Arachnids and their role in the web of life

by Theresa Peluso

People generally seem to dislike arachnids.  And yet spiders, like Charlotte in Charlotte’s Web (by E.B. White) are renowned for their industriousness, patience and persistence – all qualities we value.  The word “arachnid” comes from the Greek word “Arachne”, which was, in Greek mythology, the name of a skilled mortal weaver who challenged Athena, the goddess of wisdom, arts and crafts, to a weaving contest; this impertinence resulted in Arachne being transformed into a spider.

To research this topic, I used the worldwide “web”. “We also talk about food webs, the web of life, and refer to other complex systems as webs too. At the same time, arachnophobia – the fear of spiders – is a well-known phobia, and the same webs we admire in other circumstances are used in horror movies to depict desolation and decay.

The classification Arachnids, of which there are about 100,000 named species (although estimates of total species range up to 600,000), encompasses a considerable range of subgroups.  There are considered to be 11 orders of Arachnids around today, and 3 orders that have become extinct.  For the purposes of this article, I will use one of the simpler categorizations of Arachnids as any of five groups:  spiders, ticks and mites, scorpions, harvestmen, and pseudoscorpions.

Almost all adult arachnids have eight legs, although the front pair of legs in some species may be modified. In addition, all species have two further pairs of appendages (pedipalps (which are a little like arms) and chelicerae (claw-like mouthparts) adapted for feeding, defence and sensory perception. Their body consists of two parts, the cephalothorax and the abdomen. In addition to having an exoskeleton, arachnids have an internal structure of cartilage-like tissue, to which certain muscle groups are attached. Arachnids are mostly terrestrial – although some are aquatic – and can be found everywhere except in places that are extremely cold, such as the Far North and Antarctica.  They range in size from the mite (about 0.08 mm long) to the black scorpion of Africa (about 21 cm long).  As arachnids grow, they molt several times.  Arachnids are mostly carnivorous, but because most are unable to digest food internally, they inject their prey with digestive fluids, then suck out the liquefied remains. Most arachnids are harmless, and prey only on insects; however, as explained further on, there are a few potentially harmful species.  There are about 40,000 known species of spiders, about 32,000 known species of mites, about 900 known species of ticks, about 6,300 known species of harvestmen, about 2,000 known species of scorpions, and just over 3,000 known species of pseudoscorpions.  Globally, 47 arachnid species are assessed as critically endangered.

Spiders, the largest group of arachnids, rank seventh in total species diversity among all orders of organisms. They have 2 body segments, 8 legs, no chewing mouth parts, and no wings.  They all produce silk.  Many species have spinnerets that produce silk, used to trap insects in webs, form walls for burrows, build egg sacs, and other applications.  Most spiders can inject venom to protect themselves or to kill and liquefy prey. Most spiders have 6 to 8 eyes, although some species have fewer or none. Only about 200 of the 40,000 species of spiders have bites that can cause health problems in humans.  Some scientists are researching whether spider venom can be used as medicines or non-toxic pesticides.

Spiders have no extensor muscles in their limbs; instead, they extend their limbs using hydraulic pressure. The largest species of Canadian spider, which belongs to the Pisauridea family, can reach 15 cm, including its legs, and is able to feed on fishes and salamanders.  Dock spiders are members of this family, and those of us who have seen them can attest to their large size!  Most spiders will live for at most one or two years, although some, like tarantulas, have been known to live up to 25 years in captivity.

Because of their fascinating physiology and behaviour, spiders are a fascinating subject of study for scientists.  Also, because of their importance as predators and their association with particular habitats, they are often used in North America for ecological studies, including as indicators of biodiversity.

Of the 1,379 species of spiders identified in Canada, 714 are ranked as “secure”, 477 as “undetermined”, 62 as “may be at risk” and 56 as “sensitive”.  Last but not least, 70 species of spiders are considered “exotic” in Canada. (Information taken from the Wild Species 2010 report, published by the Canadian Endangered Species Conservation Council http://publications.gc.ca/collections/collection_2011/ec/CW70-7-2010-eng.pdf .)

The total world mass of spiders is huge — about 25 million tonnes! Collectively, these arachnids consume between 400 million and 800 million tonnes of animal prey every year.  On average they consume about 10 percent of their body weight in food, which would be like a 90-kg man eating 9 kg of meat each day.  It is estimated that one spider consumes as many as 2,000 insects in a year. For comparison, humans as a species (all 7 billion of us) and whales as a group each consume about 400 million tonnes of other animals per year.  (Information taken from New Atlas:  Study: Spiders eat more annually than weight of all human adults, by Nick Lavars, March 2017

Clearly, spiders are an essential part of our ecosystem because they regulate the density of other invertebrate herbivores and predators.  Many of the insects they eat are nuisances or dangerous and deadly to humans, such as disease-carrying mosquitoes.  Spiders also help to increase agricultural yields by eating pests that feed on farmers’ crops.  It’s unfortunate that the use of pesticides by farmers is also affecting spider abundance in their fields, thereby exacerbating the problem of pest infestations instead of fixing it.

Ticks and mites, like spiders, have bodies divided into two segments. In the adult stage, they have eight legs. Unlike other arachnids, mites and ticks go through four stages of development: egg, larva, nymph and adult. Mites and ticks vary in size from 0.08 mm to 20 mm.

Ticks, as we may know from unpleasant experience, are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians.  Most ticks belong to one of two major families, the hard ticks or soft ticks.  Tick species are found all over the world, but tend to thrive more in countries with warm, humid climates because they need a certain amount of moisture to undergo metamorphosis, and also because low temperatures restrict their development from egg to larva.  The spread of the black-legged tick, which carries the Lyme disease bacterium, Borrelia burgdorferi, to our part of Ontario has been facilitated by our warming climate.  Let’s hope that the unusually cold temperatures we’ve had this winter will drastically reduce its proliferation!

Now, is there anything positive that can be said in favour of ticks?  They carry and transmit a wide range of diseases to people and pets, diseases that can permanently disable, and even kill them.  Well, maybe it’s a small consolation to realize that ticks are food for other animals, which eat them in large quantity; for example, guinea fowl (native to Africa but raised on many continents), chickens, wild turkeys and opossums. Although many tick bites have no lasting effect, we are well advised to be very cautious about ticks in general, and to make sure we groom ourselves and our pets and livestock frequently, and use tick-control repellents and medications when needed.

What about mites?  Most species of mites feed on dead organic matter, and fungi in forests and grasslands.  This makes them beneficial decomposers, as they break down organic matter, and allow nutrients to be used by plants again. Many other species are predators of other mites and tiny insects, or are parasites of a wide range of animals. Yet other species of mites are significant pests of agricultural plants; and yet, in Massachusetts, many greenhouses that grow flowering plants for sale are using beneficial species of mites and insects to control pests.

A few species of mites physically affect domestic animals and humans.  House-dust mites are common in homes, and can cause allergies to house dust in people.  Some mites are vectors of bacterial-type diseases such as typhus fever, and of several viral diseases, and can cause certain skin diseases like scabies and rosacea.  But for the most part, they are harmless.  In fact, nearly every one of us walks around with his/her own personal collection of mites belonging to the Demodex genus.  Two of these, D.folliculorum and D.brevis, live exclusively on human faces. These mites, found in every ethnic group, especially relish parts of your body that contain oils (sebum), like your face.  These mites spend most of their time buried in our hair follicles, and are most commonly found in our eyelids, nose, cheeks, forehead and chin, but also on other parts of our body as well (which may be considered “too much information”).  Even if you manage to kill all of them, they will come back because they find your face irresistible.  The DNA of mites on East Asian people is distinctly different from that of North and South American populations.  Because these differences exist, studying the mites could tell us how our distant ancestors migrated around the planet, and reveal the relationship between different populations.

For more information, see the August 31, 2012 National Geographic article (https://www.nationalgeographic.com/science/phenomena/2012/08/31/everything-you-never-wanted-to-know-about-the-mites-that-eat-crawl-and-have-sex-on-your-face/ ).

 

Harvestmen, also called “daddy-long-legs” (although some species have short legs), differ from spiders in several ways. The name “harvestmen” comes from their being sighted in late summer and fall.  Similar to spiders, they have the same number of appendages, but only two eyes, vs the spider’s six or eight, only one body segment, and no venom or silk glands.  Perhaps as compensation, harvestmen have a pair of scent glands that secrete a smelly fluid when they’re disturbed. Their method of fertilization is direct, unlike with spiders, which go about it in a very roundabout way involving sperm webs and pedipalps. In addition, the legs of harvestmen easily detach from their bodies, and once they detach, they continue twitching to distract their predators. They are typically about 0.8 cm in length, although some can exceed 15 cm.  Harvestmen are solitary, mainly nocturnal creatures, and prefer moist, shady environments.  Harvestmen are not only completely harmless, but also beneficial because they eat a huge variety of food, preferring insects, such as aphids, beetles, caterpillars and flies, spiders, snails and slugs, as well as vegetable and fecal matter.  In turn, they are food for large spiders, predatory insects and birds.

Scorpions, like spiders, have two body segments and eight legs (including a pair of pedipalps that are basically long pincers); but they also have a six-segment tail, which contains the scorpion’s anus and its sting, including venom glands.  Of the 2,000 known scorpion species, only 30 or 40 have strong enough poison to kill a person. The venom is used for killing prey and for self-defence. Scorpions, which range in size from 9 mm to 23 cm, are found on all major land masses except Antarctica, and in all habitats, including mountains and intertidal zones, in the ground, in trees, rocks and sand.  The only scorpion species known to live in Canada is the northern or boreal scorpion (family Vaejovidae, Paruroctonus boreus), and is found in southern Saskatchewan, Alberta and British Columbia – not in Ontario.  This mildly venomous scorpion is only 30 mm long, and is not known to sting humans. As adults, most scorpions are nocturnal and solitary, usually staying in the same territory throughout their lives. They typically eat insects, but are able to survive lean times by drastically slowing their metabolism, so that they can survive on as little as one insect per year. Because of their predatory nature, they control potential pests.  As with spiders, there’s also the potential for their venom to be developed into pesticides and life-saving medications.  And as part of the web of life, they are food for other animals, such as centipedes, shrews, owls, bats, hornbills, and coyotes. Some animals, like meerkats and mongooses, are immune or resistant to their venom. Despite their scary reputation, some scorpion species are threatened by habitat loss, and by collectors as part of the pet trade.

Most of the time healthy human adults don’t need medical treatment after being stung by a scorpion.  For minor stings, ice and antihistamines or hydrocortisone cream can ease inflammation and itching.  If you are travelling to a region with highly venomous scorpions, the best approach is prevention.  Keep your feet covered, wear gloves if handling rocks, logs or vegetation, shake out your shoes before putting them on, and don’t lie on the bare ground.

The fourth – and for the purposes of this column, the last – order of arachnids is the pseudoscorpion.  They are small (from 2 to 8 mm in length), with a flat, pear-shaped body Pseudoscorpions have eight legs, as well as two very long pedipalps, or pincers; hence the “scorpion” in the name.  The pincers contain a mild venom duct, with which the pseudoscorpion pours a mildly corrosive venom on its prey to capture and immobilize it.  Pseudoscorpions may have two, four or no eyes. Like spiders, they spin silk from a gland in their jaws to make disc-shaped cocoons for mating, moulting, or keeping warm in cold weather.  Because pseudoscorpions are so small and harmless, they are rarely seen although quite common.

About 350 species of pseudoscorpions live in North America, and of these, probably fewer than 50 species in Canada. They can be found in cold regions like Northern Ontario and above the timberline in the Rocky Mountains, although they are most plentiful in warm tropical regions.  They prefer dark, humid habitats; for example, in soil and leaf litter, beneath tree bark, under rocks and logs, or in bird or mammal nests and buildings.  They are most commonly seen in spring or summer. Some like to hitch rides on animals, such as beetles or birds.  Pseudoscorpions eat many types of small insects and other insects and arachnids, including lice and mites.  None of them are harmful to people.

Although not many species of arachnids are considered endangered, we need to avoid anything that can contribute to pollution or habitat loss, as we do for other creatures in our natural world.  Because arachnids play such a critical role in controlling undesirable insects, in providing food for animals higher up in the food web, and in decomposing organic matter, we need to value them.  We can take the necessary precautions to avoid or deal with the harmful ones, and treat the others with the respect and appreciation they deserve.

In conclusion, I leave you with this quote by author Donna Lynn Hope:

Spiders are anti-social, keep pests under control, and mostly mind their own business, but they somehow summon fear in humans who are far more dangerous, deceitful, and have hurt more people.  Of the two, I’m more suspicious about the latter.