Archive for the 'Hemipterans (True Bugs)' Category

It’s a trap!

Yesterday, I was poking around a small bush of white flowers looking for insects to photograph. I noticed this butterfly hanging from the bottom of a flower, rather than sitting on top:

What are you doing under there?

I panned around to underneath the flower and found out why:

Unlucky butterfly.

The butterfly had been snared by an ambush bug (Phymatinae), which is a subgroup of assassin bugs (Reduviidae). I think the above animal is a nymph belonging to the genus Phymata. These bugs hang out underneath flower petals until unweary pollinators visit. They then lunge out and snare their prey with their enlarged raptorial appendages, piercing the exoskeleton with a syringe like rostrum.

Here is an adult of the same, or a similar, species. About one of every three flowers in this bush had an ambush bug laying in wait below.

Phymata sp

'Come a bit closer my pretties.'

If any insect-gurus can identify this exact species, it would be much appreciated.

Aphid adornment: Lateral gene transfer from fungi to aphids.

Carotenoids are integral components of animal biochemistry. These organic compounds, characterized by long hydrocarbon chains and loops, are used in photoreception, antioxidation, the immune system, and for ornamental coloration. There are over 800 known carotenoid compounds found in nature. They absorb varying wavelengths of blue and green light, causing tissue containing large quantities of carotenoids to appear green, yellow, orange, or red. This absorptive property is what makes carotenoids crucial for vision and coloration in animals.

However, there is a snag. Animals cannot produce their own carotenoids. Their genomes lack the enzymes necessary to synthesize carotenoids from smaller hydrocarbons, and therefore they must ingest carotenoids from organisms that can. Certain bacteria, archea, plants, and fungi are all capable of producing specific carotenoids. By adding these organisms to their diet, animals can fulfill their carotenoid needs.

One animal that takes advantage of carotenoid coloration is the pea aphid, Acyrthosiphon pisum. There are two primary color morphs of this aphid species, green and red (see below). The green aphid morph has large quantities of greenish-yellow carotenoids: alpha-, beta-, and gamma-carotene. The red aphid has decreased amounts of those carotenoids, and instead has gobs of the red carotenoid, torulene.

Color morphs of the pea aphid, A. pisum, and the carotenoids predominately responsible for the color variation. Adapted from Moran and Jarvik, 2010.

This polymorphism is of great ecological significance for the aphid. Parasitoid wasps prefer to lay their eggs in green aphids, and carnivorous ladybugs prefer to eat red aphids. If there is a spike in wasp parasitism, the red morphs become more predominant. If there is a spike in ladybug predation, then the green morphs become more common. By maintaining the genetic diversity of both color morphs in an aphid population, that population can guard against being wiped out by a temporal increase in parasites or predators.

However, this polymorphic color variation seen in aphids presents an interesting question. Where are the aphids getting their carotenoids? The plant phloem sap that aphids suck out of leaves is low in carotenoids, and the carotenoids produced by aphid host plants do not match those found in the aphids. In addition, the endosymbiotic bacteria within the aphids cannot be the source of the carotenoids, as there are no carotenoid biosynthesis genes in their genomes.

However, researchers were surprised to discover that the aphid’s red-green color polymorphism is inherited in a classic Mendelian autosomal manner (remember your Punnett squares, kids?), with the red allele dominant to the green allele. This indicates that the genes responsible for carotenoid-based colorations in these aphids are located within their own genomes! A search of the newly published pea aphid genome revealed the presence of several carotenoid synthases, cyclases, and desaturases. This suite of carotenoid biosynthesis genes is capable of producing the colored carotenoids necessary for the red and green aphid color morphs. A mutation in one allele of these genes prevents the production of the red carotenoid, torulene, resulting in the green aphid color morph.

These are the first carotenoid biosynthesis genes found in an animal. Indeed, no other known animal genome, including several other insect genomes, contain homologues to these genes: so where did they come from?

Carotenoid synthesis genes from various organisms. Adapted from Moran and Jarvik, 2010.

A gene tree containing the aphid carotenoid genes, as well as similar genes from bacteria, plants, and fungi can bee seen to the left. The aphid genes (blue) are completely nested within fungal carotenoid biosynthesis genes (brown). Therefore, at some point in aphid evolution, the genes for carotenoid biosynthesis were transfered from a fungi directly into the aphids genome. It is likely that a similar event occurred from bacteria (black) to plants (green) as well. The possible mechanisms for such gene transfers, especially between two eukaryotes, are poorly understood at this point.

Earlier, I wrote about a similar case of lateral gene transfer from algae to a sea slug. That was the first known transfer of genes between multicellular organisms. This new example in aphids lends support to the notion that these lateral gene transfers are more common in eukaryotes that once though. It may turn out, as genome sequencing increases at exponential rates, that the eukaryotic tree of life actually resembles (to some extent) the interconnected gene web seen in bacteria.

References:

  • Moran, N., & Jarvik, T. (2010). Lateral Transfer of Genes from Fungi Underlies Carotenoid Production in Aphids Science, 328 (5978), 624-627 DOI: 10.1126/science.1187113

Hat tip to Microecos for tweeting this.

Arthropod Roundup: Amphipods under the ice, high octane isopods, and the pea aphid genome

Quick blurbs about arthropod research and news:

  • NASA climate researchers have discovered animal life deep below the Pine Island Glacier Ice Shelf in Antarctica. The researchers drilled a hole six-hundred feet deep and eight inches wide into the glacial ice sheet about twelve miles from the open ocean. When they lowered a camera below the ice sheet, the scientists were surprised to see a Lyssianasid amphipod crustacean swim up and park on the cable. The researchers were only expecting to find microbial life under the ice sheet this far in from the open ocean. It is unknown what the primary energy source for animals living here could be. The presence of a three-inch amphipod, however, suggests a much more elaborate and dynamic ecosystem than hypothesized in this poorly understood habitat. (DSN has a video of the amphipod)
  • Limnoriid isopods, commonly called gribble worms for some reason (they neither are, nor resemble worms), have a ravenous appetite for wood. This is not unusual among arthropods; many diverse groups including termites, millipedes, and squat lobsters are capable of digesting woody plant matter. However, all these creatures process the wood with the aid of gut-dwelling symbiotic bacteria. A new study finds that the Limnoriid isopod, Limnoria quadripunctata is special in that it doesn’t rely on bacteria-produced catalysts to break down wood, but rather has the necessary glycosyl hydrolase enzymes incorporated into its genome. These enzymes are evolutionarily related to similar proteins found in arthropods, but their derived function for wood digestion in Limnoriid isopods is completely novel. The researchers, or their over-excitable university PR department, think the study of these enzymes could aid in bio fuel synthesis.
  • The gemone of the pea aphid, Acyrthosiphon pisum, has been sequenced. This is the first Hemipteran (true bug) genome and will provide clues about the evolutionary history of certain hexapod groups. This new genome could also help agriculturalists develop new techniques to control aphid pests and the spread of aphid-borne plant viruses. Researchers are also interested in the pea aphid’s, apparently, scaled down immune response system and their ability to easily switch specialization from one plant species to another.

The kiss of death: Deceptive predatory tactics of assassin bugs

Assassin bugs (Reduviidae) belong to the Hemipteran order, sometimes referred to as “true bugs.” Hemipterans also include aphids, leafhoppers, and cicadas. Like all Hemipterans, assassin bugs feed using a specialized proboscis, called a rostrum. However, unlike their vegetarian, sap-sucking cousins, assassins use their rostrum for extracting fluids from living prey.

Though most assassin bugs feed on other insects and arachnids, some species predate mammals such as bats (video) and humans. In fact, certain species of assassins are the primary vector of Chagas disease in humans. Assassin bugs are evolutionarily specialized for their predatory lifestyles in an astonishing number of ways:

  • Stylets and digestive venom: Integrated with the rostrum, assassin bugs have specialized serrated stylets for tearing into crevices in animal tissue. Once inside, the assassin injects a digestive saliva that breaks down the unlucky prey’s innards into a nutrient rich slurry; which the assassin then sucks back up through the stylet.
  • Raptorial forelegs: Thread-legged assassin bugs (Emesinae) have beefed-up forelegs equipped with sharp spines for grabbing, impaling, or pinning their prey. Liken this adaptation to the forelegs of praying mantids and “spearer” mantis shrimp.
  • The specialized foreleg of thread-legged assassin bugs (Redei, 2007; artour_a).

  • Sticky hairs: Assassin bugs of the genus Zelus also use their forelegs to capture prey. However, instead of sharp spines, they use fine hairs coated in a glue like substance. It is not known if the bugs produce their own glue, or if they obtain it from plant sap. Regardless, they use it to ambush and immobilize prey as they begin to liquidate their insides.
  • Zelus longipes has fine hairs (electron micrograph, right panel) on its forelegs which are coated with a viscous, glue-like material. This is used to immobilize prey. Click the image for a much larger version. (Photo by: Chuck Ulmer; SEM image adapted from Werner and Reid, 2001)

  • Lure signals: Feather-legged assassin bugs (Holoptilinae) lure ants to their doom with visual signals and pheromones produced in a special organ, called a trichome. Read more at Myrmician.

Wow, I really got sidetracked in that introduction. I had no idea how awesome assassin bugs were. Every bit of research I completed for this post led me to another exciting factoid.

Regardless, I need to get to the point of this post, which is a new paper about some disturbingly sinister predatory tactics in the assassin bug species, Stenolemus bituberus. This assassin bug has its work cut out for itself, as it predominately stalks some truly dangerous quarry, arachnids. The assassin bug faces the challenge of obtaining an advantageous position on the spider; from which it can launch a swift, fatal strike. The researchers found that these sneaky assassins use more than one technique to outsmart and turn the tables on their cunning prey.

The Australian based researchers placed assassin bugs on the webs of five species of spider. Through tedious observation they discovered that S. bituberus uses two contrasting methods to get the drop on web-building spiders. Both methods involve manipulation of the spider’s own web.

First, in a stalking behavior, the assassin sneaks up on the spider. In order to accomplish this, the bug walks over the web with an irregular pattern of footsteps. The spider does not notice arrhythmic motions, and the assassin is able to get within striking distance. This technique is also used by web-invading jumping spiders (and is useful when you want to avoid drawing the attention of colossal sandworms while crossing the deserts of Arrakis). In addition, the assassin bugs also make use of natural “smokescreens” such as strong gusts of wind on the webs in order to advance on the unwitting spiders.

The researchers also noted a second predatory behavior in which the assassin bugs bait and lure the spiders. They accomplish this by plucking the web in such a way as to mimic the struggles of a helplessly trapped insect. When the spider comes over to inspect and process its captive, it instead gets an carapace-full of rostrum, as the assassin bug pounces it. Also, considering the ant-luring techniques of the feather-legged assassins described above, one must wonder if chemical attractants are involved in this case as well. Watch a video of the luring and striking behavior, here.

As an interesting aside, the researchers also noted that the assassin bugs habitually tapped the spiders with their antenna just prior to the strike. This behavior is seen in other predatory arthropods, however its purpose is not clear. It is possible that the assassin bug is getting last-second distance, orientation, and identity information about the spider before launching its attack. Another possibility is that the assassin bug is hypnotizing the spider, habituating it to stimuli, so that it is less likely to respond violently when the assassin strikes for real.

Damn, these bugs are awesome.

References:

  • Wignall, A., & Taylor, P. 2010. Predatory behaviour of an araneophagic assassin bug. Journal of Ethology. DOI: 10.1007/s10164-009-0202-8
  • Wolf, K., & Reid, W. 2001 Surface Morphology of Legs in the Assassin Bug (Hemiptera: Reduviidae): A Scanning Electron Microscopy Study with an Emphasis on Hairs and Pores. Annals of the Entomological Society of America, 94(3), 457-461. DOI: 10.1603/0013-8746(2001)094[0457:SMOLIT]2.0.CO;2
  • Redei, D. 2007. New and little-known thread-legged assassin bugs from Australia and New Guinea (Heteroptera: Reduviidae: Emesinae). Acta Zoologica Academiae Scientiarum Hungaricae. 53 (4), 363–379.

Abandon Ship! Parasitoid fly larvae flee their doomed host

A new research article, published in the Proceedings of the Royal Society B discusses a unique insect endoparasitoid. This fly larva typically grows inside an aphid host until it matures and exits the aphid. However, the researchers have discovered that the larva is capable of abandoning its aphid early if the aphid is threatened by a predator. This behavior prevents the larva from going down with the ship.

The red Endaphis fugitiva larva parasitoid exiting its injured aphid host. Image adapted from Muratori et al., 2010.

Larval endoparasitism is well know among insects, with parasitoid wasps being the most common example. However, some flies also engage in endoparasitism. One such fly, Endaphis fugitiva, was describe just last year. Unlike parasitoid wasps, which inject their eggs into the body of their host insect, E. fugitiva lays its eggs on the leaves of plants with aphid infestations. The eggs hatch, and the fly larvae seek out the nearest aphid, where they use their specialized mouthparts to bore into the aphid’s abdomen. Once inside, the larva feeds off the aphid until it matures. It exits through the anus, killing the aphid, and drops to the ground. There, the larval fly forms a cocoon and metamorphosizes into an adult.

In the most recent paper, Muratori et al. have shown that the E. fugitiva larva can sense that its host is in danger and jump ship before it is consumed along with the aphid. They demonstrated that the fly larvae will exit the aphid if the aphid is injured or exposed to predation by lacewings or syrphid larva. Surprisingly, the researchers also found that early-ejecting fly larvae were still able to grow to full size and pupate into adults in the same amount of time as normal.

Muratori et al. hypothesize three possible mechanisms by which the larval fly may be able to sense its host’s imminent demise:

  • Emergency cues such as stress factors in the aphid’s haemolymph could be detected by the parasitoid larva.
  • Direct contact between the predators mouthparts and the parasitoid larva.
  • A drop in the internal pressure of the aphid as the predator begins to suck out its internal fluids could be detected by the parasitoid larva.

Watch this video of a larval fly fleeing from its host aphid as the aphid is attacked by a lacewing (Via Nature). What a way for the poor aphid to go. Just as a predator starts sucking out your internal fluids, a massive fly larva shreds your innards and bursts out your anus.

References:

Muratori, F.B., Borlee, S. & Messing, R.H., 2010. Induced niche shift as an anti-predator response for an endoparasitoid. Proc. R. Soc. B, published online before print January 13, 2010

Muratori, F.B., Gagne, R.J. & Messing, R.H., 2009. Ecological traits of a new aphid parasitoid, Endaphis fugitiva (Diptera: Cecidomyiidae), and its potential for biological control of the banana aphid Pentalonia nigronervosa (Hemiptera: Aphididae). Biological Control, 50(2), 185-193.


I have moved.
Arthropoda can now be found here.

Michael Bok is a graduate student studying the visual system of mantis shrimp.

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