The production of milk to nourish a baby is a characteristic of mammals, or members of the class Mammalia. The female has glands that have the impressive ability to make a liquid that supplies all of the nutritional needs of the baby. Scientists have recently discovered that a particular species of jumping spider also makes a nutritious liquid that supports the growth of its young. Although the liquid doesn't have the same composition as mammalian milk, the researchers refer to it as milk in their report to the public. It appears to be essential for the development of the spiderlings.
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| An adult male jumping spider (Phidippus audax) Credit: Opoterser, CC BY-SA 3.0 license |
Some Features of Jumping Spiders
Like other spiders, jumping spiders belong to the class Arachnida. They are members of the family Salticidae within the class Arachnida. People may be most familiar with the types of spiders that they encounter in homes, which belong to several different families. The family Salticidae is the largest family in terms of number of species, however.
The animals in the family have excellent eyesight compared to other spiders and are great jumpers, as their name suggests. They have eight eyes: two pairs in the front of their head and two pairs on the top of the head. The two frontmost eyes are very large compared to the others, as shown in the photo above. The rest of the eyes can be seen in the photo below. Most jumping spiders are three quarters of an inch or less in length.
The animals in the family have excellent eyesight compared to other spiders and are great jumpers, as their name suggests. They have eight eyes: two pairs in the front of their head and two pairs on the top of the head. The two frontmost eyes are very large compared to the others, as shown in the photo above. The rest of the eyes can be seen in the photo below. Most jumping spiders are three quarters of an inch or less in length.
The spiders are carnivorous and feed on insects. Scientists have noticed that some species drink nectar in addition to eating insects. Jumping spiders produce silk, but they don’t make webs. The silk is used to make a dragline. Scientists investigating the dragline have shown that it does more than just support the body. The spiders use it to control their orientation in space as well as to stabilize their body when they jump.
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| An adult female jumping spider (Colonus sylvanus) Credit: Kaldari, public domain license |
A Jumping Spider That Resembles an Ant
The research into spider milk was performed by scientists at the Xishuangbanna Tropical Botanical Garden, which is part of the Chinese Academy of Sciences. The spider that they studied was Toxeus magnus, a species of jumping spider that resembles an ant.
Some spiders mimic ants, which helps to protect them from an attack by a predator. The ants produce formic acid, which can irritate and injure the predator. Spiders don’t produce the acid, but having a body that resembles an ant’s can discourage predators from attacking them. Since they are insects, ants have six legs, not eight like a spider. One species of jumping spider that resembles an ant sometimes holds its front pair of legs in the air, making them look like a pair of insect antennae.
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| A male Toxeus magnus Credit: Sarefo, CC BY-SA 3.0 License |
The Epigastric Furrow
The body of a spider consists of two segments: the cephalothorax and the abdomen. The segments are joined by a narrow stalk called the pedicel. The underside or ventral surface of the abdomen has a horizontal furrow just behind the openings to the book lungs and the epigyne in between them. The furrow is known as the epigastric furrow and releases the milk. The illustration below shows the openings to the book lungs and the epigyne, or epigynum, but not the furrow behind them.
The book lungs, or respiratory organs, contain plate-like structures that look like the pages of a book. The organs contain hemolymph, which is the liquid that replaces blood in spiders. The hemolymph absorbs oxygen and releases carbon dioxide. The epigyne is the opening that leads to the female's reproductive system.
The book lungs, or respiratory organs, contain plate-like structures that look like the pages of a book. The organs contain hemolymph, which is the liquid that replaces blood in spiders. The hemolymph absorbs oxygen and releases carbon dioxide. The epigyne is the opening that leads to the female's reproductive system.
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| Ventral surface of a spider Credit: James Henry Emeron, public domain license |
Spider “Milk”
With the aid of a microscope, the researchers discovered that the female Toxeus magnus released droplets of liquid from her epigastric furrow. The spiderlings fed on this liquid for forty days, although they also started hunting for food themselves when they were twenty days old. The spiderlings were considered to be adults at fifty-two days of age.
The researchers discovered that if spiderlings that had just emerged from the egg were prevented from drinking the "milk", they stopped their development and died within ten days. This indicated that the milk was essential for both their development and their survival in the early stage of their life.
The Importance of Maternal Care
Interestingly, the researchers found that after twenty days (when the spiderlings started finding food for themselves as well as drinking milk), the youngsters apparently didn't experience any more benefits from the milk provision on its own. They did benefit from receiving the milk combined with the care of their mother during this time, however.
In addition to feeding the young, the spider mothers took good care of the nest and their offspring. They removed the waste released by the youngsters and repaired any damage to the nest. This activity increased the survival rate of the spiderlings and enabled them to reach the normal body size of an adult.
Interestingly, the researchers found that after twenty days (when the spiderlings started finding food for themselves as well as drinking milk), the youngsters apparently didn't experience any more benefits from the milk provision on its own. They did benefit from receiving the milk combined with the care of their mother during this time, however.
In addition to feeding the young, the spider mothers took good care of the nest and their offspring. They removed the waste released by the youngsters and repaired any damage to the nest. This activity increased the survival rate of the spiderlings and enabled them to reach the normal body size of an adult.
It’s interesting that an animal that many people consider to be relatively primitive has such seemingly advanced behaviour. More research into spider milk and how and exactly where it's produced is needed, but the preliminary results are intriguing. Researchers are finding some surprising features in the animal kingdom. Nature is always fascinating
References
Jumping spider facts from the Menunkatuck Audubon Society (Part of the National Audubon Society)
Jumping spiders mimic ants to defy predators from Cornell University
Mammal-like milk in a jumping spider from the Science Daily news service
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