Venomous and poisonous arthropods: what makes them different?

After talking about venomous mammals, fishes and lizards, ‘All you need is Biology’ brings you this post about venomous and poisonous arthropods. We will try to explain you what makes them different and which arthropods produce some kind of toxic substance (and how they do it). It will probably surprise you!

Venomous vs poisonous animals

Although some people normally use these words interchangeably, they really mean the same? The answer is NO.

A venomous animal develops specialized organs or elements (such as fangs, teeth or stings) to actively inoculate venom inside the body of their victim as an offensive or defensive mechanism. On the other hand, a poisonous animal does not develop these type of organs, but specialized tissues or glands that produce toxins that are released passively as a defensive system; others acquire these substances from their diet. Sometimes, the toxin is not produced in any specific organ, but integrated within body tissues as a defense against predation.

Despite these differences, once in the body venoms and toxins can cause similar damage, which depends on their mode of action, the assimilated amount and the victim’s features. In humans, effects caused by these substances range from irritation, inflammation or redness to severe systemic damage in cases of powerful poisons.

Venomous and poisonous arthropods

Arachnids

Arachnids (subphylum Cheliceromorpha) include two of the better known venomous arthropods: spiders and scorpions. Both groups develop specialized organs to inoculate venomous substances which use either to hunt and defend themselves against predators or potential enemies.

  • Spiders

The specialized organs for venom inoculation in spiders are the chelicerae, a pair of preoral appendices typical of Cheliceromorpha which they use to grab the food. Spiders’ chelicerae, which are fang-shaped, are related to basal venom glands. These fangs have an internal duct that finish in a terminal opening through which venom is released and injected inside victims’ bodies like a hypodermic needle.

Spiders have the most evolved form of chelicerae: jackknife chelicerae. The two parts of the chelicerae come together like a folding knife, and when threatening to attack, the spiders rise the chelicerae and open the angle of the fangs.

Spider’s chelicerae. Public domain image (CC0) obtained from pixabay.

Some of the most dangerous spiders for humans are the Australian funnel-web spiders (genera Atrax, Hadronyche and Illawarra). Their venom is toxic to sodium channels, which results in the massive release of neurotransmitters.

“Funnel web spider” of the species Hadronyche cerberea. Have you noticed the drop of venom in its chelicer?. Picture by Alan Couch on Flickr (CC 2.0).
  • Scorpions

The most distal part of the scorpion tail, the telson (an additional segment found in several arthropods), has become a venomous organ that ends in a stinger. Like chelicerae in spiders, telson in scorpions is related to venom glands that contain toxic substances.

Scorpion of the species Centruroides vittatus, common in the middle of EUA and in the north of Mexico. In red, telson ended in a sting. Public domain image (CC0).

Scorpion venom is usually rich in neurotoxins that alter both the central and the peripheral nervous system of the victim by dissociating the parasympathetic and sympathetic nervous systems. In humans, the effects of their sting vary from intense local pain (with minor inflammation) to cardiac arrhythmias and acute pulmonary edema, like in the Indian species Hottentotta tamulus, which is considered one of the most venomous scorpions in the world.

BE CAREFUL! Neither all arachnids nor related groups are venomous; e. g. harvestmen, camel spiders and whip spiders (Amblypygi) ARE NOT venomous.

From left to right: harvestman (Daniel Jolivet on Flickr, CC 2 .0), camel spider (CC 3.0) and whip spider (Geoff Gallice on Flickr).

Myriapoda

The subphylum Myriapoda is divided in two classes: Diplopoda (millipedes) and Chilopoda (centipedes), and both produce toxic substances.

  • Millipedes

Millipedes, which have an elongated body composed of a lot of segments with two pairs of legs (rarely just one pair), are detritivores and inoffensive. However, they release toxins (alkaloids, benzoquinones, phenols) as a defensive mechanism to prevent predation. Some of these released substances are caustic and can burn the exoskeleton of other arthropods or cause skin and mucous inflammation in bigger animals.

Millipede toxins are produced inside repugnatorial or odoriferous glands and then excreted through small micropores located at both sides of the body when being crushed or feeling threatened.

At the first sight, micropores are difficult to see. Picture by Thomas Shahan on Flickr (CC 2.0).

TRIVIA: black lemurs from Madagascar (Eulemur macaco) grab and bite millipedes to stimulate their secretions, and then rub them all over their body. It is thought that lemurs cover themselves on millipede’s toxins since these work as insect repellent.

If you want to learn some more about this behaviour, don’t miss the following video. We recommend you to stay until the end…the final result will probably surprise you!

  • Centipedes

Centipedes also have a segmented body like millipedes; however, each segment has just a pair of legs. While millipedes are detritivores, centipedes are carnivorous arthropods that hunt their preys actively. To do so, they have developed two large forcipules originated from the first pair of legs which can inject venom contained in glands in the trunk of the animal. They also bite when feeling threatened.

Forcipules of Scolopendra cingulata, by Eran Finkle (CC 3.0).

The Scolopendra genus causes the most severe injuries. However, despite causing an intense pain when stinging, almost all envenomations caused by centipedes spontaneously resolve without complications.

Insects

Despite their diversity, there exist just a few cases of venomous/poisonous insects (class Insecta).

  • Beetles

Some beetle families (Coleoptera order), such as Meloidae, Oedemeridae and Staphylinidae (Paederus and Paederidus genera) contain toxins within their hemolymph which are released by compression as a defensive strategy against predators. These substances cause skin burns, redness and inflammation in humans.

Sptaphylinidae of the species Paederus littoralis, from Spain, France and Italy. Picture by Alvesgaspar (CC 4.0).

Meloidae and Oedemeridae hemolymph contain cantharidine, while the one of Paederus and Paederidus contains pederine, a substance that is exclusive of females of these beetles and of certain marine sponges, and which is thought to be produced by symbiont bacteria.

  • Bugs

Although some bugs (suborder Heteroptera) are better known for being disease vectors, they also cause different types of skin injuries in humans due to the release of caustic and inflammatory substances as a defense when being compressed (e. g. Pentatomidae family) or by the injection of salivary enzymes that are normally used to kill and dissolve preys (e. g. Belostomatidae family).

Belostomatidae. Public domain image (CC0).
  • Hymenopterans

Most of wasps, bees and ants (Hymenoptera order) produce toxins as a defensive mechanism. In most of those cases, females develop a stinger at the end of the abdomen resulting from the evolution of the ovipositor (Aculeata infraorder); however, there are also some groups that defend themselves by biting.

Ants (Formicidae family) usually attack by biting, but some species, such as those in the group of the fire ants (Solenopsis spp.) and the bullet ants (Paraponera spp., Dinoponera spp.), also have stingers like bees and wasps. Formic acid probably is the best-known toxin produced by ants, but is unique to the Formicinae subfamily; fire ants, for example, inject piperidine alkaloids. The sting of the bullet ants, which are distributed throughout center and south America, is considered the most painful sting for humans caused by an insect according to the Schmidt Index (which considers it to be as painful as a gunshot!).

Red ant of the species Solenopsis invicta (left, public domain image (CC0)) and bullet ant of the species Paraponera clavata (right, April Nobile / © AntWeb.org / CC BY-SA 3.0).

Females of most of bees and wasps within the Aculeata group develop an abdominal stinger. Their venom is usually rich in phospholipases, producing effects ranging from local inflammation to severe anaphylactic reactions (when suffering of hypersensibility or after being attacked by thousands of insects, as it has happened several times with the killer bee in America). The sting of the tarantula hawk (Pepsis formosa) from Mexico and southern USA, is considered the second most painful after the one of the bullet ant.

Pepsis formosa, a tarantula hawk. Public domain image (CC0).
  • Butterflies and moths

A lot of butterflies and moths (Lepidoptera order) produce toxins either during their larval stages, adulthood or both as a defensive mechanism against predation.

Sometimes, caterpillars are covered by urticant bristles or hairs that cause skin lesions (erucism), as in the case of the pine processionary (Thaumetopoea pityocampa), a harmful plague for pines which is very spread in southern Europe and America.

Pine processionary caterpillar nest, by John H. Ghent (CC 3.0).

On the other hand, adults of some species, like those of the monarch butterfly (Danaus plexippus) and Zygaena spp., both showing flashy colors (aposematism, a type of animal mimicry), develop toxins within their corporal tissues to prevent predation. The monarch butterfly obtains these substances by feeding on toxic plants of the Asclepias genus.

Zygaena transalpina, by gailhampshire (CC 2.0).

.             .             .

Have you found this information interesting? Do you know any other venomous or poisonous arthropod? Feel free to leave your comments below!

References

The main image is of public domain (CC0) and was downloaded from Pixabay.

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