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Arachnids - Class: Arachnida
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The Class of animals known as Arachnida contains Spiders, Ticks, Mites, and Scorpions as well as a few lesser known kinds of crawlers. Fascinating to some, in many others they evoke very strong emotional reactions. And with good reason perhaps, as Australia is home to quite a few of the... shall we say, 'more interesting' varieties of Arachnid. And some of these, according to eye-witness reports, can grow to blood-curdling proportions.
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On these pages we'll do our best to showcase some of these legendary creatures as well as the more mundane ones. As with Insects', we do not as yet know the full extent of the Arachnid story. More species are discovered on a regular basis and of many species already described, not much is known to date.
We do hope though that by putting these creatures on display here we may help waylay some of our collective prejudices against them. |
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Often elusive, Arachnids are mostly terrestrial creatures with unique body structures. They include a wide range of species such as spiders, scorpions, ticks, mites, and more. One of the most distinctive features of arachnids is their eight legs, unlike insects which have six. In addition to these, they have two more pairs of appendages that have evolved for various purposes such as feeding, defense, and sensory perception. The first pair, known as the chelicerae, are primarily used for feeding and defense. The second pair, the pedipalps, may serve a variety of functions including feeding, locomotion, and in many cases, reproduction. In certain arachnid species, the foremost pair of legs has adapted to perform sensory functions. In others, different appendages may grow large enough to resemble additional pairs of legs. Unlike insects, arachnids do not possess antennae or wings.
The body of an arachnid is typically divided into two main segments, or tagmata: the cephalothorax and the abdomen. These two segments are fused together to form a compact body structure. This is in contrast to insects which have three distinct body segments: the head, thorax, and abdomen. |
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The respiratory systems of Arachnids consist of two types of organs: book lungs and tracheae. The book lungs are so named because they have thin, leaf-like structures arranged in a way that resembles the pages of a book. Gaseous exchange is achieved within these structures. In contrast, the tracheae are tube-like structures that transport air directly to the tissues.
Additionally to these respiratory organs, arachnids also have an 'open circulatory system'. This system involves a fluid called hemolymph, which is analogous to the blood in vertebrates. The hemolymph circulates within spaces in the tissues, known as sinuses. This combination of book lungs, tracheae, and an open circulatory system allows arachnids to efficiently exchange gases and deliver oxygen to their tissues, enabling them to survive in a variety of environments. |
Arachnids have a diverse diet that varies depending on the group and species. Some arachnids, like spiders and scorpions, primarily consume insects or other arachnids. However, they are also known to eat larger animals such as lizards, mammals, birds, and amphibians. Ticks and some mites are parasites that live on another creature and take nutrients from their host and they have mouthparts to suit. Some arachnids even consume plants and fungi. For instance, certain species of mites extract juices from plants or consume fungi.
Many arachnids produce venom that paralyzes their prey. Once the prey is immobilized, the arachnid injects digestive juices into the animal, which liquefies the animal’s insides. The arachnid then sucks up the liquefied insides of the animal. Other arachnids use their chelicerae and pedipalps to crush their prey before using enzymes to further break down the tissues. |
Reproductive behaviours in Arachnids can be quite complex and diverse, with many species exhibiting unique mating rituals and strategies. Typically, the male arachnid will produce a sperm packet, which he then transfers to the female during mating. This process varies widely among different species. In some cases, the male will place the sperm packet on a web or other substrate, and the female will pick it up. In other cases, the male will use specialized appendages to directly transfer the sperm packet to the female. Once the female has received the sperm, she stores it in a special structure called a spermatheca until she is ready to lay her eggs. Only then will she fertilize the eggs with the stored sperm. Eggs are usually laid in a cocoon somewhere protected or carried around by the female until they hatch. Many species also provide extended care for the hatchlings.
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References and links:
> Allen Carson Cohen, A.C., (1998). 'Solid-to-Liquid Feeding: The Inside(s) Story of Extra-Oral Digestion in Predaceous Arthropoda.' American Entomologist, Volume 44, Issue 2, Summer 1998, Pages 103–117. https://doi.org/10.1093/ae/44.2.103
> Framenau, V.W., Baehr, B.C. and Zborowski, P., (2014). 'A Guide to the Spiders of Australia.' New Holland Publishers, Chatswood, N.S.W.
> Schierwater, B. and DeSalle, R., (2021). 'Invertebrate Zoology: A Tree of Life Approach.' CRC Press. ISBN 978-1-4822-3582-1.
https://web.pdx.edu/~smasta/ArachnidInfo.html
https://en.wikipedia.org/wiki/Book_lung
https://en.wikipedia.org/wiki/Hemolymph
https://en.wikipedia.org/wiki/Pedipalp
https://en.wikipedia.org/wiki/Tagma
Header photo:
Erik Beringen.
Erik Beringen.