Friends, enemies or just neighbours

Let’s face it, it’s hard to find a group of animals that elicits more disgust and phobias in humans than arachnids. This completely disproportionate fear, especially in our communities, is exploited in films (from Arachnophobia to the Harry Potter series (Aragog), not forgetting Lord of the Rings (Shelob) or Spiderman, literature or even painting (Odilon Redon). The reality is that humans, depending on where they live, often come into contact with dozens of species of arachnids without realising it. In Belgium, our homes are natural homes to various species of spiders and mites, even when perfectly maintained.

Just neighbours

Most of us believe we live in barren houses where we’re the only tenants. Nothing could be further from the truth! In 2016, a group of researchers explored the biodiversity found in fifty individual houses around Raleigh, state capital of North Carolina in the USA. The results, published in a scientific journal, showed that the houses were home to between 32 and 211 species of arthropods (100 species on average), of which about 19% were spiders. Of the 554 rooms that were searched from top to bottom, only five didn’t contain any species of anthropods. The majority of these species were not harmful; humans are simply in touch with them without knowing it. It is obvious that many species of arachnids are present in our homes as well. Mostly spiders and mites are found indoors, but other groups (Opiliones) are also found around our homes. In biology, species that develop a tolerance towards the presence of humans, ranging from occasional or circumstantial cohabitation to preference for or even complete dependence on the presence of humans, are called anthropophiles (a form of synanthropy). The most common spiders in our country include house spiders, daddy long legs and false widow spiders. Less spectacular but just as present, dust mites are everywhere in our homes. Dust mites (Pyroglyphidae) are a group of 13 species found in homes around the world. Their abundance is influenced by various factors such as temperature and humidity, sometimes causing medical problems, as we will see below. Their food consists of mould and human dander (skin flakes). The average human actually sheds enough dead skin cells in one day to maintain a population of 10,000 dust mites for six months.

Domestic house spider (Tegenaria domestica)

The domestic house spider (Agelenidae) is a common house spider found in Europe but has been introduced to many other countries around the world. Its body is modest in size (6–9 mm for males and 7–12 mm for females) but its leg span (including legs) can be substantial (up to 50 mm). It usually weaves a relatively horizontal web extended by a tube inside it. This nocturnal species is completely harmless to humans. It feeds on any arthropod that falls or ventures into its web. When they go in search of a female to reproduce, the males often cause a stir among members of the household, especially when they happen to fall into the bathtub with no way of escape.

The Pholcus Phalangioides (Pholcus phalangioides)

Resembling Opiliones because of its long, slender legs, the Pholcus Phalangioides (Pholcidae) is a typical house spider, often found in cellars and typically on ceilings. Its body measures between 5 and 8 mm. Like the domestic house spider, the Pholcus Phalangioides
is found in Europe but also in many other countries. Totally harmless to humans, it flees when disturbed or vibrates its web so fast that it almost disappears from view.

Cupboard spider (Steatoda grossa)

The cupboard spider (Theridae) is cosmopolitan. Found in homes in Europe, America and Australia, it is often found in dark corners, to such an extent that it is less noticeable than the two previous species. The males measure 5 to 10 mm and the females 6 to 10 mm. Their abdomen is globular and their morphology resembles the black widow’s. This resemblance leads to a large number of calls each year from people who believe the famous spiders to be residing in their homes. The species is in no way aggressive and always seeks to escape. However, when handled or disturbed, it can bite, although the bite doesn’t have any serious consequences. Resembling the black widow spider,without being as dangerous as it ,this spider is a popular species on film production sets.
In Sam Raimi’s 2002 film Spiderman, for example, it was the Steatoda grossa that gave Peter Parker his superpowers by biting him.

Did you know that there are about 700 species of spiders in Belgium and only a few of them (no more than a dozen) are able to pierce the skin of a human being by biting it? None of them are deadly of course!


Arachnids are not the most popular group of animals among humans. Andwhile it is true that a tiny fraction of the venomous species in this clade are certainly deadly, many researchers are meticulously studying the chemical compounds of venoms to explore their therapeutic properties. In both scorpions and spiders alike, the potential is wide-ranging and antifungal, antibacterial, antiviral, anti-tumour and analgesic therapeutic applications have been introduced. Expectations for the future of these new molecules are such that a European project, VENOMICS, has been launched by various countries including Belgium. Although no drug has yet been put on the market, intensive international efforts are under way and promising publications have proliferated. For example, a study by Brazilian researchers published in February 2019 in the reliable Journal of Sexual Medicine revealed that they had successfully synthesised a protein (PnPP-19) from the venom of the dangerous banana spider (Phoneutria nigriventer) and integrated it into a gel capable of treating erectile dysfunction, particularly among patients who cannot be treated with Viagra!

Arthropods, and in particular insects, cause extensive damage to agricultural crops. They are also vectors for serious diseases such as malaria,which affects about 216 million people each year and kills 445,000. Insecticides used today exhibit resistance problems or cause significant environmental damage. The development of “bioinsecticides” is a key issue for the future. Arachnids are among the most efficient venomous predators of insects. Important research is therefore under way to isolate and undertake documentary analysis of tomorrow’s bioinsecticides. In 2014, the American company VESTARON CORPORATION patented the first insecticide derived from spider venom. Marketed under the name SPEAR T, it is widely used in greenhouses.

While agricultural destruction is caused by insects, it is also caused by certain arachnids, in particular mites. Mites pierce the epidermis of host plants to suck out the cell contents. Depending on the species, they can attack various types of plants and many parts of them. Integrated pest management is increasingly incorporating the use of mites which prey on mites to limit losses. A Belgian company, BIOBEST GROUP, which began its operations by producing bumblebee hives for pollination in greenhouses, is currently developing IPM methods to increase predatory mites by feeding them before pests arrive.


While there is no question that arachnids include several deadly venomous species, they are far from being among the most harmful to human populations. While spider envenomation is hard to count, there are an estimated 1.5 million cases of scorpion envenomation each year, causing between 2,000 and 3,000 deaths. Serious bites and stings are particularly geographically localised. Thus, serious cases in Europe have numbered a few dozen in recent years, almost exclusively in the Mediterranean region where a cousin of the black widow, the Malmignatte (Latrodectus (mactans) tredecimguttatus), resides. Let’s face it, compared to the 95,000 to 120,000 deaths per year from snake bites or the two million people killed indirectly by mosquito bites, it is helpful to place spiders and scorpions in perspective. In Belgium, we don’t have any dangerous species of spiders and the vast majority of those we encounter don’t even have the capacity to penetrate human skin with their fangs.

The real enemies are actually to be found in the group of mites. This group, which also includes ticks, has numerous species of medical or veterinary importance, not to mention crop pests. Let’s start with dust mites which we have talked about. These are found throughout our homes and are believed to be responsible for about 45% of allergies and cause many asthma attacks when their concentration becomes too high. Human scabies, caused by the Sarcoptes mite (Sarcoptes scabiei), is a disease present in many countries and is actively spreading (+- 300 million new cases each year). Even though it is benign, scabies doesn’t heal spontaneously. Then there are all the relatively serious diseases that are transmitted indirectly by this group, such as Lyme disease, considered to be the most serious arthropod-borne disease in Europe and North America. With this disease, the tick (Ixodes ricinus or other species) transmits Spirochaetes bacteria (Borrelia sp.) If treated quickly and effectively with antibiotics for two to four weeks (90% of cases), the disease won’t leave any sequelae. Unfortunately, if left untreated, the disease can develop over several years. It can then take on cutaneous, articular or neurological forms, either acutely or chronically. Other diseases include Mediterranean spotted fever, Q fever, tick-borne encephalitis, babesiosis and scrub typhus.

The various diseases caused directly or indirectly by this group of arachnids aren’t limited to humans, but can also affect animals, both pets and livestock, with serious and severe economic consequences.

Finally, this group includes major crop pests. It is estimated that they destroy between 18 and 26% of agricultural crops each year, for an estimated annual cost of $470 billion.

Did you know that in popular usage, the term tarantula is synonymous with Mygalomorphae, the large hairy spiders? This is a mistake due to the fact that, in English, tarantula is in fact the term for Mygalomorphae. However, in French, the tarantula is actually a much smaller wolf spider (Lycosa tarantula) from the Taranto region of Italy.