollen is in contact with the pistil and seeds develop through self-pollination. Almost all of them will be consumed by the larva in the end.

Frostweed duped

As a consequence, the Canada Frostweed plant produces less offspring. A yellow flower produces on average about forty seeds, and a larva saves only one or two of them. Reproduction, however, is not in immediate danger. This is because the plant not only produces a small number of yellow flowers that open, unless caterpillar disturbs the process, but also a large number of flowers without yellow petals and only four or five stamens, which appear later in the year. These flowers never open and produce seeds through self-pollination. While they make less seeds than yellow, open flowers (only six or seven seeds per flower), there are much more of them. So seeds are produced anyway.

But seeds of open flowers that develop after cross-pollination are necessary for the exchange of hereditary material. In plant populations with a high infestation rate, such exchange is limited, and genetic variation is low.

Larva safer?

The researchers don’t mention what benefit a larva gains by intervening in the flowering process. If the flower opened and had been pollinated normally, seeds to be consumed would have appeared as well. Perhaps in a closed flower, the larva is safer from predators and parasites.

Willy van Strien

Photo: Homer D. House, 1918 (Wikimedia Commons)

Hillier, N.K., E. Evans & R.C. Evans, 2018. Novel insect florivory strategy initiates autogamy in unopened allogamous flowers. Scientific Reports 8: 17077. Doi:10.1038/s41598-018-35191-z

Stealthy moth

Bats receive attenuated echo of their calls

The moth Bunaea alcinoe applies acoustic camouflage

Scales and furry coat of the moth Bunaea alcinoe thwart the searching method of hunting bats, as Zhiyuan Shen and colleagues show. That is how it manages to make itself undetectable.

Moths, which fly around at night, have to deal with agile enemies: bats. These predators find their prey by emitting high (ultrasonic) calls and hearing the echo of their sound when it is reflected by a wall, a tree – or a moth. This ‘echolocation’ enables them to localize a moth and find out in which direction it goes, and then they can catch it.

Moths have developed different ways to escape from these enemies. Some hear the bats’ sounds and quickly change direction; others produce a sound by themselves with which they startle or confuse a hunting bat; still others have wings that distort the echo.

Another defence strategy is to absorb part of the bats’ sounds, preventing their reflection. That idea is applied by Bunaea alcinoe, the cabbage tree emperor moth which lives in Africa, as Zhiyuan Shen and colleagues show.


The numerous scales that cover the wings of this moth are responsible for the absorption. The scales, which look like leaves on a pedicel under a microscope, have a regular, very open nanostructure. Thanks to this structure, they can vibrate exactly at the frequencies of the bats’ sound, the researchers show. The sound waves are transferred to the scales, where they are extinguished by friction: acoustic camouflage against bat echolocation.

Butterflies, which are active during daytime and are not hunted by enemies that use echolocation, have scales on their wings with a different nanostructure, that do not vibrate at the frequency of bat calls.

Another feature is the striking hair growth on the backside of the moths. That furry coat also absorbs sound, in the way curtains and carpets do.

As a result, bats have difficulty finding this prey, Bunaea alcinoe, as only a part of the sound of their calls is reflected when it hits a moth.

Willy van Strien

Photo: Paul Wursten (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Explanation by researchers on YouTube

Shen, Z., T.R. Neil, D. Robert, B.W. Drinkwater & M.W. Holderied, 2018. Biomechanics of a moth scale at ultrasonic frequencies. PNAS, online Nov. 12. Doi: 10.1073/pnas.1810025115
Neil, T.R., Z. Shen, B.W. Drinkwater, D. Robert & M.W. Holderied, 2018. Stealthy moths avoid bats with acoustic camouflage. Journal of the Acoustical Society of America 144: 1742. Doi:  10.1121/1.5067725
Zeng, J., N. Xiang, L. Jiang, G. Jones, Y. Zheng, B. Liu & S. Zhang, 2011. Moth wing scales slightly increase the absorbance of bat echolocation calls. PLoS ONE 6(11): e27190. Doi:10.1371/journal.pone.0027190

Complex cooperation

Weaver ant will rescue nestmate from spider web

a weaver ant on its nest

Floria Uy and colleagues show that workers of the weaver ant care about their nestmates: when an ant gets entangled in a spider’s web, others are willing to help it. The ants were already known for their ‘living sewing machine’.

The weaver ant or green tree ant, Oecophylla smaragdina, is common in the tropical parts of Asia and Australia. Its presence is clearly visible because of its nests, which stand out as balls of leaves in bushes and trees. The arboreal nests are the result of a special piece of group work, involving not only workers, but also larvae.

weaver ants building their nestTo construct a nest, the ants must glue the leaf edges together. Workers will line up, grasp the edges of two leaves (or two pieces of a long leaf) and draw them together. If the distance between the leaf edges is large, they grab each other and form living chains to bridge the gap. By then shortening the chain, they pull the leaf edges towards each other.


Living sewing machine

They then attach the edges firmly to each other with a ‘living moveable sewing machine’, as described by Ross Crozier. While many workers hold the leaf edges, others join with a mature larva between their jaws.weaver ants use larval silk to construct the nest The workers stimulate the larvae to spin a silk thread and move them zigzag between the leaf edges, stitching the leaves together.

In many ant species, larvae spin silk to make a cocoon in which they pupate. But in the weaver ant, their silk is used for nest construction.

A colony of weaver ants consists of multiple nests in several trees; many ants are running on trails between these nests. In peripheral nests of the colony, soldiers live that guard the boundaries of the territory and defend it against conspecific ants from foreign colonies. On average, a colony will live for eight years and can have as many as half a million inhabitants.

That is a lot, and you would be inclined to think that an ant’s life does not matter that much.

Yet, the ants are concerned when a nestmate that is in danger, Floria Uy and colleagues now show. They discovered a second example of complex cooperation within this species.

Ant in distress

If a weaver ant is trapped in a spider’s web, conspecifics may bite the threads of the web and free the victim, as Uy showed. But an alternative scenario is also possible: ants may attack a worker that is entangled in a spider’s web and kill it. When will the ants rescue a conspecific, and when will they kill it?

Uy, who conducted experiments on the Solomon Islands, put a number of ants next to an ant trail after having wrapped them in spider’s silk; in some cases, the victim was a nestmate of the ants on the trail, in other cases it was from a foreign colony.

She found that ants will always help a nestmate in distress. But for ants from a different colony, the outcome is uncertain: some are rescued, others are killed. That some of them are killed is to be expected: to the residents on the trail, they are intruders and accordingly, they are treated aggressively. The fact that some non-nestmates in distress are helped instead is surprising.

It may be a mistake, the researchers hypothesize. In the study area, ants from neighbouring colonies have a greater chance of being rescued than ants from distant colonies. Ants recognize nestmates and colony mates by smell, and neighbouring colonies may have a rather similar odour.

Willy van Strien

Large: weaver ant on nest. Rushen (via Flickr, CC BY-SA 2.0)
Small, middle: workers building the nest. Sean.hoyland (Wikimedia Commons, Public Domain)
Small, below: nest with weaver ants. Bernard DUPONT (via Flickr, CC BY-SA 2.0)

Watch weaver ants building a nest

Uy, F.M.K., J.D. Adcock, S.F. Jeffries & E. Pepere, 2018. Intercolony distance predicts the decision to rescue or attack conspecifics in weaver ants. Insectes Sociaux, online Nov. 3. Doi: 10.1007/s00040-018-0674-z
Crozier, R.H., P.S. Newey, E.A. Schlüns & S.K.A. Robson, 2010. A masterpiece of evolution – Oecophylla weaver ants (Hymenoptera: Formicidae). Myrmecological News 13: 57-71

Role pattern erased

Twisted-wing parasites change the behaviour of host wasps

The paper wasp Polistes dominula is host to a manipulating parasite, Xenos vesparum

The life cycle of the parasite Xenos vesparum is closely linked to that of the wasps in which it lives. It modifies their behaviour in such a way that it meets its needs, as Laura Beani and colleagues demonstrate.

It is often creepy as well as fascinating to see how parasites control their host. A nice example is Xenos vesparum, parasite of the European paper wasp (Polistes dominula). Its manipulation skills are being unravelled by Laura Beani and her colleagues.

The parasite, which belongs to the insect group of twisted-wing parasites, has a bizarre life cycle, with a striking difference between males and females. In the larval stage, the parasite lives within a wasp host. Males pupate in their host; the front part of the pupae extrudes trough the cuticle between the plates of the host’s abdomen. When adult males emerge, they leave their host to live freely; within a day, they die.

Females live much longer. They remain in their host and don’t pupate, but turn into a ‘bag’ filled with egg cells and a fat supply. Only their cephalothorax, into which head and thorax are fused together, is tough and visible between the plates of the host’s abdomen. Usually only one parasite, either male or female, will mature in a parasitized wasp.

Male and female parasite must mate on the wasp in which the female lives. They do it fast.

Wasp colony

Xenos parasites effectively exploit the annual cycle of their host. In March, fertilized wasp queens, which have spent the winter in groups, awaken. Every queen occupies a place to establish a colony. She builds an open nest and lays the first eggs, which will produce workers. Before these eggs have developed into adults, the queen also has to collect food and take care of the brood. But later, from May, she is just laying eggs, while the workers, who don’t reproduce themselves, do the rest of the work.

In summer, the colony is flourishing with a maximum of fifty wasps, and it is time for the next step. The queen now starts laying eggs that will develop into males and sexual females, future queens. Males and sexual females (gynes) appear in July-August.


Then the queen has finished her task. She stops and the colony collapses. The gynes leave the nest and in early autumn, they aggregate in groups that attract males. Mating follows. As winter approaches, the fertilized gynes search for a sheltered place, again aggregating; they often cluster in buildings, for example under roof tiles. There they hibernate and wait for the spring. Males and worker wasps die before winter. In March, the new queens awaken from winter diapause and the cycle starts again.

The European paper wasp is a common species, and it is not as annoying as the common wasp, Vespula vulgaris.

Trumpet creeper

The parasite disturbs the role pattern of its host. But not immediately. In May, tiny parasite larvae penetrate into worker wasp larvae, which appear to be little affected by the presence of the parasite. Only when the hosts have developed into a pupa, the parasite larvae undergo a growth spurt and mature.

And then the manipulation starts: parasitized workers do not stick to their role. They are lazy and at the age of one week, they will leave the nest.

Beani, doing research in Tuscany, describes how in early summer the parasitized worker wasps are mainly to be found on trumpet creeper bushes; the trumpet creeper, originating from North America, has naturalized in Europe. It produces a lot of nectar, which the parasitized wasps enjoy. Healthy, non-parasitized wasps spend much less time on this plant. Because the hosts deserted the nest and moved to trumpet creeper, the parasites easily find a partner with which they can mate. In the wasp nest, mating would be impossible, as parasite males would immediately be chased off by healthy workers.

Castration by Xenos

Parasite embryos develop within the fertilized parasite females in a wasp’s body and new parasite larvae emerge at the end of July. A female parasite releases more than three thousand larvae which all need a host to develop. When healthy foraging wasps pass by, larvae cling to them, are transported to the wasps’ nest and start searching for wasp larvae. Among infected wasp larvae, there will now be putative males and sexual females, which were destined to reproduce. But they will never do the job, as the parasite castrates them.


From mid-July on, parasitized wasps (workers, males and gynes) form groups outside the nests, just like healthy young sexual females will do later in the season: the role pattern is erased. They gather on high plants and later on buildings, usually places where healthy males gather every year or where future queens use to overwinter. The parasitized wasps are inactive, the parasites have much opportunity to mate.

When healthy sexual wasp females fly out and aggregate, they often join these groups of parasitized wasps.

At the end of the season, when the gynes have been fertilized and gather at places to hibernate, wasps that contain a fertilized parasite female will join them. Parasite females safely spend the winter in a wasp body, in a group of wasps on a sheltered place. Wasps that carried a parasite male have no function anymore; they die in autumn.


When healthy young queens leave to establish a colony in spring, parasitized wasps are left behind. A few weeks later, when the first wasp larvae have hatched in wasp nests, the parasites release their larvae. They then apply a last manipulative trick: they induce their host wasp to deliver the mature larvae in several young wasp nests. There are still no adult workers to defend these nests and the queen is often gone to collect food. From within her host, the parasite female drops larvae in the nests. She also drops some larvae on plants, as a foraging wasp may come along and take them with it.

And so the Xenos parasite completes the circle – with enforced cooperation of the host.

Willy van Strien

Photo: ©Hans Hillewaert (Wikimedia Commons, Creative Commons BY-SA 4.0)

Xenos peckii mating on YouTube

Beani, L., F. Cappa, F. Manfredini & M. Zaccaroni, 2018. Preference of Polistes dominula wasps for trumpet creepers when infected by Xenos vesparum: A novel example of co-evolved traits between host and parasite. PLoS ONE 13:e0205201. Doi: 10.1371/journal.pone.0205201
Beani, L., R. Dallai, D. Mercati, F. Cappa, F. Giusti & F. Manfredini, 2011. When a parasite breaks all the rules of a colony: morphology and fate of wasps infected by a strepsipteran endoparasite. Animal Behaviour 82: 1305e1312. Doi:10.1016/j.anbehav.2011.09.012
Beani, L., 2006. Crazy wasps: when parasites manipulate the Polistes phenotype. Annales Zoologici Fennici 43: 564-574.
Hughes, D.P., J. Kathirithamby, S. Turillazzi & L. Beani, 2004. Social wasps desert the colony and aggregate outside if parasitized: parasite manipulation? Behavioral Ecology 15: 1037-1043. Doi:10.1093/beheco/arh111

Airmobile brigade

Hovering guards of bee colony position themselves orderly

Hovering guards defend the nest of Tetragonisca angustula

It is difficult for a robber bee to stealthy approach a colony of the stingless bee Tetragonisca angustula, because hovering guards will detect it. These guards arrange themselves in an organized manner, Kyle Shackleton and colleagues show.

Workers of the stingless bee Tetragonisca angustula defend their colony extraordinary well against their enemies. Some workers are dedicated guards; they are heavier than other workers and have longer legs. Such specialised soldier caste is not known in other bee species. And while during the day always guards are standing in or near the nest entrance, there are often also some hovering in front of it to keep an eye on the access route, especially in the afternoon. Such an airmobile brigade is unique too.


The most important enemy is the robber bee Lestrimelitta limao. Robber bee workers do not collect nectar and pollen from flowers themselves, but get it from colonies of other species. They also steal food that is prepared for the larvae and nest constructing material. Tetragonisca angustula, with its large colonies, is vulnerable. No wonder, then, that there are guards that keep an eye on what is near the nest. It is important to deal with an approaching single robber bee at once, because it is a scout. It will recruit hundreds of others for a raid that will last for hours or days.

As more hovering guards are active, such a flying intruder is detected earlier and intercepted at a greater distance from the nest. The guards recognize the robber bee from its odour and colour; it is black and smells like lemon. The guards wrestle it to the ground by clamping to an antenna or wing. They are not able to kill it, because it is three times heavier than a they are. But they may stop it.

Maximal field of view

Often only a few hovering guards are hanging in front of the nest. Kyle Shackleton and colleagues now show that these guards do not choose their position randomly, but in a coordinated way. If two guards  are hovering, there will usually be one on the left and one of the right side of the access route to the nest. In case of three guards, it rarely occurs that all of them hover at the same side. And four guards mostly are distributed evenly; sometimes sometimes three guards hover at the one side and one at the other side, and it hardly happens that all four guards are at the same side. Because of this coordinated distribution, the hovering guards have a maximal field of view and they will discover an approaching flying enemy as fast as possible.

In case of immediate danger, more guards will be hovering in front of the nest. An even distribution between left and right is less important in that case, because together they will have a good view anyway. There is no surveillance at night, for in the evening the bees close the nest entrance with wax.

Willy van Strien

A Tetragonisca angustula hovering guard bee next to a nest-entrance. Bibafu (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Shackleton, K., D.A. Alves & F.L.W. Ratnieks, 2018. Organization enhances collective vigilance in the hovering guards of Tetragonisca angustula bees. Behavioral Ecology 29: 1105-1112. Doi: 10.1093/beheco/ary086
Grüter, C., C. Menezes, V.L. Imperatriz-Fonseca & F.L.W. Ratnieks, 2012. A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee. PNAS 109: 1182-1186. Doi: 10.1073/pnas.1113398109
Grüter, C., M.H. Kärcher & F.L.W. Ratnieks, 2011. The natural history of nest defence in a stinngless bee, Tetragonisca angustula (Latreille) (Hymenoptera: Apidae), with two distinct types of entrance guards. Neotropical Entomology 40: 55-61. Doi: 10.1590/S1519-566X2011000100008
Van Zweden, J.S., C. Grüter, S.M. Jones & F.L.W. Ratnieks, 2011. Hovering guards of the stingless bee Tetragonisca angustula increase colony defensive perimeter as shown by intra- and inter-specific comparisons. Behavioral Ecology and Sociobiology 65: 1277-1282. Doi: 10.1007/s00265-011-1141-2