With a bulky abdomen, striated yellow and black, giant spiders are impressive for their voracity.Accustomed to eating crickets, cockroaches and other insects, these spiders have at times been seen consuming much larger prey, such as lizards and even small birds.Arachnid experts have long wondered how such a feat is possible. Work coordinated by biochemist Adriana Lopes, of the Butantan Institute in São Paulo, is beginning to answer this question.
As soon as a potential meal becomes entangled in the web, it is immobilized by an injection of poison. The giant spiders (Nephilingis cruentata) then cover it in a regurgitated thick, brownish liquid that is capable of dissolving tissue and turning it into a pasty glop. This is the giant spider’s method of slowly feeding on its prey, by consuming small pre-digested parts. For a long time it was thought that the liquid cast on the prey was the poison itself, produced by a gland near the spider’s mouth. By studying the digestion of Nephilingis cruentata in detail, Lopes’s group found that the liquid actually is the digestive fluid, which is rich in enzymes that help digest prey.
Easily found in gardens, near lamps, in corners of walls or construction sites, giant spiders have a marked sexual dimorphism.Females in general are much larger than males.Some can reach 4 centimeters (cm) in length. Males, on the other hand, measure on average about 0.5 cm.Despite their size, these creatures are large enough to make the hair of anyone who has a slight fear of spiders stand on end, even though their venom is harmless to humans.In insects, however, the poison acts on the nervous system and causes paralysis, but not death.Prey are usually alive when covered by the fluid and they begin to be partially digested.
A few years ago, curiosity about the digestion of Nephilingis cruentata led Lopes’s team to begin analyzing the chemical composition of the spiders’ digestive fluid. On a quick search around the Butantan Institute, they collected 10 specimens. Back in the laboratory, the researchers fed them and induced them to produce the digestive fluid through mechanical and electrical stimuli. They then characterized the chemical composition of the samples. The researchers found that the digestive fluid was secreted and synthesized in cells of the gut and that it was very rich in enzymes that break down or turn proteins, fats and sugars into smaller molecules. Such a process means that the molecules can be converted into energy more easily. In all, they characterized almost 400 enzymes, which were described in a study published in the September 2016 issue of the scientific journal BMC Genomics.
Among the carbohydrases, enzymes that digest carbohydrates (sugars), Lopes’s group identified large concentrations of chitinases, which specialize in degrading chitin, the natural polymer responsible for the hardness of the arthropod exoskeleton.Among the proteolytic enzymes that degrade proteins, the astacins were synthesized in greater amounts.These enzymes are commonly found in most living things.But, according to Lopes, this is the first time the production of such a wide variety of astacins has been observed at such high levels.“We identified 25 types of astacins in the digestive fluid of these spiders,” she says.
Using bioinformatics, the researchers did a phylogenetic study of these enzymes. Specific software analyzed the DNA sequences that make up the genes containing the recipe for astacins of the giant spider and compared them with those produced by other spiders and other arthropods. The results suggest that spiders that are evolutionarily more primitive produce fewer astacins than those that have emerged more recently. The data presented in the article are still preliminary, explains Lopes, but allow room for some interpretations. One is that the two-stage digestion—an extracorporeal and an intracellular one—would be something that evolved over millions of years, allowing these spiders to go for long periods without feeding. “Nephilingis cruentata can go without food for up to a year,” says Lopes. “An overproduction of these enzymes in these spiders would be justified by the need for a digestive system that makes the most of all nutrients.”
Evidence obtained by Lopes’s group reinforces this hypothesis. She and her collaborators found that after the spider ingests the entire prey, a second phase of digestion begins, which now takes place inside the cells. Inside the cells of the gut, the part of the nutrients that was not transformed by the digestive fluid is transported into digestive vacuoles; these are intracellular compartments filled with enzymes that break down proteins, most likely formed by the fusion of lysosomes with nutrient-containing vesicles. Under microscopic analysis, the researchers identified large amounts of sugars and lipids stored in reserve cells connected to the giant spider’s gut. It could be this reserve that provides the nutrients needed to keep these spiders alive during long periods of food scarcity.
In a line of research parallel to Lopes’s work, biologist Hilton Japyassú at the Biology Institute of the Federal University of Bahia (UFBA) is investigating the flexible behavior supposedly fixed in Nephilingis cruentata.He says these spiders are able to memorize information and learn from their experiences, thereby perfecting basic instincts such as hunting and building a web.Japyassú began studying the species almost 10 years ago while still at the Arthropod Laboratory of the Butantan Institute.In his research, he found that these spiderscan change their hunting behavior or web construction according to the size of the prey they intend to capture.
Giant spiders produce two types of silk thread, one dry and one more viscous.To weave the thread into a web, they use different spinnerets, structures associated with the silk glands located at the back of the abdomen.The two types of silk are used in distinct parts of web construction.The dry threads, for example, give shape to the web’s overall structure, which consists of the radial threads, like spokes of a wheel;the dry spiral, which is built from the center to the edges;the frame threads, which support the entire spiral;and the refuge, which is connected to the central region where spiders wait for their prey.The viscous threads, on the other hand, make up the adhesive spiral of the web and are responsible for the capture.
When spiders catch large prey, they cut the threads supporting the web, causing it to envelop their future meal and limit its movements.Small prey are immobilized with injection of the poison, which paralyzes them.Part of this plasticity derives from their memories of previous predatory events.According to Japyassú, spiders are able to remember different aspects of their prey, such as size or type, and also to remember how many were previously captured.One indication of this is that the overall dimensions, shape, and spacing between web spirals take into account the frequency and size of captured prey.
The hunt begins when the spider directs its attention to certain parts of the web, usually those areas where prey most often become trapped. It watches over these areas by holding some threads of the web taut with its front legs.This tension allows the spider to sense certain types of vibration and detect even the most subtle ones.“The hungrier it is, the tauter the spider holds the threads,” says Japyassú. “In this way, previously imperceptible vibrations, produced by small prey, become the object of its attention.” The next step in the hunt is the capture.As soon as an insect falls into the web, the spider runs toward the prey, approaches it and, depending on the circumstances, eats it immediately or wraps it in more web strands before taking it to a refuge where previously captured prey are stored.
By analyzing how these spiders hunt, Japyassú found that these tactics show phylogenetic signs. This means that certain behaviors have evolved over time, being modified and transmitted systematically to the behavioral repertoire of other spiders in response to stimuli from the environment in which they live. It is as if the organization of these behavioral tactics is facilitated in the spider’s brain. What would explain how certain behaviors are refined? For Japyassú, it would be the ability to learn, characteristic of the brain. “As the spider adds new experiences, certain behaviors are enhanced in response to the challenges posed by the environment.”
FUZITA J.F. et al. High throughput techniques to reveal the molecular physiology and evolution of digestion in spiders. BMC Genomics. V. 17 (716), pp. 1-19. 2016.
JAPYASSÚ, H.F. et al. Predatory plasticity in Nephilingis cruentata (Araneae: Tetragnathidae): Relevance for phylogeny reconstruction. Behaviour.V.139 (4), pp.529-44.2002.