Wolf spider. Photo by R. Kraft, Public Domain
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I haven’t done a blog about spiders in a couple of years. The last one, I think, was in August 2018 when I wrote about the electrical properties of spider silk. The proteins that make up spider silk are negatively charged (lots of ionized amino acids!), and flying insects generate positive charges via their wing movements! Opposites attract, and, so, these charge differences can draw insects to the spiders’ sticky webs! For this week’s blog, I have pulled together some recent papers that continue explore unexpected aspects of spider silk and spider behavior!
“Net casting” is a behavior exhibited by the 68 species in the Deinopidae family of spiders. These spiders are found in the tropics all around the world including Florida in the United States. Hunting at night, the net-casting spider makes a web that it stretches out between its four front legs. As it hangs motionless from its perch on a tree or shrub branch it senses ground prey (appropriately sized insects) via its excellent sense of vision (large eyes are a feature of all of the species in this family!) and drops down and entangles the captured insect in its web. The net-caster is also able to sense flying insect prey (especially mosquitoes) and can carry out an elaborately acrobatic leap and back-flip to ensnare passing mosquitoes in its web.
Ogre-faced spider. Photo by B. Dupont. Wikimedia Commons
In a paper published recently (October 29, 2020) in Current Biology, researchers from Cornell, Northwestern, the University of Chicago and the University of Nebraska, looked at one of the net-casting species, Deinopis spinosa (also called the “ogre-faced spider”) in order to determine how these net-caster sense their flying insect prey.
They found that the ogre-faced spider finds its flying prey not by vison but, instead, via sound vibrations. It is well known, though, that spiders do not have ears or other obvious organs for sound perception. Many spiders are, however, known to detect sounds through their mechanical impacts on the silk strands of their webs, but the ogre-spider’s precision in its perception of the location, direction and speed of the passing insect suggest that they were perceiving and evaluating its sound vibrations directly through the air! A number of experiments confirmed that air transmitted sound vibrations were the trigger for the ogre-faced spider’s remarkable leaping and grabbing behavior.
The authors of this paper feel that the leg structure that senses the mechanical vibrations on a spiders’ web is also the organ that the ogre-faced spider uses to sense air-borne sound waves. A spider’s leg is divided into seven segments. The second to the last segment is called the “metatarsus.” On the distal end of the metatarsus is grooved area that is full of specialized setae (hairs) called “trichobothria.” These trichobothria are know to be the sensory transducers for web vibrations and are now thought to have a similar role in reacting to air-borne sounds.
The ogre-faced spider, then, “hears” with its legs!
Spider leg segments. Figure by J.H.Emerton, Wikimedia Commons
It is not known which leg or legs are used in this sensory system, but the ogre-faced spider is sensitive to a wide range of vibrational frequencies. Some of the low frequency sounds correspond to the sounds generated by the wings of flying insects. These frequencies trigger the jumping and grabbing behavior the spider uses to catch its flying prey. Some of the higher frequencies, though, correspond to the sounds made by foraging birds! These frequencies cause the spider to remain motionless on its perch! The ogre-faced spider, then, may be using its ability to “hear” to both find prey and avoid its own predators!
In another paper also published in Current Biology (August 17, 2020) researchers at Georgia Tech described a group of spider species known as the “sling-shot” spiders (three genera in the family Theridiosomatidae). These spiders make a silk that is incredibly strong and elastic. They construct a cone-shaped web that is anchored to a single-stranded tension line of silk. They then use their legs to pull the tension line taut and wait until a suitable insect (often a mosquito) flies into range. They then release the tension line and with an acceleration of 130 g’s and a velocity of 4 meters per second ride their web into the air to ensnare their insect prey.
Sling-shot spider. Photo by L.E.Reeves
The spider on the web maintains its hold on the tension line and after a sling-shot event can pull the web back into position and re-arm the firing mechanism! The sling-shot web, then, can be used over and over again. The ability of the spider silk strand to store and then rapidly release the considerable potential energy of the sling-shot mechanism and its durability and potential for repeated use are properties that are being explored by bioengineers at Georgia Tech and elsewhere! Sheila Patek, a biologist at Duke University who studies biological spring mechanisms but who was not involved in this study, commented on this research, “it just opens up a myriad of areas of research in evolution, neuroscience, materials science, and human engineering.”
Toxeus magnus (male), Photo by Sarefo, Wikimedia Commons
And, finally, in a paper published in Science (November 29, 2018) the remarkable physiology and behavior of a group of social spiders was described. Only a very small percentage of spider species are capable of living in groups. One such social spider, though, is Toxeus magnus, a jumping spider from Taiwan.
Female T. magnus spiders live inside protective nests with their abundant broods of spider-lings. Observations on these nests have indicated that the very small spider-lings do not leave the nest to forage for food. Only the mother spider goes out and finds prey. The mother, though, when she returns to the nest does not regurgitate or deposit food for her offspring. The apparently non-feeding spider-lings, however, grow rapidly and develop into adult forms.
Closer observation inside the nest revealed that the young spider-lings and also a number of relatively mature young spiders (who do make trips outside the nest to find their own food) feed on nutrient-rich secretions produced by the mother spider. These secretions contain abundant sugars and fats and extremely high amounts of protein (four times the protein found in cow’s milk!). The secretions are released by the adult female spider through her epigastric furrow (the abdominal opening through which eggs are released). The spider-lings cluster around the adult female’s furrow and avidly drink up this “spider’s milk.”
Spider-lings feed exclusively on these secretions for the first twenty days of their lives and may continue to occasionally “suckle” on the mother spider until they are fully grown and close to reproductive maturity. These secretions are essential for the survival and growth of the spider-lings. When denied access to them newly hatched spider-lings die within ten days.
Mammals, of course, are the class of vertebrates defined by the mammary glands of females and their ability to make milk to feed their young. Many other animals, though, like this jumping spider, provide nutrition for young life stage individuals. Cockroaches, pigeons, doves, flamingos and penguins all make nutrient rich secretions which they then feed to their young, and some fish and amphibians also produce nutrient-rich skin secretions upon which their young then feed. The consequences of this maternal care include increased interactions between generations and increased strength of social bonding. Cultural transmission of knowledge may also increase as a consequence of this early life stage dependence and nurturing. It would be interesting to study the “non-mammalian” animals that care for their young to see how their ecological and evolutionary pathways are affected.